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Design problem for scientific lamp current source.
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Geoff
Nov 13, 2012, 5:45:00 PM11/13/12
to
http://i251.photobucket.com/albums/gg302/triode101/sink.jpg
This circuit is to drive a scientific lamp which is low pressure
neon of strike volts 600V and typ run volts of about 200V. The
design intent is DC current from 0 to 60mA. This circuit
traditionally runs to about 30mA with no problem and is well
proven at these currents. Normally it is not darlington pair but
it is something I have tried..
The trouble I have with this circuit is that at above currents
of about 35-40mA the lamp starts to flicker indicating a higher
current than is dialed in by the pot. An oscilloscope at the
opamp output confirms that at the point where it starts to
flicker, the loop starts to try to lower the current since the
opamp output volts drops. The onset can be made to be a periodic
dip (tens of hz dip about a volt) before the lamp begins to
flicker. When the lamp flickers on and off more severely, the
opamp tries to go negative to compensate. This to me shows that
the output transistor is conducting, not the failure of the loop
control.. I have played around with the drive impedance, ie
with/without the 1k base resistor. The point of flicker can be
less severe with the resistor, or latches hard without. Maybe
the VCEO spec is being stretched here. I have tried a variac and
reduced the volts to about 580V, still does it but onset is
higher current (50mA). I have tried a BUL 416 transistor also. I
might just be applying too much voltage, but I can't see why,
might have to look at the op curves a bit harder. Maybe I could
use a MOSFET or IGBT.
Tim Wescott
Nov 13, 2012, 6:19:16 PM11/13/12
to
sure to dissipate the heat, or what?
Given that the data sheet says nothing about the transistor cutoff
frequency or input capacitance, I suspect that it's not anything to write
home about. That slow of oscillation is surprising, but the 1N4007 is
going to slow down the speed that you can turn things off, because at
that point you'll be working the C-B capacitance of the predriver against
the 1k-ohm resistor.
I'd look at the base voltage of the drive transistor vs. the opamp
voltage, and see if you learn anything.
I'd also see if I could find some higher voltage devices. I know they'll
be thin on the ground at that voltage level, but there should be
something, and then you wouldn't have to worry about your 700V transistor
misbehaving with your 800V supply.
Assuming that you could stabilize it, a MOSFET or IGBT should slide right
in there.
--
My liberal friends think I'm a conservative kook.
My conservative friends think I'm a liberal kook.
Why am I not happy that they have found common ground?
Tim Wescott, Communications, Control, Circuits & Software
http://www.wescottdesign.com
Bill Sloman
Nov 13, 2012, 6:21:27 PM11/13/12
to
On Nov 14, 9:45 am, Geoff <pub...@email.com> wrote:
> http://i251.photobucket.com/albums/gg302/triode101/sink.jpg
>
> This circuit is to drive a scientific lamp which is low pressure
> neon of strike volts 600V and typ run volts of about 200V. The
> design intent is DC current from 0 to 60mA. This circuit
> traditionally runs to about 30mA with no problem and is well
> proven at these currents. Normally it is not darlington pair but
> it is something I have tried..
This is a slightly Russian circuit.
> http://i251.photobucket.com/albums/gg302/triode101/sink.jpg
>
> This circuit is to drive a scientific lamp which is low pressure
> neon of strike volts 600V and typ run volts of about 200V. The
> design intent is DC current from 0 to 60mA. This circuit
> traditionally runs to about 30mA with no problem and is well
> proven at these currents. Normally it is not darlington pair but
> it is something I have tried..
A more subtle approach is to use a transformer to apply the 600V
striking voltage to the lamp for long enough to get the discharge
going - less than a microsecond for a glow discharge, some
microseconds for an arc - which lets you get away with a supply
voltage a lot closer to the 200V running voltage of the lamp.
> The trouble I have with this circuit is that at above currents
> of about 35-40mA the lamp starts to flicker indicating a higher
> current than is dialed in by the pot. An oscilloscope at the
> opamp output confirms that at the point where it starts to
> flicker, the loop starts to try to lower the current since the
> opamp output volts drops. The onset can be made to be a periodic
> dip (tens of hz dip about a volt) before the lamp begins to
> flicker. When the lamp flickers on and off more severely, the
> opamp tries to go negative to compensate. This to me shows that
> the output transistor is conducting, not the failure of the loop
> control.. I have played around with the drive impedance, ie
> with/without the 1k base resistor. The point of flicker can be
> less severe with the resistor, or latches hard without. Maybe
> the VCEO spec is being stretched here. I have tried a variac and
> reduced the volts to about 580V, still does it but onset is
> higher current (50mA). I have tried a BUL 416 transistor also. I
> might just be applying too much voltage, but I can't see why,
> might have to look at the op curves a bit harder. Maybe I could
> use a MOSFET or IGBT.
need to look at the frequency compensation. That 10nF capacitor is
decoupling the op amp from the output stage at higher frequencies, but
it may not be the right value to do what you want, and it could be
contributing extra phase shift where you don't actually want it.
Get hold of a text-book that talks about stabilising negative feedback
loops and read it carefully. "The Art of Electronics" by Horowitz and
Hill might be one such textbook - though it covers a lot more than
just negative feedback.
Some of our regular posters have published more specialised texts, and
may be prepared to stick their necks out further.
--
Bill Sloman, Sydney
--
Bill Sloman, Sydney
Tim Williams
Nov 13, 2012, 7:05:13 PM11/13/12
to
SOA says you can only run 600V up to a hair over 10mA, maybe 12mA. BJTs
are a bad idea for dropping high voltages due to second breakdown.
Why not choose a tube like 6L6GC? Current production, doesn't take much
heater power, and takes the voltage easily. Okay, a MOSFET would be
smaller and cheaper (though probably about even if you include the
heatsink, come to think of it).
Note you're trying to carry a maximum of (800V - 200V) * 0.06A = 36W,
which is a moderate amount for a small transistor. Just in case, you'd be
better off with a TO-247 package, or two TO-220s in parallel.
Switching methods are much more efficient, even if you can't run the thing
directly on AC (I assume that's part of the design requirement).
Tim
--
Deep Friar: a very philosophical monk.
Website: http://seventransistorlabs.com
"Geoff" <pub...@email.com> wrote in message
news:XnsA10ABEB...@88.198.244.100...
are a bad idea for dropping high voltages due to second breakdown.
Why not choose a tube like 6L6GC? Current production, doesn't take much
heater power, and takes the voltage easily. Okay, a MOSFET would be
smaller and cheaper (though probably about even if you include the
heatsink, come to think of it).
Note you're trying to carry a maximum of (800V - 200V) * 0.06A = 36W,
which is a moderate amount for a small transistor. Just in case, you'd be
better off with a TO-247 package, or two TO-220s in parallel.
Switching methods are much more efficient, even if you can't run the thing
directly on AC (I assume that's part of the design requirement).
Tim
--
Deep Friar: a very philosophical monk.
Website: http://seventransistorlabs.com
"Geoff" <pub...@email.com> wrote in message
news:XnsA10ABEB...@88.198.244.100...
George Herold
Nov 13, 2012, 7:10:03 PM11/13/12
to
If you have voltage to throw away you can always try putting in more
R.
And sure why not a high voltage FET.
George h.
Geoff
Nov 13, 2012, 7:18:19 PM11/13/12
to
Tim Wescott <t...@seemywebsite.com> wrote in
news:ptSdnZVX6KzpSD_N...@web-ster.com:
I am looking at MOSFETS now.
news:ptSdnZVX6KzpSD_N...@web-ster.com:
Geoff
Nov 13, 2012, 7:20:53 PM11/13/12
to
Bill Sloman <bill....@ieee.org> wrote in
news:69b959c3-b683-486c...@vb8g2000pbb.googlegr
oups.com:
Your point about separating the run and strike ccts is well
taken. I might go that way if I can't find another solution
using the present transformer.
I don';t think it is a stability problem because at the point
of breaking down, the volts to the base are only dipping,
indicating the loop is trying to prevent the output from
conducting.
news:69b959c3-b683-486c...@vb8g2000pbb.googlegr
oups.com:
taken. I might go that way if I can't find another solution
using the present transformer.
I don';t think it is a stability problem because at the point
of breaking down, the volts to the base are only dipping,
indicating the loop is trying to prevent the output from
conducting.
Geoff
Nov 13, 2012, 7:24:38 PM11/13/12
to
"Tim Williams" <tmor...@charter.net> wrote in
news:k7un7q$hfd$1...@dont-email.me:
> SOA says you can only run 600V up to a hair over 10mA,
> maybe 12mA. BJTs are a bad idea for dropping high voltages
> due to second breakdown.
>
> Why not choose a tube like 6L6GC? Current production,
> doesn't take much heater power, and takes the voltage
> easily. Okay, a MOSFET would be smaller and cheaper
> (though probably about even if you include the heatsink,
> come to think of it).
>
> Note you're trying to carry a maximum of (800V - 200V) *
> 0.06A = 36W, which is a moderate amount for a small
> transistor. Just in case, you'd be better off with a
> TO-247 package, or two TO-220s in parallel.
>
> Switching methods are much more efficient, even if you
> can't run the thing directly on AC (I assume that's part of
> the design requirement).
>
> Tim
>
I agree I'm pushing the limits of the SOA. Now that this is
becoming clear that it is the problem, I can tackle it in the
best way I can. I had thought about a tube too. For the moment I
might try a MOSFET in a TO247 pack. The SOA is a bit more
friendly at the high voltage end.
Switch mode is out of the question at present, I know the
customer has a huge dislike, so I am doing it the way he wants.
news:k7un7q$hfd$1...@dont-email.me:
> SOA says you can only run 600V up to a hair over 10mA,
> maybe 12mA. BJTs are a bad idea for dropping high voltages
> due to second breakdown.
>
> Why not choose a tube like 6L6GC? Current production,
> doesn't take much heater power, and takes the voltage
> easily. Okay, a MOSFET would be smaller and cheaper
> (though probably about even if you include the heatsink,
> come to think of it).
>
> Note you're trying to carry a maximum of (800V - 200V) *
> 0.06A = 36W, which is a moderate amount for a small
> transistor. Just in case, you'd be better off with a
> TO-247 package, or two TO-220s in parallel.
>
> Switching methods are much more efficient, even if you
> can't run the thing directly on AC (I assume that's part of
> the design requirement).
>
> Tim
>
becoming clear that it is the problem, I can tackle it in the
best way I can. I had thought about a tube too. For the moment I
might try a MOSFET in a TO247 pack. The SOA is a bit more
friendly at the high voltage end.
Switch mode is out of the question at present, I know the
customer has a huge dislike, so I am doing it the way he wants.
lang...@fonz.dk
Nov 13, 2012, 7:26:42 PM11/13/12
to
On Nov 14, 1:05 am, "Tim Williams" <tmoran...@charter.net> wrote:
> SOA says you can only run 600V up to a hair over 10mA, maybe 12mA. BJTs
> are a bad idea for dropping high voltages due to second breakdown.
>
> Why not choose a tube like 6L6GC? Current production, doesn't take much
> heater power, and takes the voltage easily. Okay, a MOSFET would be
> smaller and cheaper (though probably about even if you include the
> heatsink, come to think of it).
>
> Note you're trying to carry a maximum of (800V - 200V) * 0.06A = 36W,
> which is a moderate amount for a small transistor. Just in case, you'd be
> better off with a TO-247 package, or two TO-220s in parallel.
>
> Switching methods are much more efficient, even if you can't run the thing
> directly on AC (I assume that's part of the design requirement).
>
> Tim
>
an upside down buck similar to this should be doable:
> SOA says you can only run 600V up to a hair over 10mA, maybe 12mA. BJTs
> are a bad idea for dropping high voltages due to second breakdown.
>
> Why not choose a tube like 6L6GC? Current production, doesn't take much
> heater power, and takes the voltage easily. Okay, a MOSFET would be
> smaller and cheaper (though probably about even if you include the
> heatsink, come to think of it).
>
> Note you're trying to carry a maximum of (800V - 200V) * 0.06A = 36W,
> which is a moderate amount for a small transistor. Just in case, you'd be
> better off with a TO-247 package, or two TO-220s in parallel.
>
> Switching methods are much more efficient, even if you can't run the thing
> directly on AC (I assume that's part of the design requirement).
>
> Tim
>
http://www.irf.com/product-info/datasheets/data/irs2980spbf.pdf
could make one with current sense on the low side
-Lasse
Geoff
Nov 13, 2012, 7:27:45 PM11/13/12
to
>
> Why the Darlington?
>
>
I had adapted the circuit from a similar one. I am not using the
darlington now. The problem persists, probably second breakdown.
> If you have voltage to throw away you can always try
> putting in more R.
>
> And sure why not a high voltage FET.
>
> George h.
Tim Wescott
Nov 13, 2012, 7:30:59 PM11/13/12
to
oscillation where you want to see steady operation. The only question is
where the oscillation is coming from, which is why I was suggesting
looking at other points in the circuit than just the op-amp output.
The circuit topology is correct assuming that there's not a lot of
coupling between the collector and base of the transistors, that the
Darlington stage isn't too slow. That 10nF cap is in the right place,
but Bill is correct that it may be too small.
Jamie
Nov 13, 2012, 9:16:00 PM11/13/12
to
tricky with so many slow components in line.
Using darlington adds to the problem with the storage time in the
trannies and the use of that diode in the base with nothing but a 1k
to pull it up isn't helping much.
That circuit needs the integrator/miller cap greatly reduced, diode
removed and possibly a Cap lead network from the feed back to give it
a little derivative in the - feed back. At least that will help with the
phase error and reduce os-kill-La-Trons! :)
MeSelf, I think a PWM would be nice but then again, we don't know what
the adverse effect of flicker would be..
Jamie
Bill Sloman
Nov 13, 2012, 9:33:18 PM11/13/12
to
On Nov 14, 11:31 am, Tim Wescott <t...@seemywebsite.com> wrote:
> On Wed, 14 Nov 2012 00:20:53 +0000, Geoff wrote:
> > Bill Sloman <bill.slo...@ieee.org> wrote in
> On Wed, 14 Nov 2012 00:20:53 +0000, Geoff wrote:
> >news:69b959c3-b683-486c...@vb8g2000pbb.googlegroups.com:
presumably been put in there to do.
--
Bill Sloman, Sydney
Spehro Pefhany
Nov 13, 2012, 9:35:21 PM11/13/12
to
On Wed, 14 Nov 2012 00:24:38 +0000 (UTC), the renowned Geoff
<pub...@email.com> wrote:
>
>
>I agree I'm pushing the limits of the SOA. Now that this is
>becoming clear that it is the problem, I can tackle it in the
>best way I can. I had thought about a tube too. For the moment I
>might try a MOSFET in a TO247 pack. The SOA is a bit more
>friendly at the high voltage end.
>
>Switch mode is out of the question at present, I know the
>customer has a huge dislike, so I am doing it the way he wants.
Where does the 800V come from?
<pub...@email.com> wrote:
>
>
>I agree I'm pushing the limits of the SOA. Now that this is
>becoming clear that it is the problem, I can tackle it in the
>best way I can. I had thought about a tube too. For the moment I
>might try a MOSFET in a TO247 pack. The SOA is a bit more
>friendly at the high voltage end.
>
>Switch mode is out of the question at present, I know the
>customer has a huge dislike, so I am doing it the way he wants.
One thing I've done in the past is to make a weak voltage doubler that
drops down to about half when you start drawing serious current. The
effective series impedance would help stabilize the circuit, and it
would dissipate a lot less power.
With your circuit, you could try a 10K 100W resistor instead of the
330R. They even come in TO247 if that's what you really want.
Best regards,
Spehro Pefhany
--
"it's the network..." "The Journey is the reward"
sp...@interlog.com Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog Info for designers: http://www.speff.com
tm
Nov 13, 2012, 9:43:29 PM11/13/12
to
"Spehro Pefhany" <spef...@interlogDOTyou.knowwhat> wrote in message
news:mc06a81kbaukpk9kj...@4ax.com...
> On Wed, 14 Nov 2012 00:24:38 +0000 (UTC), the renowned Geoff
> <pub...@email.com> wrote:
>
>>
>>
>>I agree I'm pushing the limits of the SOA. Now that this is
>>becoming clear that it is the problem, I can tackle it in the
>>best way I can. I had thought about a tube too. For the moment I
>>might try a MOSFET in a TO247 pack. The SOA is a bit more
>>friendly at the high voltage end.
>>
>>Switch mode is out of the question at present, I know the
>>customer has a huge dislike, so I am doing it the way he wants.
>
> Where does the 800V come from?
>
> One thing I've done in the past is to make a weak voltage doubler that
> drops down to about half when you start drawing serious current. The
> effective series impedance would help stabilize the circuit, and it
> would dissipate a lot less power.
>
> With your circuit, you could try a 10K 100W resistor instead of the
> 330R. They even come in TO247 if that's what you really want.
>
>
That's exactly how many HeNe gas laser power supplies work. The first stage
> <pub...@email.com> wrote:
>
>>
>>
>>I agree I'm pushing the limits of the SOA. Now that this is
>>becoming clear that it is the problem, I can tackle it in the
>>best way I can. I had thought about a tube too. For the moment I
>>might try a MOSFET in a TO247 pack. The SOA is a bit more
>>friendly at the high voltage end.
>>
>>Switch mode is out of the question at present, I know the
>>customer has a huge dislike, so I am doing it the way he wants.
>
> Where does the 800V come from?
>
> One thing I've done in the past is to make a weak voltage doubler that
> drops down to about half when you start drawing serious current. The
> effective series impedance would help stabilize the circuit, and it
> would dissipate a lot less power.
>
> With your circuit, you could try a 10K 100W resistor instead of the
> 330R. They even come in TO247 if that's what you really want.
>
>
of the CW multiplier have large capacitors. Then it has enough stages with
smaller capacitors to make the start voltage. A proper current limiting
ballast resistor establishes the run current.
Owen Roberts
Nov 13, 2012, 9:50:18 PM11/13/12
to
Take a look at the I-V curve for the lamp. You may be entering a
region of oscillation in the plasma.
Steve
region of oscillation in the plasma.
Steve
Geoff
Nov 13, 2012, 10:31:56 PM11/13/12
to
Spehro Pefhany <spef...@interlogDOTyou.knowwhat> wrote in
news:mc06a81kbaukpk9kj...@4ax.com:
> On Wed, 14 Nov 2012 00:24:38 +0000 (UTC), the renowned
> Geoff <pub...@email.com> wrote:
>
>>
>>
>>I agree I'm pushing the limits of the SOA. Now that this is
>>becoming clear that it is the problem, I can tackle it in
>>the best way I can. I had thought about a tube too. For the
>>moment I might try a MOSFET in a TO247 pack. The SOA is a
>>bit more friendly at the high voltage end.
>>
>>Switch mode is out of the question at present, I know the
>>customer has a huge dislike, so I am doing it the way he
>>wants.
>
> Where does the 800V come from?
>
> One thing I've done in the past is to make a weak voltage
> doubler that drops down to about half when you start
> drawing serious current. The effective series impedance
> would help stabilize the circuit, and it would dissipate a
> lot less power.
>
> With your circuit, you could try a 10K 100W resistor
> instead of the 330R. They even come in TO247 if that's what
> you really want.
>
>
> Best regards,
> Spehro Pefhany
50VA. The caps are a pair of 450V 47uF. I think that is a good
idea too, effectively reducing the supply regulation.
Geoff
Nov 13, 2012, 10:34:22 PM11/13/12
to
Owen Roberts <o...@case.edu> wrote in
news:9c56b78d-d228-48a1...@kt16g2000pbb.googleg
roups.com:
> Take a look at the I-V curve for the lamp. You may be
> entering a region of oscillation in the plasma.
>
> Steve
>
>
>
>
I might just do that too. There is a chance it acts like a
relaxation oscillator. The lamp does not misbehave with a simple
R in series with a HV supply at 50mA or so.
news:9c56b78d-d228-48a1...@kt16g2000pbb.googleg
roups.com:
> Take a look at the I-V curve for the lamp. You may be
> entering a region of oscillation in the plasma.
>
> Steve
>
>
>
>
relaxation oscillator. The lamp does not misbehave with a simple
R in series with a HV supply at 50mA or so.
Robert Baer
Nov 14, 2012, 12:08:27 AM11/14/12
to
and/or supply for that current range.
Very possible the lamp is in a negative resistance region.
BTW, the secondary ionization voltage for neon is 40.9V and the third
ionization voltage for neon is 63.2V - meaning you better see about 63V
across it when operating properly.
Bill Sloman
Nov 14, 2012, 12:17:59 AM11/14/12
to
the cathode by positive ion bombardment needs quite a large voltage
drop across the anode glow region.
--
Bill Sloman, Sydney
Tim Wescott
Nov 14, 2012, 12:54:25 PM11/14/12
to
region -- the V-I curve that I'm familiar with for lamps like this has a
large "constant voltage" region; if he's operating in that region then it
should be easy to pull a constant current.
If he's getting out to the edge of that region, though, then life gets
more difficult, or impossible: I think when you get close to the turn-off
current there's some hysteresis in the V-I curve that's going to make it
impossible to hold a steady current.
Jon Elson
Nov 14, 2012, 4:48:30 PM11/14/12
to
Geoff wrote:
>
>
> http://i251.photobucket.com/albums/gg302/triode101/sink.jpg
>
> This circuit is to drive a scientific lamp which is low pressure
> neon of strike volts 600V and typ run volts of about 200V. The
> design intent is DC current from 0 to 60mA. This circuit
> traditionally runs to about 30mA with no problem and is well
> proven at these currents. Normally it is not darlington pair but
> it is something I have tried..
>
200 V at 60 mA is 12 W! Must be a big neon lamp!
>
>
> http://i251.photobucket.com/albums/gg302/triode101/sink.jpg
>
> This circuit is to drive a scientific lamp which is low pressure
> neon of strike volts 600V and typ run volts of about 200V. The
> design intent is DC current from 0 to 60mA. This circuit
> traditionally runs to about 30mA with no problem and is well
> proven at these currents. Normally it is not darlington pair but
> it is something I have tried..
>
> The trouble I have with this circuit is that at above currents
> of about 35-40mA the lamp starts to flicker indicating a higher
> current than is dialed in by the pot.
My guess is that the lamp exhibits negative resistance at this
> of about 35-40mA the lamp starts to flicker indicating a higher
> current than is dialed in by the pot.
current. You will find it pretty difficult to control the current
through a negative resistor. I think the only hope is to add
series resistance greater than the value of the negative resistance.
Jon
Jon Elson
Nov 14, 2012, 4:53:36 PM11/14/12
to
and any simple control loop will be confounded by this.
Jon
bloggs.fred...@gmail.com
Nov 14, 2012, 4:48:14 PM11/14/12
to
On Tuesday, November 13, 2012 5:45:00 PM UTC-5, Geoff wrote:
> http://i251.photobucket.com/albums/gg302/triode101/sink.jpg
You seem to be missing the obvious for the circuit and that is you need an isolation resistor between the OA output and transistor base such that the rolloff pole formed by that resistor and the worst case transistor Miller capacitance is at least a decade higher in frequency than the RC feedback of the 10n and 1K (really). There's nothing exotic about a crummy VCS for a neon lamp, scientific or not, it is hack simple. A lot of your circuit values are screwy.
> http://i251.photobucket.com/albums/gg302/triode101/sink.jpg
You seem to be missing the obvious for the circuit and that is you need an isolation resistor between the OA output and transistor base such that the rolloff pole formed by that resistor and the worst case transistor Miller capacitance is at least a decade higher in frequency than the RC feedback of the 10n and 1K (really). There's nothing exotic about a crummy VCS for a neon lamp, scientific or not, it is hack simple. A lot of your circuit values are screwy.
Martin Brown
Nov 14, 2012, 5:04:04 PM11/14/12
to
On 14/11/2012 21:48, Jon Elson wrote:
> Geoff wrote:
>>
>> http://i251.photobucket.com/albums/gg302/triode101/sink.jpg
>>
>> This circuit is to drive a scientific lamp which is low pressure
>> neon of strike volts 600V and typ run volts of about 200V. The
>> design intent is DC current from 0 to 60mA. This circuit
>> traditionally runs to about 30mA with no problem and is well
>> proven at these currents. Normally it is not darlington pair but
>> it is something I have tried..
>>
> 200 V at 60 mA is 12 W! Must be a big neon lamp!
That's about the rating of a few elemental reference low pressure
> Geoff wrote:
>>
>> http://i251.photobucket.com/albums/gg302/triode101/sink.jpg
>>
>> This circuit is to drive a scientific lamp which is low pressure
>> neon of strike volts 600V and typ run volts of about 200V. The
>> design intent is DC current from 0 to 60mA. This circuit
>> traditionally runs to about 30mA with no problem and is well
>> proven at these currents. Normally it is not darlington pair but
>> it is something I have tried..
>>
> 200 V at 60 mA is 12 W! Must be a big neon lamp!
emission lamps. A low pressure sodium lamp is pretty bright even at 12W.
>> The trouble I have with this circuit is that at above currents
>> of about 35-40mA the lamp starts to flicker indicating a higher
>> current than is dialed in by the pot.
> My guess is that the lamp exhibits negative resistance at this
> current. You will find it pretty difficult to control the current
> through a negative resistor. I think the only hope is to add
> series resistance greater than the value of the negative resistance.
by dropping about half the HT voltage across the external resistor after
the lamp strikes and the extra series resistance should then discourage
it from becoming a negative resistance relaxation oscillator.
--
Regards,
Martin Brown
Bill Sloman
Nov 14, 2012, 5:17:53 PM11/14/12
to
current source, sensitive - in the first instance - to the current
through the sensing resistor. The voltage drop across the lamp does
control the base-collector voltage, but that has very little effect
(less than 0.1%) on the base-emitter voltage which the op amp sees.
If the discharge turned right off, then there would be a problem. If
the control loop is oscillating, and drove the controlled current down
to zero this could happen, but stopping the control loop from
oscillating would be the area to concnetrate on, rather than the non-
linearity of the lamp when operating way off the desired stable state.
--
Bill Sloman, Sydney
Jim Thompson
Nov 14, 2012, 6:08:50 PM11/14/12
to
On Wed, 14 Nov 2012 15:48:30 -0600, Jon Elson <jme...@wustl.edu>
wrote:
In my disco days I dimmed neon by feeding the transformer whole cycles
of AC... dimming by skipping full cycles.
(Duty-cycle schemes with TRIAC's proved to be transformer killers :-)
...Jim Thompson
--
| James E.Thompson, CTO | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| Phoenix, Arizona 85048 Skype: Contacts Only | |
| Voice:(480)460-2350 Fax: Available upon request | Brass Rat |
| E-mail Icon at http://www.analog-innovations.com | 1962 |
I love to cook with wine. Sometimes I even put it in the food.
wrote:
of AC... dimming by skipping full cycles.
(Duty-cycle schemes with TRIAC's proved to be transformer killers :-)
...Jim Thompson
--
| James E.Thompson, CTO | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| Phoenix, Arizona 85048 Skype: Contacts Only | |
| Voice:(480)460-2350 Fax: Available upon request | Brass Rat |
| E-mail Icon at http://www.analog-innovations.com | 1962 |
I love to cook with wine. Sometimes I even put it in the food.
lang...@fonz.dk
Nov 14, 2012, 6:14:15 PM11/14/12
to
On 14 Nov., 23:04, Martin Brown <|||newspam...@nezumi.demon.co.uk>
wrote:
and why not increase the 68R
-Lasse
wrote:
-Lasse
Bill Sloman
Nov 14, 2012, 8:19:18 PM11/14/12
to
constant current source, and could be expected to cope with a negative
resistance across the lamp without difficulty. If the current source
is not properly frequency compensated, and thus oscillates, it could
turn the lamp right off, and restarting the discharge could be messy.
--
Bill Sloman, Sydney
Bill Sloman
Nov 14, 2012, 8:27:06 PM11/14/12
to
On Nov 14, 11:05 am, "Tim Williams" <tmoran...@charter.net> wrote:
> SOA says you can only run 600V up to a hair over 10mA, maybe 12mA. BJTs
> are a bad idea for dropping high voltages due to second breakdown.
>
> Why not choose a tube like 6L6GC? Current production, doesn't take much
> heater power, and takes the voltage easily. Okay, a MOSFET would be
> smaller and cheaper (though probably about even if you include the
> heatsink, come to think of it).
>
> Note you're trying to carry a maximum of (800V - 200V) * 0.06A = 36W,
> which is a moderate amount for a small transistor. Just in case, you'd be
> better off with a TO-247 package, or two TO-220s in parallel.
>
> Switching methods are much more efficient, even if you can't run the thing
> directly on AC (I assume that's part of the design requirement).
Switching supplies always leave some ripple on the lamp output, which
> SOA says you can only run 600V up to a hair over 10mA, maybe 12mA. BJTs
> are a bad idea for dropping high voltages due to second breakdown.
>
> Why not choose a tube like 6L6GC? Current production, doesn't take much
> heater power, and takes the voltage easily. Okay, a MOSFET would be
> smaller and cheaper (though probably about even if you include the
> heatsink, come to think of it).
>
> Note you're trying to carry a maximum of (800V - 200V) * 0.06A = 36W,
> which is a moderate amount for a small transistor. Just in case, you'd be
> better off with a TO-247 package, or two TO-220s in parallel.
>
> Switching methods are much more efficient, even if you can't run the thing
> directly on AC (I assume that's part of the design requirement).
may not be acceptable in the application.
Enough filtering can reduce this to what ought to be acceptable levels
- though you may end up still seeing the effects if convection
currents inside the lamp, or something equally odd - but a big
transistor on a big heatsink is usually a cheaper and easier solution
than a switching supply.
--
Bill Sloman, Sydney
George Herold
Nov 14, 2012, 9:35:45 PM11/14/12
to
George H.
Mark
Nov 14, 2012, 9:42:40 PM11/14/12
to
> My guess is that the lamp exhibits negative resistance at this
> current. You will find it pretty difficult to control the current
> through a negative resistor. I think the only hope is to add
> series resistance greater than the value of the negative resistance.
>
> Jon
Mark
bloggs.fred...@gmail.com
Nov 14, 2012, 11:59:07 PM11/14/12
to
josephkk
Nov 17, 2012, 12:47:50 PM11/17/12
to
On Tue, 13 Nov 2012 18:30:59 -0600, Tim Wescott <t...@seemywebsite.com>
wrote:
>
>>>
>> Your point about separating the run and strike ccts is well taken. I
>> might go that way if I can't find another solution using the present
>> transformer.
>>
>> I don';t think it is a stability problem because at the point of
>> breaking down, the volts to the base are only dipping, indicating the
>> loop is trying to prevent the output from conducting.
>
>Well, it is by definition a stability problem because you're seeing
>oscillation where you want to see steady operation. The only question is
>where the oscillation is coming from, which is why I was suggesting
>looking at other points in the circuit than just the op-amp output.
>
>The circuit topology is correct assuming that there's not a lot of
>coupling between the collector and base of the transistors, that the
>Darlington stage isn't too slow. That 10nF cap is in the right place,
>but Bill is correct that it may be too small.
If an integrator is really the correct topology Bill may be right, however
i do not think it is the correct topology; i would replace it with 1n.
Moreover a 100R resistor in place of the diode should do far better at
turning off the transistor / Darlington.
It is very much to the point to look at the transistor base when it gets
wonky.
?-)
wrote:
>
>>>
>> Your point about separating the run and strike ccts is well taken. I
>> might go that way if I can't find another solution using the present
>> transformer.
>>
>> I don';t think it is a stability problem because at the point of
>> breaking down, the volts to the base are only dipping, indicating the
>> loop is trying to prevent the output from conducting.
>
>Well, it is by definition a stability problem because you're seeing
>oscillation where you want to see steady operation. The only question is
>where the oscillation is coming from, which is why I was suggesting
>looking at other points in the circuit than just the op-amp output.
>
>The circuit topology is correct assuming that there's not a lot of
>coupling between the collector and base of the transistors, that the
>Darlington stage isn't too slow. That 10nF cap is in the right place,
>but Bill is correct that it may be too small.
i do not think it is the correct topology; i would replace it with 1n.
Moreover a 100R resistor in place of the diode should do far better at
turning off the transistor / Darlington.
It is very much to the point to look at the transistor base when it gets
wonky.
?-)
dagmarg...@yahoo.com
Nov 17, 2012, 1:32:35 PM11/17/12
to
On Nov 14, 4:48 pm, bloggs.fredbloggs.f...@gmail.com wrote:
If he needed the drive, then an emitter-follower to Vc, not a
darlington.
--
Cheers,
James Arthur
Geoff
Dec 9, 2012, 5:57:42 PM12/9/12
to
Geoff <pub...@email.com> wrote in
news:XnsA10ABEB...@88.198.244.100:
>
>
> http://i251.photobucket.com/albums/gg302/triode101/sink.jpg
>
> This circuit is to drive a scientific lamp which is low
> pressure neon of strike volts 600V and typ run volts of
> about 200V. The design intent is DC current from 0 to 60mA.
> This circuit traditionally runs to about 30mA with no
> problem and is well proven at these currents. Normally it
> is not darlington pair but it is something I have tried..
>
perfectly well. Confirming that the problem was indeed
transistor breakdown, as I suspected. I swapped it for a
2SK1317 when I had stock and it went well too. For curiositys
sake I checked the lamp voltage at a few currents. Basically
it had a positive resistance at low currents and at higher (>
40mA) currents tended towards constant voltage ie from 194 to
210 mA range. These lamps do eventually run out of gas
pressure, and will begin to turn off/on under these
conditions. But this is not what I saw, which was a huge
increase in current. The astronomy market now has a new
product anyway.
news:XnsA10ABEB...@88.198.244.100:
>
>
> http://i251.photobucket.com/albums/gg302/triode101/sink.jpg
>
> This circuit is to drive a scientific lamp which is low
> pressure neon of strike volts 600V and typ run volts of
> about 200V. The design intent is DC current from 0 to 60mA.
> This circuit traditionally runs to about 30mA with no
> problem and is well proven at these currents. Normally it
> is not darlington pair but it is something I have tried..
>
> The trouble I have with this circuit is that at above
> currents of about 35-40mA the lamp starts to flicker
> indicating a higher current than is dialed in by the pot.
> currents of about 35-40mA the lamp starts to flicker
> indicating a higher current than is dialed in by the pot.
> An oscilloscope at the opamp output confirms that at the
> point where it starts to flicker, the loop starts to try to
> lower the current since the opamp output volts drops. The
> onset can be made to be a periodic dip (tens of hz dip
> about a volt) before the lamp begins to flicker. When the
> lamp flickers on and off more severely, the opamp tries to
> go negative to compensate. This to me shows that the output
> transistor is conducting, not the failure of the loop
> control.. I have played around with the drive impedance, ie
> with/without the 1k base resistor. The point of flicker can
> be less severe with the resistor, or latches hard without.
> Maybe the VCEO spec is being stretched here. I have tried a
> variac and reduced the volts to about 580V, still does it
> but onset is higher current (50mA). I have tried a BUL 416
> transistor also. I might just be applying too much voltage,
> but I can't see why, might have to look at the op curves a
> bit harder. Maybe I could use a MOSFET or IGBT.
>
I grabbed a IRFPE50 which had a marginal SOA and it worked
> point where it starts to flicker, the loop starts to try to
> lower the current since the opamp output volts drops. The
> onset can be made to be a periodic dip (tens of hz dip
> about a volt) before the lamp begins to flicker. When the
> lamp flickers on and off more severely, the opamp tries to
> go negative to compensate. This to me shows that the output
> transistor is conducting, not the failure of the loop
> control.. I have played around with the drive impedance, ie
> with/without the 1k base resistor. The point of flicker can
> be less severe with the resistor, or latches hard without.
> Maybe the VCEO spec is being stretched here. I have tried a
> variac and reduced the volts to about 580V, still does it
> but onset is higher current (50mA). I have tried a BUL 416
> transistor also. I might just be applying too much voltage,
> but I can't see why, might have to look at the op curves a
> bit harder. Maybe I could use a MOSFET or IGBT.
>
perfectly well. Confirming that the problem was indeed
transistor breakdown, as I suspected. I swapped it for a
2SK1317 when I had stock and it went well too. For curiositys
sake I checked the lamp voltage at a few currents. Basically
it had a positive resistance at low currents and at higher (>
40mA) currents tended towards constant voltage ie from 194 to
210 mA range. These lamps do eventually run out of gas
pressure, and will begin to turn off/on under these
conditions. But this is not what I saw, which was a huge
increase in current. The astronomy market now has a new
product anyway.
Geoff
Dec 9, 2012, 5:59:21 PM12/9/12
to
low currents and at higher (>
> 40mA) currents tended towards constant voltage ie from 194 to
> 210 mA range.
EDIT: 194 to 210 Volts.
> 40mA) currents tended towards constant voltage ie from 194 to
> 210 mA range.
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