Resistor for neon indicator lamp
George Herold
working the other day. eemax won't provide any component level
support, but I've ordered a new board for $45.00.
When I opened up the unit, after switching off the circuit breaker on
the 240V AC line, I observed that a big (maybe 3-5 watt) (metal film?)
resistor was discolored and was an open circuit. The markings look
like 100 ohms, but because of the discoloration it's hard to be sure.
I say metal film because the resistor is pale blue in color. The
resistor feds a neon indicator bulb... (And probablly more of the
circuitry.)
My question. Is 100 ohms a good value as a current limiting resistor
for a small neon lamp running off 240 V AC? (60 Hz if that matters.)
I'm not sure what the I-V curve for the lamp will look like. (Is the
one shown here OK?)
http://en.wikipedia.org/wiki/Neon_lamp
Thanks for any advice.
George H.
jf...@my-deja.com
> My question. Is 100 ohms a good value as a current limiting resistor
> for a small neon lamp running off 240 V AC? (60 Hz if that matters.)
> I'm not sure what the I-V curve for the lamp will look like. (Is the
> one shown here OK?)
for a value between 100K and 470K. At 100K, a half-watt resistor
would be marginal, and I would go for one rated for 1 watt. Within
this range, the value is probably not too critical.
Gareth
On 01/03/2010 21:53, George Herold wrote:
> rved that a big (maybe 3-5 watt) (metal film?)
> resistor was discolored and was an open circuit. The markings look
> like 100 ohms, but because of the discoloration it's hard to be sure.
> I say metal film because the resistor is pale blue in color. The
> resistor feds a neon indicator bulb... (And probablly more of the
> circuitry.)
>
> My question. Is 100 ohms a good value as a current limiting resistor
> for a small neon lamp running off 240 V AC? (60 Hz if that matters.)
> I'm not sure what the I-V curve for the lamp will look like. (Is the
> one shown here OK?)
100 ohms is much too small. It's more likely to be 100 k ohms or even
higher.
--
-----------------------------------------------------------------------
To reply to me directly:
Replace privacy.net with: totalise DOT co DOT uk and replace me with
gareth.harris
Jon Kirwan
Assuming the neon bulb is still okay, why not measure the
break down? I'd start with perhaps 100k with your 240V. If
the breakdown is around 100V, the current will be in the area
of 1mA, which is probably safe enough. And 100mW, or so, so
you won't burn something up.
At say 2 watts, you would be talking in the area of 10k, my
guess. Not 100 ohms.
Jon
D Yuniskis
> I've got a tankless electric water heater from eemax. that stopped
> working the other day. eemax won't provide any component level
> support, but I've ordered a new board for $45.00.
>
> When I opened up the unit, after switching off the circuit breaker on
> the 240V AC line, I observed that a big (maybe 3-5 watt) (metal film?)
> resistor was discolored and was an open circuit. The markings look
> like 100 ohms, but because of the discoloration it's hard to be sure.
> I say metal film because the resistor is pale blue in color. The
> resistor feds a neon indicator bulb... (And probablly more of the
> circuitry.)
>
> My question. Is 100 ohms a good value as a current limiting resistor
> for a small neon lamp running off 240 V AC? (60 Hz if that matters.)
Yikes! You should be looking at more like 1mA through the lamp.
Are you sure about the connections in the circuit?
William Sommerwerck
> for a small neon lamp running off 240V AC?
It is, if you want the neon lamp to light up whitish purple for less than an
second, then explode.
You need something more in the range of 150K to 200K, possibly even larger.
George Herold
Yes the lamp looks like the picture in the wiki article. If I
believe the curve there, then I've got something like 160-200 volts
across the lamp at 20mA. So I've got (call it) 80 V to drop on the
resistor at 20mA.. I get 3 k ohm at a watt or two. Hmm maybe what
looks like brown black brown is a discolored red black red. (I always
have hated the way red and brown look almost the same on the pale blue
body of the metal film resistors.)
100k seems like it won't allow enough current through to keep the lmap
lite. (But I've never worked with neon lamps.)
George H.
George Herold
Thanks, I'm beginning to think I should just leave it alone and wait
for the replacement PCB. Now sense 'frying' someting else.
George H.
George Herold
>
> - Show quoted text -
Thanks Jon. This value seemed way to small to me too.
George H.
George Herold
>
> - Show quoted text -
Well I can see that the big (3 to 5 watt) resistor feds the neon
lamp. I don't know what else the circuit does.... I didn't bother to
try and trace the whole thing out. Perhaps I can look at it more
closely tonight and see if I can make something more out of it.
Thanks,
George H.
George Herold
> On Mon, 1 Mar 2010 13:53:13 -0800 (PST), George Herold
> <ggher...@gmail.com>wrote:
>
>
>
>
>
>
> - Show quoted text -
Thanks I'll look up an NE2 and see what I can make out of it.
George H.
Don Klipstein
I don't believe the voltages in that curve in Wiki - I would plan on 70
volts and peak current around 3 milliamps (peak voltage of 240 VAC is
about 340 volts).
If the glass bulb's overall length is 3/4 inch, then the neon lamp is
probably an NE-2H. I would use an 82K or 100K 2-watt resistor for an
NE-2H at 240 volts AC. I would err on the high side (100K) to maximize
life of the lamp. I suspect the toated resistor is a 100K 2 watt whose
yellow band was browned by being toasted.
- Don Klipstein (d...@misty.com)
Don Klipstein
100 ohms in series with a neon lamp at 240 volts AC: Not that I would
try this for the answer, but I suspect both the neon lamp and the resistor
will likely be severely damaged. Either or both may explode, and the neon
lamp may implode.
- Don Klipstein (d...@misty.com)
Jamie
John Larkin
Numbers more like 330K to 560K are more reasonable. Higher currents
will sputter the gas and make the thing fail sooner.
Better yet, use an LED.
John
George Herold
> In <ef993f27-eac5-404c-a3f8-2d1224188...@j27g2000yqn.googlegroups.com>,
>
> - Show quoted text -
Excellent, Thanks Don. I like the faded yellow stripe idea. The bulb
is about 1/2 that size. It's standing straight up and the tip of the
glass is 0.4" (10mm) above the circuit board.
George H.
George Herold
>
> - Show quoted text -
Well somehow this must also feed power to some other part of the
circuit. There's also an LM324 and small triac Q601e3 on the pcb.
George H.
Don Klipstein
George Herold wrote:
>On Mar 1, 5:48�pm, d...@manx.misty.com (Don Klipstein) wrote:
>> In <ef993f27-eac5-404c-a3f8-2d1224188...@j27g2000yqn.googlegroups.com>,
>> George Herold wrote:
<SNIP to here>
>> >Yes the lamp looks like the picture in the wiki article. � If I
>> >believe the curve there, then I've got something like 160-200 volts
>> >across the lamp at 20mA. �So I've got (call it) 80 V to drop on the
>> >resistor at 20mA.. I get 3 k ohm at a watt or two. �Hmm maybe what
>> >looks like brown black brown is a discolored red black red. �(I always
>> >have hated the way red and brown look almost the same on the pale blue
>> >body of the metal film resistors.)
>>
>> >100k seems like it won't allow enough current through to keep the lmap
>> >lite. �(But I've never worked with neon lamps.)
>>
>> � I don't believe the voltages in that curve in Wiki - I would plan on 70
>> volts and peak current around 3 milliamps (peak voltage of 240 VAC is
>> about 340 volts).
>>
>> � If the glass bulb's overall length is 3/4 inch, then the neon lamp is
>> probably an NE-2H. I would use an 82K or 100K 2-watt resistor for an
>> NE-2H at 240 volts AC. I would err on the high side (100K) to maximize
>> life of the lamp. �I suspect the toated resistor is a 100K 2 watt whose
>> yellow band was browned by being toasted.
>
>Excellent, Thanks Don. I like the faded yellow stripe idea. The bulb
>is about 1/2 that size. It's standing straight up and the tip of the
>glass is 0.4" (10mm) above the circuit board.
That sounds just a little small for the A1B (mini NE-2) and the A1C
(mini NE-2H), so I suspect your lamp is one of those. Given the size of
the resistor, I think it's the latter.
If your neon lamp kicked the bucket along with the resistor (more likely
it survived, at least well enough to glow from 240 volts AC), Radio Shack
may still carry these (a litle incorrectly calling them NE-2H).
- Don Klipstein (d...@misty.com)
John Fields
>I've got a tankless electric water heater from eemax. that stopped
>working the other day. eemax won't provide any component level
>support, but I've ordered a new board for $45.00.
>
>When I opened up the unit, after switching off the circuit breaker on
>the 240V AC line, I observed that a big (maybe 3-5 watt) (metal film?)
>resistor was discolored and was an open circuit. The markings look
>like 100 ohms, but because of the discoloration it's hard to be sure.
>I say metal film because the resistor is pale blue in color. The
>resistor feds a neon indicator bulb... (And probablly more of the
>circuitry.)
>
>My question. Is 100 ohms a good value as a current limiting resistor
>for a small neon lamp running off 240 V AC?
---
No.
For a small neon lamp like an NE-2, (GE 3AD) with a recommended current
of 0.3mA and a nominal holding voltage of about 60V, peak, that's about
42VRMS, so the resistor would have to drop about 200V at 300�A:
Vs - Vne 200V
R = ---------- ~ ------- ~ 670k oms
Ine 3e-4A
and it would dissipate:
P = IE = 3e-4A * 200V ~ 0.06 watts
If that resistor were 100 ohms, then with 200V dropped across it
there'd be 200 amperes through it _and_ the neon lamp, which they'd both
hate for a little while.
OTOH, if the resistor is 100k it'll drop about 200V with a neon lamp in
series, which will let about 2mA through when the lamp fires.
That'll cause the resistor to dissipate about 0.4 watts, which doesn't
explain why it got toasted, since even into a short it would only
dissipate about:
E� 240�V
P = --- = ------- = 0.576 watts
R 1e5R
Just for grins, let's say it's a 3 watt resistor running full bore in
series with a load (like a high current neon lamp) drawing 5mA.
Then the value of the resistor would be:
E� 200�V
R = --- = ------- = 13,333 ohms ~ 13k
P 3W
and if there was some circuitry downstream from the lamp, which shorted,
then the resistor would dissipate:
E� 240�V
P = --- = ------- = 4.43 watts
R 13e3R
Not huge but, anyway, without a schematic it's all just conjecture.
---
(60 Hz if that matters.)
>I'm not sure what the I-V curve for the lamp will look like. (Is the
>one shown here OK?)
>
>http://en.wikipedia.org/wiki/Neon_lamp
---
Only in a very general way; here's a much better one for your
application:
JF
George Herold
> On Mon, 1 Mar 2010 13:53:13 -0800 (PST), George Herold
>
>
> JF
Thanks John, I must admit it's hard to understand how a 3 watt 100k
ohm resistor across the 240 V lines would blow even if everything down
stream of it was shorted. (As your calculation shows.) A new circuit
board is in the mail, but this won't solve the mystery. The board has
been redesigned and is no longer analog but digital. Once I put the
new board in I'll look at the busted one and see if I can figure
anything out.
Say could you resend that link to the neon bulb I-V curve? What ever
you posted got turned into gobble-de-gook by google.
"d7iqo5lphgp0k15o8fh2ccal5eoi0c2...@4ax.com"
George H.
Jon Kirwan
Don Klipstein
George Herold wrote in part:
<SNIP previously quoted material>
>Thanks John, I must admit it's hard to understand how a 3 watt 100k
>ohm resistor across the 240 V lines would blow even if everything down
>stream of it was shorted. (As your calculation shows.) A new circuit
>board is in the mail, but this won't solve the mystery. The board has
>been redesigned and is no longer analog but digital. Once I put the
>new board in I'll look at the busted one and see if I can figure
>anything out.
I have seen resistors fail in ways that cause their resistance to
decrease until they get toasted and then go open circuit.
My experience has been that this happens more with carbon composition
resistors. Those tend to be cylinders with untapered ends. Many times,
the composition is exposed. The composition may absorb moisture and
experience a decrease in resistance, especially if it is operated only
intermittently. This is more likely to be a problem where there is
exposure to moisture or high humidity or salty coastal air.
And carbon's resistivity has a negative temperature coefficient.
Preferably the resistor degrades rapidly and then goes open circuit
quickly by the time it allows anything else to get overloaded to a
damaging extent.
A possible other explanation is that a severe line voltage surge caused
something in the resistor to carbonize. The resistor becomes more
conductive and hotter. The carbonization may then go into a runaway
situation, leading to sizzle-sizzle and then to *pow*, *poof* or *phut*.
- Don Klipstein (d...@misty.com)
John Fields
<gghe...@gmail.com> wrote:
>Thanks John, I must admit it's hard to understand how a 3 watt 100k
>ohm resistor across the 240 V lines would blow even if everything down
>stream of it was shorted. (As your calculation shows.) A new circuit
>board is in the mail, but this won't solve the mystery. The board has
>been redesigned and is no longer analog but digital. Once I put the
>new board in I'll look at the busted one and see if I can figure
>anything out.
>
>
>Say could you resend that link to the neon bulb I-V curve? What ever
>you posted got turned into gobble-de-gook by google.
---
Ah, Google...
Google groups doesn't support binaries so I'll email it to you.
JF
George Herold
> In <36079a9a-c27f-4b4b-859f-c5ea6260d...@s36g2000prf.googlegroups.com>,
Thanks Don, This certianly wasn't a carbon composition. (Though I
was digging through a pile of 1-2 Watt 100-300k ohm carbon C's at work
today.) It doesn't seem like putting 100k in there can hurt anything,
though I don't expect it to work either... I figure something else
might have happened also.
George H.
George Herold
> On Tue, 2 Mar 2010 12:34:55 -0800 (PST), George Herold
>
Wow. Thanks John
Jon Kirwan
by the usual post-a-message-on-the-public-cork-board method,
sometimes called a newsgroup.
Let me know.
Jon
John Fields
---
You're welcome. :-)
If you're interested in neon glow lamps, GE's 1965 glow lamp manual
(pretty much the Bible) is here:
http://www.donsbulbs.com/cgi-bin/r/t.pl/library.html
JF
George Herold
>
> - Show quoted text -
Excellent, lots of reading there!
George H.