build a simple 5v power supply for digital circuit

[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 ;

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]

Thanks Phil,
for all the answers
robb

"robb" ;
** Just use the basic circuit, no mods are needed.

** Depends on the amp draw.
4700 uF is good for 1.5 A, 470uF for 150 mA.

** 0.1 uF ( 100nF) , 63 volt plastic film is all you need to know.
** Rarely needed - use a IN4001 if you like.

** No such animal.

** Rarely needed.

** None.

** Large PSUs only.
Hundreds of watts, not 5 watts.
........ Phil

Thanks again Phil, (in case you missed top post)
for all the answers
robb

 

[Fred Abse]

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.

It has a marked effect on the *peak* current, however.

 

[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]

would be (0.5m/0.25V)*1A=2000u, make that 4700u...

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.

 

[robb]

;

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]

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]

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

Hint: The 7805 forces 5V across R1. Where does the current through
R1 go?

...Jim Thompson

 

[robb]

(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]

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 ?

If you'd read the whole article, it would have answered your question.

 

[Fred Abse]

"Fred Abse"



** Bollocks.

You really think so?

Try this:

Version 4
SHEET 1 880 680
WIRE 128 16 -304 16
WIRE 128 112 128 16
WIRE 128 112 80 112
WIRE 176 112 128 112
WIRE 288 112 240 112
WIRE 416 112 288 112
WIRE 80 144 80 112
WIRE 176 144 176 112
WIRE -304 208 -304 16
WIRE 80 224 80 208
WIRE 240 224 240 112
WIRE 240 224 80 224
WIRE 288 224 288 112
WIRE 416 224 416 112
WIRE 176 256 176 208
WIRE 240 256 176 256
WIRE 80 272 80 224
WIRE 176 272 176 256
WIRE 80 368 80 336
WIRE 128 368 80 368
WIRE 176 368 176 336
WIRE 176 368 128 368
WIRE 240 384 240 256
WIRE 288 384 288 288
WIRE 288 384 240 384
WIRE 416 384 416 304
WIRE 416 384 288 384
WIRE -304 400 -304 288
WIRE 128 400 128 368
WIRE 128 400 -304 400
FLAG 288 384 0
SYMBOL cap 272 224 R0
SYMATTR InstName C1
SYMATTR Value {C}
SYMBOL res 400 208 R0
SYMATTR InstName R1
SYMATTR Value 10
SYMBOL voltage -304 192 R0
WINDOW 123 0 0 Left 0
WINDOW 39 0 0 Left 0
SYMATTR InstName V1
SYMATTR Value SINE(0 10 60)
SYMBOL diode 64 144 R0
SYMATTR InstName D1
SYMBOL diode 160 272 R0
SYMATTR InstName D2
SYMBOL diode 160 208 M180
WINDOW 0 24 72 Left 0
WINDOW 3 24 0 Left 0
SYMATTR InstName D3
SYMBOL diode 64 336 M180
WINDOW 0 24 72 Left 0
WINDOW 3 24 0 Left 0
SYMATTR InstName D4
TEXT -32 440 Left 0 !.step param C 1000e-6 10000e-6 1000e-6
TEXT 56 560 Left 0 !.tran 0 25e-3 15e-3 uic
TEXT 80 496 Left 0 !.opt plotwinsize=0




[Transient Analysis]
{
Npanes: 1
{
traces: 1 {34603010,0,"I(D1)"}
X: ('m',0,-1.80420021658953e-005,0.001,0.00997780209162187)
Y[0]: (' ',0,-1,1,11)
Y[1]: ('_',0,1e+308,0,-1e+308)
Amps: (' ',0,0,0,-1,1,11)
Log: 0 0 0
GridStyle: 1
}
}

 

[robb]

If you'd read the whole article, it would have answered your question.

thanks for reply stephen,

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]

thanks for reply stephen,

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

When in doubt, return to my hint ;-)

...Jim Thompson

 

[Jamie]

(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

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]

[...]make that 4700u...

Fred, [...] your last name is ringing a bell

http://en.wikipedia.org/wiki/Fred_Bloggs

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]

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