build a simple 5v power supply for digital circuit
robb
*7805* regulator power supply that is {AC in, bridge, 7805, 5v
out}
my questions are concerning the various mods/enhance such as ;
filter caps - between bridge and regulator, Is this load
stabilizer also ? How to calculate proper size ?
osc stability caps - .1 uF between V5+ and V0
load stabilizer - caps between V5+ and V0 , How to estimate
size ?
back emf diodes - diode between V5+ amd V0 , how to choose diode
?
short circuit protection diodes - is tis same as back emf diode
?
bleeder resistors - where, what and why ?
decoupling caps - same as osc stability caps ?
? name descrp ? - diode inline/series on V+ out of rectifier ,
What is purpose ?
soft start - inductor caps and resistor, When to use ?
Are any of these mods incompatible with one another or
complicates other mods values ?
which of these enhancements have most bang for trouble ?
a good link that explains thes econcepts and how to estimate
values etc would be great too .
thanks for any help,
robb
Phil Allison
"robb"
> yes i googled and found lots of different mods to the basic 5v
> *7805* regulator power supply that is {AC in, bridge, 7805, 5v
> out}
>
> my questions are concerning the various mods/enhance such as ;
** Just use the basic circuit, no mods are needed.
> filter caps - between bridge and regulator, Is this load
> stabilizer also ? How to calculate proper size ?
** Depends on the amp draw.
4700 uF is good for 1.5 A, 470uF for 150 mA.
> osc stability caps - .1 uF between V5+ and V0
> load stabilizer - caps between V5+ and V0 , How to estimate
> size ?
** 0.1 uF ( 100nF) , 63 volt plastic film is all you need to know.
> back emf diodes - diode between V5+ amd V0 , how to choose diode
> ?
** Rarely needed - use a IN4001 if you like.
> short circuit protection diodes
** No such animal.
> bleeder resistors - where, what and why ?
** Rarely needed.
> ? name descrp ? - diode inline/series on V+ out of rectifier ,
> What is purpose ?
** None.
> soft start - inductor caps and resistor, When to use ?
** Large PSUs only.
Hundreds of watts, not 5 watts.
........ Phil
Nobody
> yes i googled and found lots of different mods to the basic 5v
> *7805* regulator power supply that is {AC in, bridge, 7805, 5v
> out}
>
> my questions are concerning the various mods/enhance such as ;
>
> filter caps - between bridge and regulator, Is this load
> stabilizer also ? How to calculate proper size ?
The characteristic of a capacitor is I=C*dV/dt.
From this, you can deduce that the voltage ripple is roughly T*I/C, where
T is the period of the AC waveform. For a 60Hz supply which has been
full-wave rectified, T=1/120s = ~8.3ms. So, if you are drawing 1A and can
tolerate at most 3V ripple, you would need ~2800uF.
Using a higher secondary voltage will allow you to tolerate more ripple,
but will increase the power dissipation of the regulator.
Also, a larger capacitor will result in a smaller conduction angle and
increased RMS current (and thus power dissipation) in the transformer and
rectifier.
Phil Allison
"Nobody"
>
>
> Also, a larger capacitor will result in a smaller conduction angle and
> increased RMS current (and thus power dissipation) in the transformer and
> rectifier.
** Complete BULLSHIT !!
The value of the filter cap has almost no effect on the rms current
igure - allowing only that the ripple voltage is under 25% of the peak
value.
This is one of the silliest and most oft repeated myths in all PSU design.
Go try it out if you doubt this.
...... Phil
robb
for all the answers
robb
"Phil Allison" <phila...@tpg.com.au> wrote in message
news:5qhre9F...@mid.individual.net...
>
> "robb"
> > yes i googled and found lots of different mods to the basic
5v
> > *7805* reg. power supply that is {AC in, bridge, 7805, 5v
Thanks again Phil, (in case you missed top post)
for all the answers
robb
Fred Bloggs
> yes i googled and found lots of different mods to the basic 5v
> *7805* regulator power supply that is {AC in, bridge, 7805, 5v
> out}
>
> my questions are concerning the various mods/enhance such as ;
>
> filter caps - between bridge and regulator, Is this load
> stabilizer also ? How to calculate proper size ?
You can't go wrong by making 2piFCR >= 25 or so. R is an estimate of
equivalent load resistance which in your case should be about 8V/Imax,
gives a conservative C, Imax is maximum load on the 5V output, F is
120Hz for a full wave.
> osc stability caps - .1 uF between V5+ and V0
Yes- a ceramic 0.1u would be a standard for high frequency bypass...and
this is in parallel with your electrolytic.
> load stabilizer - caps between V5+ and V0 , How to estimate
> size ?
A conservative estimate is to select an electrolytic that prevents the
output from going out of regulation when stressed with a no load to full
load step increase in current. 5% of 5V is 0.25V so to be conservative
you would want the e-cap to hold up a full load step increase for 500us
without drooping more than that. Example, if full load is 1A then you
have C,min>= (0.5u/0.25V)*1A=2uF, use a 4.7uF, e-cap values change a lot
with temp and initial tolerance. This is in addition to your 0.1u high
frequency bypass...
> back emf diodes - diode between V5+ amd V0 , how to choose diode
> ?
Yeah- that would not hurt- you mean a diode from the regulator output
pin to its input pin with anode on the output. A 4001 would be good for
this.
> short circuit protection diodes - is tis same as back emf diode
> ?
The back emf diode is the same as the short circuit protection diode-
prevents blowing the regulator if the bridge filter cap sputters or goes
short for some other reason.
> bleeder resistors - where, what and why ?
Bleeders allow the high energy capacitors to discharge when the circuit
is powered down- it would not hurt to put these at least on the bridge
filter capacitor- usually a 1 MegaOhm or so...
> decoupling caps - same as osc stability caps ?
Yep...
> ? name descrp ? - diode inline/series on V+ out of rectifier ,
> What is purpose ?
Not sure- that is just another form of back emf diode, and it would make
sense if a second circuit was loading the bridge filter circuit in
addition to the 7805...drawback is it reduces headroom, which is the
differential voltage across the regulator circuit input and output...
> soft start - inductor caps and resistor, When to use ?
You don't need it for a 7805 circuit...
>
> Are any of these mods incompatible with one another or
> complicates other mods values ?
Sure- there are tons of complicated mods you could do. Are you compiling
a compendium, or is it you're not confused enough?
>
> which of these enhancements have most bang for trouble ?
Real smart question, now you're starting to sound like a lamebrain..
>
> a good link that explains thes econcepts and how to estimate
> values etc would be great too .
The 7805 or LM340 series datasheets would be a good start. Most of the
manufacturers now link to applicable application notes for the parts. It
was so hard for people to figure things out for themselves...
And don't forget about the stepdown transformer rating, this could be
important. To make a long story short, you need an 8VAC with RMS current
rating at 2x the numerical DC maximum loading current. Then there's all
that rigmarole about heat sinking the IC, max working voltages and
ripple currents for the caps, and other stuff computed for worst case
conditions which typically range over +/-20% nominal and some other
things like fusing. Anything less gets you a pos that flakes out. Come
to think of it, just buy a surplus open frame...or use a plug-in...
Fred Bloggs
> A conservative estimate is to select an electrolytic that prevents the
> output from going out of regulation when stressed with a no load to full
> load step increase in current. 5% of 5V is 0.25V so to be conservative
> you would want the e-cap to hold up a full load step increase for 500us
> without drooping more than that. Example, if full load is 1A then you
> have C,min>= (0.5u/0.25V)*1A=2uF, use a 4.7uF, e-cap values change a lot
> with temp and initial tolerance. This is in addition to your 0.1u high
> frequency bypass...
would be (0.5m/0.25V)*1A=2000u, make that 4700u...
Jamie
Fred, You sound like some one I may have run into
over the years, your last name is ringing a bell and
I just can't seem to bring it to the top of my head.
If you wouldn't mind, could you tell me a brief
location and history? I'm in
CT , USA btw.
--
"I'd rather have a bottle in front of me than a frontal lobotomy"
http://webpages.charter.net/jamie_5
Phil Allison
"Fred Abse"
Phil Allison wrote:
>
>> The value of the filter cap has almost no effect on the rms current
>> - allowing only that the ripple voltage is under 25% of the peak value.
>
** Bollocks.
...... Phil
JeffM
>>[...]make that 4700u...
>>
Jamie wrote:
>Fred, [...] your last name is ringing a bell
robb
"Fred Bloggs" <nos...@nospam.com> wrote in message
news:4749BF27...@nospam.com...
>
> > yes i googled and found lots of different mods to the basic
5v
> > *7805* regulator power supply that is {AC in, bridge, 7805,
5v
> > out}
> >
> > my questions are concerning the various mods/enhance such as
;
>
,
> > What is purpose ?
>
> Not sure- that is just another form of back emf diode, and it
would make
> sense if a second circuit was loading the bridge filter circuit
in
> addition to the 7805...drawback is it reduces headroom, which
is the
> differential voltage across the regulator circuit input and
output...
>
i think that is where i saw an example, a +/- 5v supply using
7805 and 7905 regulators
> And don't forget about the stepdown transformer rating, this
could be
> important. To make a long story short, you need an 8VAC with
RMS current
> rating at 2x the numerical DC maximum loading current. Then
there's all
> that rigmarole about heat sinking the IC, max working voltages
and
> ripple currents for the caps, and other stuff computed for
worst case
> conditions which typically range over +/-20% nominal and some
other
> things like fusing. Anything less gets you a pos that flakes
out. Come
> to think of it, just buy a surplus open frame...or use a
plug-in...
currently just taking simple approach of using a 18/24 Vac 1 Amp
wall-wart while experimenting with 5v psu
thanks for time and answers to the questions
robb
robb
"robb" <so...@where.on.net> wrote in message
news:13k7a00...@corp.supernews.com...
> yes i googled and found lots of different mods to the basic 5v
> *7805* regulator power supply
> \that is {AC in, bridge, 7805, 5v out}
>
I came across this universal psu design while searching for 5V
psu info ...
is there a problem with the universal psu schematic (page 2)
shown in the following document ?
more specifically with the selectable voltage part of the
schematic
http://www.qsl.net/lu3mgp/electronica/UniversalPowerSupply.pdf
also
i am trying to understand how that PSU can produce higher voltage
than 5V while using a 7805 5V regulator and no path for the
bridge rectifier (+) voltage to bypass the 5V regulator ?
thanks for any help
robb
Jim Thompson
Hint: The 7805 forces 5V across R1. Where does the current through
R1 go?
...Jim Thompson
--
| James E.Thompson, P.E. | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| Phoenix, Arizona Voice:(480)460-2350 | |
| E-mail Address at Website Fax:(480)460-2142 | Brass Rat |
| http://www.analog-innovations.com | 1962 |
America: Land of the Free, Because of the Brave
robb
5v
> > *7805* regulator power supply
> > that is {AC in, bridge, 7805, 5v out}
(images on a.b.s.e)
i built and experimenting with the 5v DC psu
i came across a strange (to me) oscope waveform.
using most basic psu {18vac in, bridge, 7805, 5v out}
with a (470 Ohm + standard LED) power indicator on the 5v side
1st
i oscope the (bridge +) and (7805 out) and i see typical
expected waves... that is bridge + positive 16 v sine peaks
squashed together and 7805 out is 5v DC but with small dips at
intervals cooresponding to positive sine peak dips
2nd (**problem here **)
i add .1uF cap between 7805 out and ground (high frequency
decoupling ) and the bridge + output changes to some high
frequency fuzz on the tops of positive sine peaks.
When i zoom in on the ?fuzz? it is a series of about 1 uSec wide
spikes that are space about 10 uSec apart and the magnitude
follows shape of the sine wave ? (see pic in a.b.s.e) thats the
best this amateur can dexcribe
3rd ( ? problem fix ? )
all of these things by themselves will make fuzz go away....
- add a .1uF cap between the -/+ bridge output
- remove the LED
- add smoothing cap to either side
any ideas to help understand what this fuzz is ?
thanks for any help ,
robb
Stephen J. Rush
If you'd read the whole article, it would have answered your question.
robb
"Stephen J. Rush" <sjr...@comcast.net> wrote in message
news:A-Cdnf1oyO2Krtba...@comcast.com...
your correct, i originally only read the circuit description so i
went back and read entire article
and really the only part relevant to my questions was the circuit
description.
if there was an error in the schematic i think they would fix it
before printing ? and i am sure the circuit description answers
the question of how the circuit works for those familiar with
divider junctions and regulator potential driving ?
but my request is to understand how this variable voltage works.
AFAIK the (Pin 2) is 0V ground and the (Pin 3) is 5V out and the
job of the 7805 is to maintain that relationship however in this
application it talks about changing the voltage of (Pin 2)
ground. i did not see where that was possible in the datasheet
for a LM7805.
but more imporatant is the schematic (? error ?) that confuses me
i thought electricity takes path of least resistance ? that is
if i take a 0 Ohm wire wire and i put a 1k Ohm resistor in
parrallel with thta wire then the resistance would be 0 Ohms for
tha t parralell circuit ?
in the schematic there is a piece (a segment) of wire in the
bottom right corner of the schematic that ultimately connects
the (-) output of the bridge rectifier to the (Pin 2) wire of
the 7805 and connected along that wire is the ? resistor divider
stuff ? it is the wire that goes by and around word "VARIABLE"
to the right side?
Now i do not see how it is that any resistance value selected in
the divider stuff is going to change the resistance between BR1
(-) out and the IC1 (G - Pin 2) ? because all the divide stuff
appears to be ? shunted ? shorted by the lower right wire segment
?
any ways even if it was an error i do not see how any
differential in voltage at (Pin 2) is generated using the BR1 (-)
line out as that represents (0 V) to me as well ?
thanks agian for reply,
robb
Jim Thompson
When in doubt, return to my hint ;-)
Jamie
Some 78xx regulators oscillate a bit especially with out
the proper load on them.
even then, you may need to place some by pass caps on it
as you did.
Jamie
Ok, thanks for clearing that up for me..
I still can't get that out of my head how ever,
it's possible the spelling of whom I'm thinking
of, may not be the same of course.
Thanks again.
Spehro Pefhany
Yeah, there's an error. The connection between the (-) output and R1
should not be there.
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
robb
"Jim Thompson" <To-Email-Use-Th...@My-Web-Site.com>
wrote in message
news:pl5mk31138ppmuhii...@4ax.com...
I did not miss your hint, it just seems to have a depth i will
need to figure out.
it seems to be a trick question and i do not want to get it wrong
:)
thanks for help
robb
i will take a wild guess and say the current goes to BR1 (-) and
(Pin 2) ? yes ?
robb
"Spehro Pefhany" <spef...@interlogDOTyou.knowwhat> wrote in
message news:cinmk3dr3oielksfc...@4ax.com...
> On Mon, 26 Nov 2007 13:48:51 -0500, the renowned "robb"
> <so...@where.on.net> wrote:
>
> >
> >"robb" <so...@where.on.net> wrote in message
> >news:13k7a00...@corp.supernews.com...
> >> yes i googled and found lots of different mods to the basic
5v
> >> *7805* regulator power supply
> >> \that is {AC in, bridge, 7805, 5v out}
> >>
> >
> >I came across this universal psu design while searching for 5V
> >psu info ...
> >is there a problem with the universal psu schematic (page 2)
> >shown in the following document ?
> >more specifically with the selectable voltage part of the
> >schematic
> >
> >http://www.qsl.net/lu3mgp/electronica/UniversalPowerSupply.pdf
> >
> >
>
and R1
> should not be there.
hi, thanks Sphero,
wait a minute are you toying with me ?
well i can not figure which connection you are talking about iis
it the same as the one i described ?
thanks for help,
robb
Phil Allison
** I know so - you ASD fucked, fuckwit.
> Try this:
** Drop dead.
..... Phil
Spehro Pefhany
<so...@where.on.net> wrote:
http://server2.hostingplex.com/~zstoretr/fix.gif
Jim Thompson
Once you apply Spehro's correction eliminating a short, the current
flows through the selected resistors below R1... R4 or R5 or R6 or R7
or R8 or R9 or pot R2
robb
"Spehro Pefhany" <spef...@interlogDOTyou.knowwhat> wrote in
> On Mon, 26 Nov 2007 19:41:09 -0500, the renowned "robb"
> <so...@where.on.net> wrote:
> >"Spehro Pefhany" <spef...@interlogDOTyou.knowwhat> wrote in
> >message news:cinmk3dr3oielksfc...@4ax.com...
> >> On Mon, 26 Nov 2007 13:48:51 -0500, the renowned "robb"
> >> <so...@where.on.net> wrote:
> >> >
> >> > "robb" <so...@where.on.net> wrote in message
> >> > news:13k7a00...@corp.supernews.com...
2)
> >> > shown in the following document ?
> >> > more specifically with the selectable voltage part of the
> >> > schematic
> >> >
> >> >
http://www.qsl.net/lu3mgp/electronica/UniversalPowerSupply.pdf
> >>
> >> Yeah, there's an error. The connection between the (-)
> >> output and R1 should not be there.
> >
iis
> > it the same as the one i described ?
> >
>
>
not the wire i was thinking, but i was close
thanks for the correction
robb
robb
"Jamie" <jamie_ka1lpa_not_v...@charter.net> wrote
in message news:bMI2j.55$rp7...@newsfe07.lga...
>
> Some 78xx regulators oscillate a bit especially with out
> the proper load on them.
> even then, you may need to place some by pass caps on it
> as you did.
>
7805
and is it typical that the oscillation shows up on the bridge out
?
thanks for reply,
robb
Jamie
board lay and lead run lengths plays a roll in that.
I assume you had a electrolytic cap on the bridge +/- outputs?
in any case, that does not stop the high freq you saw in there, you
don't always see this problem in all 78xx, I've seen it in some
depending on who made the component where is others, they work fine
with out any compensation. Also, if you constructed the bridge your
self instead of using a ready made unit, it's smart to put some small
bypass caps across the diodes..
Other issues is when voltages on the output side exceed the voltage
on the input side.. the internals of the regulator have an issue with
this how ever, like I said before, it depends on who made the component.
I've seen some that included the bleeder diode.
In a case like that, a bleeder diode from the output to the
input is a desirable component to use. For a + type, the anode would
be on the output, cathode on the input. Basically, that simply causes
a by pass of the regulator if the output voltage happens to be higher
than the input voltage by 0.7V or more.
Hope that did something for you.
ehsjr
>>"robb" <so...@where.on.net> wrote in message
>>news:13k7a00...@corp.supernews.com...
>>
>>> yes i googled and found lots of different mods to the basic
>
> 5v
>
>>> *7805* regulator power supply
>>> that is {AC in, bridge, 7805, 5v out}
>
>
> (images on a.b.s.e)
>
> i built and experimenting with the 5v DC psu
The images do not show a filter cap after the bridge and
before the 7805. Add a large cap (4700uF shown) across
the brideg output:
--------
| +|-----+------in[7805]out---
| Bridge | | + |
| | [4700uF] |
| | | |
| -|-----+-----------+
--------
Ed
T
spef...@interlogDOTyou.knowwhat says...
> On Mon, 26 Nov 2007 13:48:51 -0500, the renowned "robb"
> <so...@where.on.net> wrote:
>
> >
> >"robb" <so...@where.on.net> wrote in message
> >news:13k7a00...@corp.supernews.com...
> >> yes i googled and found lots of different mods to the basic 5v
> >> *7805* regulator power supply
> >> \that is {AC in, bridge, 7805, 5v out}
> >>
> >
> >I came across this universal psu design while searching for 5V
> >psu info ...
> >is there a problem with the universal psu schematic (page 2)
> >shown in the following document ?
> >more specifically with the selectable voltage part of the
> >schematic
> >
> >http://www.qsl.net/lu3mgp/electronica/UniversalPowerSupply.pdf
> >
> >
> >also
> >i am trying to understand how that PSU can produce higher voltage
> >than 5V while using a 7805 5V regulator and no path for the
> >bridge rectifier (+) voltage to bypass the 5V regulator ?
> >
> >thanks for any help
> >robb
>
> Yeah, there's an error. The connection between the (-) output and R1
> should not be there.
>
>
> Best regards,
> Spehro Pefhany
>
I noticed that when I looked at the schematic. I'm planning on building
this one since it's a variable power supply, hell I might just build two
of them because I've got a couple products in mind that use 5V for logic
and 12V to drive things like steppers, etc.
T
says...
If you look at the schematic the connection between the lower end of R1
and the other end of C3 should not exist.
T
spef...@interlogDOTyou.knowwhat says...
Nice graphic! Thanks for that, I used a plain black pen to cross out the
connection on my schematic.
pom
///
>> I came across this universal psu design while searching for 5V
>> psu info ...
>> is there a problem with the universal psu schematic (page 2)
>> shown in the following document ?
>> more specifically with the selectable voltage part of the
>> schematic
>>
>> http://www.qsl.net/lu3mgp/electronica/UniversalPowerSupply.pdf
>>
>>
nobody mentioned it but the schematic is faulty!
Negative output should not be connected to the "mass" leg of the
regulator. Else, output voltage cannot be different from 5V - other
effects neglected
pom
Phil Allison
"pom" <p...@free.fr>
>>>
> Hello
> nobody mentioned it but the schematic is faulty!
> Negative output should not be connected to the "mass" leg of the
> regulator. Else, output voltage cannot be different from 5V - other
> effects neglected
http://www.qsl.net/lu3mgp/electronica/UniversalPowerSupply.pdf
** The design is purest crap.
Output regulation wil be poor, as will hum rejection - since Iq is a highly
variable quantity.
Bound to be HF unstable at some settings and loads too.
That ain't no way to treat a 7805.
Just sub in a LM317 and nearly all the troubles go.
......... Phil
Spehro Pefhany
<phila...@tpg.com.au> wrote:
>
>"pom" <p...@free.fr>
>>>>
>> Hello
>> nobody mentioned it but the schematic is faulty!
>> Negative output should not be connected to the "mass" leg of the
>> regulator. Else, output voltage cannot be different from 5V - other
>> effects neglected
>
>http://www.qsl.net/lu3mgp/electronica/UniversalPowerSupply.pdf
>
>** The design is purest crap.
>
>Output regulation wil be poor, as will hum rejection - since Iq is a highly
>variable quantity.
It's actually fairly stable with output current, less so with input
voltage and temperature. Probably better than most switchable AC
adapters wot consist primarily of a badly made multi-tap transformer
and a crappy slide switch with PCB contacts. But it could be <1%
rather than <10%, as you point out.
>Bound to be HF unstable at some settings and loads too.
I have yet to see a 7805 oscillate under any reasonable conditions.
>That ain't no way to treat a 7805.
It should probably have a larger output cap and a diode eg. 1N5403
across the 7805 and probably across the output too since we don't know
what will get hooked up to it.
>Just sub in a LM317 and nearly all the troubles go.
>
>
>
>......... Phil
>
Yup. Of course you have to recalculate the resistor values, taking
into account that you should draw a minimum of 5mA from the '317 to
keep it in regulation. Eg. 237R and 715R for 5V.
Another potential problem with the original circuit is that the output
voltage will spike as high as Vin - 1.xV during the switch transitions
unless it's a shorting type rotary switch.
That's typically considered an undesirable characteristic in a power
supply.
Phil Allison
"Spehro Pefhany"
"Phil Allison"
>>"pom" <p...@free.fr>
>>>
>>> Hello
>>> nobody mentioned it but the schematic is faulty!
>>> Negative output should not be connected to the "mass" leg of the
>>> regulator. Else, output voltage cannot be different from 5V - other
>>> effects neglected
>>
>> http://www.qsl.net/lu3mgp/electronica/UniversalPowerSupply.pdf
>
>>
>>** The design is purest crap.
>>
>>variable quantity.
>
>
> It's actually fairly stable with output current, less so with input
> voltage and temperature.
** Natsemi data shows Iq may change by up to 0.5 mA in the range of normal
output currents up to 1 amp,
by up to 1mA over the nominal input voltage range & by 1 mA over the
allowed temp range.
Adds up to 2.5 mA.
2.5 mA flowing in 1.5 kohms = 3.75 volts.
That IS craaaaap !!
>>Bound to be HF unstable at some settings and loads too.
>
> I have yet to see a 7805 oscillate under any reasonable conditions.
** A comment about you and other circuits.
Waste of type.
>>That ain't no way to treat a 7805.
>
> It should probably have a larger output cap....
** There is effectively no output stability cap.
That 0.01uF hanging off the + output finds it way back to the 7805's ground
terminal via a crippling amount of series resistance.
>>Just sub in a LM317 and nearly all the troubles go.
>>
>
> Yup.
** Just follow the data sheet for the LM317 for the needed R values.
The only way that horribly crude idea of converting a LM7805 into a variable
reg can be even half satisfactory is * IF * the input voltage is itself
regulated and the load current fixed.
...... Phil
Spehro Pefhany
<phila...@tpg.com.au> wrote:
>
>The only way that horribly crude idea of converting a LM7805 into a variable
>reg can be even half satisfactory is * IF * the input voltage is itself
>regulated and the load current fixed.
It's more than okay to put a zener or LED on the GND lead.
It's also okay if you don't mind blowing all kinds of current in the
resistor network (the Fairchild data sheet recommends > 25mA rather
than the 5mA the Poptronics circuit has), which would reduce the
effect of Iq variations by about 3:1.
They also show an op-amp, but that has potential stability issues that
might actually exist in the real world.
Phil Allison
"Spehro Pefhany"
"Phil Allison"
>>
>>The only way that horribly crude idea of converting a LM7805 into a
>>variable
>>reg can be even half satisfactory is * IF * the input voltage is itself
>>regulated and the load current fixed.
>
> It's more than okay to put a zener or LED on the GND lead.
** Completely different matter.
Remaining within context is a real trial for you.
> It's also okay if you don't mind blowing all kinds of current in the
> resistor network (the Fairchild data sheet recommends > 25mA rather
> than the 5mA the Poptronics circuit has),
** The Poptronics abortion runs at circa 11mA.
5 / 470 = ???
......... Phil
Fred Bloggs
And the circuit description is demented, the output topology is not a
'voltage divider', he's putting some ridiculous amount of current
through that LED, the whole thing is bad. No wonder that generation of
kids just threw their hands up and quit electronics.
Fred Bloggs
Spehro Pefhany wrote:
> On Tue, 27 Nov 2007 23:33:37 +1100, the renowned "Phil Allison"
> <phila...@tpg.com.au> wrote:
>
>
>>The only way that horribly crude idea of converting a LM7805 into a variable
>>reg can be even half satisfactory is * IF * the input voltage is itself
>>regulated and the load current fixed.
>
>
> It's more than okay to put a zener or LED on the GND lead.
>
> It's also okay if you don't mind blowing all kinds of current in the
> resistor network (the Fairchild data sheet recommends > 25mA rather
> than the 5mA the Poptronics circuit has), which would reduce the
> effect of Iq variations by about 3:1.
>
> They also show an op-amp, but that has potential stability issues that
> might actually exist in the real world.
>
>
> Best regards,
> Spehro Pefhany
The pos is intractable as shown. Note that S2 is more likely a
break-before-make, opening the 7805 return for even a few hundred usecs
is not a good idea, output heads for the upper limits, then filtering
the output with a reasonable value of capacitance is also problematic
with that dammed switch too.
Spehro Pefhany
<phila...@tpg.com.au> wrote:
>
>"Spehro Pefhany"
> "Phil Allison"
>
>>>
>>>The only way that horribly crude idea of converting a LM7805 into a
>>>variable
>>>reg can be even half satisfactory is * IF * the input voltage is itself
>>>regulated and the load current fixed.
>>
>> It's more than okay to put a zener or LED on the GND lead.
>
>
>** Completely different matter.
>
> Remaining within context is a real trial for you.
My context (in this thread)is hobby-level power supply circuits.
Switching zeners rather than resistors is not an unreasonable
approach, but we're still left with the spiking-during-switching
issue.
>> It's also okay if you don't mind blowing all kinds of current in the
>> resistor network (the Fairchild data sheet recommends > 25mA rather
>> than the 5mA the Poptronics circuit has),
>
>
>** The Poptronics abortion runs at circa 11mA.
>
> 5 / 470 = ???
Right, okay, bad memory, more like a 2:1 difference.
>......... Phil
Eeyore
Spehro Pefhany wrote:
> Another potential problem with the original circuit is that the output
> voltage will spike as high as Vin - 1.xV during the switch transitions
> unless it's a shorting type rotary switch.
If it was done the sensible way by adding resistors in parallel that problem
could at least be minimised.
Graham
Jamie
Yes, it was mentioned, you're just a little late :)
PinkFloyd43
> "robb" <so...@where.on.net> wrote in message
> news:13k7a00...@corp.supernews.com...
>> yes i googled and found lots of different mods to the basic 5v
>> *7805* regulator power supply
>> \that is {AC in, bridge, 7805, 5v out}
>>
>
> psu info ...
> is there a problem with the universal psu schematic (page 2)
> shown in the following document ?
> more specifically with the selectable voltage part of the
> schematic
>
> http://www.qsl.net/lu3mgp/electronica/UniversalPowerSupply.pdf
>
>
> i am trying to understand how that PSU can produce higher voltage
> than 5V while using a 7805 5V regulator and no path for the
> bridge rectifier (+) voltage to bypass the 5V regulator ?
>
> thanks for any help
> robb
>
>
>
>
>
what more than one person believes to be a decent 5V PS?
Jamie
You know, that's a good catch. Power supplies to many of us
are like car's. We assume to much..
You get in your car to drive it, you shouldn't have to
question if you have an engine in it! You just naturally assume
that. But then again, that analogy maybe flawed if you live in a
shady neighborhood :)
Jamie
In my opinion, you would be better off with fixed post regulators of
every voltage you need all branching off from the same bridge and filter
cap..
If you want a variable out, you can create one from one of the higher
voltage fixed post regulators with 2 Resistors and a pot.
Some Links.
http://www.tkk.fi/Misc/Electronics/circuits/psu_5v.html
http://www.paia.com/KRUKits/K4/K4.pdf
http://www.aaroncake.net/circuits/supply6.asp
http://members.tripod.com/michaelgellis/power2.html
http://www.educypedia.be/electronics/circuitssupplies.htm
Phil Allison
"Phil Allison"
> "Fred Abse"
>> Phil Allison wrote:
>>>>
>>>>> The value of the filter cap has almost no effect on the rms current
>>>>> figure - allowing only that the ripple voltage is under 25% of the
>>>>> peak value.
>>>>
>>>> It has a marked effect on the *peak* current, however.
>>>
>>>
>>> ** Bollocks.
>>>
>>
>> You really think so?
** The following section has just been added to this popular audio design
site to debunk the notion posted by Fred Abse.
http://sound.westhost.com/power-supplies.htm#myth
...... Phil