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basic power supply

I want to built a basic power supply with 6 7[89]?? regulators. I have
a line transformer capable of supplying 31VDC (15.5 + center-tap,
scrap transformer). However, I do not know where I should connected
the required capacitors (that is, I know they are required (and why),
but I do not how to do it). Any help?
 
J

Jon Kirwan

Jan 1, 1970
0
I want to built a basic power supply with 6 7[89]?? regulators. I have
a line transformer capable of supplying 31VDC (15.5 + center-tap,
scrap transformer).

You probably mean VAC. Transformers on their own don't work well with
DC. Where did you find the term 31VDC written?

By the way, I love the idea of learning some electronics by trying a
hand at building a power supply. So this is a great project to work
on. You can start by ignoring some details that would probably be
more confusing that helpful and later get better at it by bringing in
those details later on when you are ready for them.
However, I do not know where I should connected
the required capacitors (that is, I know they are required (and why),
but I do not how to do it). Any help?

Okay. So you have a transformer in hand and what to know what to add
to it. Do you want a power supply that allows you to vary the
voltage? Or do you just want a fixed output? Do you know what the
rating of the transformer's secondary (output) is, regarding amps? How
many different voltage outputs, regulated, are you looking for? Just
one? Or more?

If the transformer doesn't already have a bridge rectifier or other
rectifiers on it, you will need to get two or four diodes. Since your
secondary is center-tapped, you can go either way. Usually, after the
diodes "rectify" the AC into deeply rippled DC, you can add
appropriately sized capacitors with the right polarity hookup _across_
(in parallel with, as often written) the + and - sides of the output
from the diodes to help filter the voltage ripple to a somewhat
steadier level before it reaches the regulator circuit. Usually,
these are electrolytic capacitors that have a particular polarity you
must be careful to follow.

What other resources do you have? Voltmeter, at least? What other
parts laying about?

Jon
 
I want to built a basic power supply with 6 7[89]?? regulators. I have
a line transformer capable of supplying 31VDC (15.5 + center-tap,
scrap transformer).

You probably mean VAC.  Transformers on their own don't work well with
DC.  Where did you find the term 31VDC written?

By the way, I love the idea of learning some electronics by trying a
hand at building a power supply.  So this is a great project to work
on.  You can start by ignoring some details that would probably be
more confusing that helpful and later get better at it by bringing in
those details later on when you are ready for them.


Ah.. yes it is VAC, not VDC. sorry. And yes, I do have some bridge
rectifiers (left over from a project using relays, where the relays
only allowed switching in 1 direction, but I needed to be able to use
2). 15.5VAC is written on the transformer, but it's center-tapped and
the markings seem to indicate that is between two adjacent taps (and
not the end taps). And this isn't really something that I'm doing for
one of my first projects - I'm building an 8-bit TTL-based calculator
right now. I just posted it in basics because it's a basic power
supply - nothing fancy. So gimme the details, if you want.
Okay. So you have a transformer in hand and what to know what to add
to it. Do you want a power supply that allows you to vary the
voltage? Or do you just want a fixed output? Do you know what the
rating of the transformer's secondary (output) is, regarding amps? How
many different voltage outputs, regulated, are you looking for? Just
one? Or more?

If the transformer doesn't already have a bridge rectifier or other
rectifiers on it, you will need to get two or four diodes. Since your
secondary is center-tapped, you can go either way. Usually, after the
diodes "rectify" the AC into deeply rippled DC, you can add
appropriately sized capacitors with the right polarity hookup _across_
(in parallel with, as often written) the + and - sides of the output
from the diodes to help filter the voltage ripple to a somewhat
steadier level before it reaches the regulator circuit. Usually,
these are electrolytic capacitors that have a particular polarity you
must be careful to follow.

What other resources do you have? Voltmeter, at least? What other
parts laying about?

Jon

I want variable voltage from the regulators, which I understand can be
done by either putting a pot on the output (relatively obvious) to
lower it, or floating the regulator above ground with a resistor
between ground and what the reg "thinks" is ground. I know that
electrolytic caps have a polarity (though I believe I have blown at
least one by hooking up a not-so-well-marked car radio up backwards),
so that's not an issue.

I found two pairs lying around which are 30uF and 1uf. Will that be
good enough?
I have plenty of resources - true-RMS multimeter with transistor
checker and (!) capacitance measurement, 100MHz oscilloscope (it cost
about 1/4 of the online price at a place called Stewart Smith's (I
live in Syracuse, New York)), home-built parallel port logic analyzer
(might even be powered off of this supply!), and of course the
mandatory connection to the net.

I have an old HP 524C counter which I have gotten numerous golden-
years-of-electronics-no-clue-what-semiconductors-are-age components,
including the above caps. And there are two electronics stores in
town, so no problem getting parts.
 
J

Jon Kirwan

Jan 1, 1970
0
I want to built a basic power supply with 6 7[89]?? regulators. I have
a line transformer capable of supplying 31VDC (15.5 + center-tap,
scrap transformer).

You probably mean VAC.  Transformers on their own don't work well with
DC.  Where did you find the term 31VDC written?

By the way, I love the idea of learning some electronics by trying a
hand at building a power supply.  So this is a great project to work
on.  You can start by ignoring some details that would probably be
more confusing that helpful and later get better at it by bringing in
those details later on when you are ready for them.

Ah.. yes it is VAC, not VDC. sorry.

Hehe. I had worried a little.
And yes, I do have some bridge
rectifiers (left over from a project using relays, where the relays
only allowed switching in 1 direction, but I needed to be able to use
2).

Okay. I'm still not clear on the amps available. Do you have a way
of finding out? Or, alternately, can you say what you want to be able
to support?
15.5VAC is written on the transformer, but it's center-tapped and
the markings seem to indicate that is between two adjacent taps (and
not the end taps).

It might help to measure from the ends, as well. In other words, work
out the details. I gather you have a meter, so use it. Doesn't take
that much to hook it up to a snipped off AC cord (I have those laying
around from stuff I throw away.) Alligator clip it, if you have to.
And this isn't really something that I'm doing for
one of my first projects - I'm building an 8-bit TTL-based calculator
right now. I just posted it in basics because it's a basic power
supply - nothing fancy. So gimme the details, if you want.

Well, I'll do what I can. I learn as I go, too, being a hobbyist. The
"big boys" will jump in when they feel I screw up badly, I'm sure.
I want variable voltage from the regulators, which I understand can be
done by either putting a pot on the output (relatively obvious)

Yes, but really, really terrible, too. In the sense of -- "it just
ain't done" except in VERY low current situations (your usual
'divider' situation.) Certainly, not for a power supply.
to
lower it, or floating the regulator above ground with a resistor
between ground and what the reg "thinks" is ground. I know that
electrolytic caps have a polarity (though I believe I have blown at
least one by hooking up a not-so-well-marked car radio up backwards),
so that's not an issue.

Yeah. I've hooked them up to AC power outlets back as a kid just to
watch them blow up. Bad gases, I heard later. But the smell isn't
the kind of thing you stick your nose into, anyway, so I mostly
avoided getting too much into my system back then. And I didn't blow
up more than a small number, even then. Gets boring.
I found two pairs lying around which are 30uF and 1uf. Will that be
good enough?

For some of the IC regulators, perhaps. What bothers me is that you
may need something larger than 30uF to keep the ripple down leading to
your regulator if the current is much of anything. The dV is easily
seen from the I=C dV/dt equation, as dV = (I/C) dt. Your dt is pretty
much determined by your AC Hz and the choice you make for the diode
topology. With 60Hz and a bridge, you are talking about 120Hz bumps
coming out. That's 8.33ms per. And the capacitor may need to supply
current for a substantial part of that time. Assume for a moment that
it is 72% of the time or about 6ms. Then you have a ripple dV of some
200*I. If you can accept a 2V ripple, that means your load current
needs to be 10mA or less. Basically, a heavy load suggests bigger
caps to keep the ripple down to something manageable. And if you
accept a lot of input ripple, just to keep the caps smaller, then your
power supply regulator needs to be better at ignoring the ripple. If
it is a linear one besides, you still need to make sure that the
lowest point in the ripple is still large enough that it provides the
output voltage required plus whatever overhead is required by the
regulation system.

So it's important to know what you plan for the regulator (switcher vs
linear), what you hope to get out as a maximum load current, and over
what range of output voltages, plus some idea of what you are willing
to accept as ripple on your regulated output, too. Among other things
I'm sure I will be told about.
I have plenty of resources - true-RMS multimeter with transistor
checker and (!) capacitance measurement, 100MHz oscilloscope (it cost
about 1/4 of the online price at a place called Stewart Smith's (I
live in Syracuse, New York)), home-built parallel port logic analyzer
(might even be powered off of this supply!), and of course the
mandatory connection to the net.

Well, that sounds great. You have some decent equipment and hopefully
have reasonable understanding about applying them (I learn myself over
time to use what I have better.)
I have an old HP 524C counter which I have gotten numerous golden-
years-of-electronics-no-clue-what-semiconductors-are-age components,
including the above caps. And there are two electronics stores in
town, so no problem getting parts.

Okay.

Are you interested in working through the design thinking? Or just
want a circuit and don't care to understand why?

And in the meantime, can you measure that transformer and let us know
what range of output voltages are okay with you (both the low end as
well as the high end -- does it really need to be able to go down to
0.2V or to 0.00V, for example?) Also, what compliance you want for
the current? (Which gets back to maybe telling us just how heavy that
transformer is or if it has a rating on it, somewhere, or part
number.)

Jon
 
E

Ecnerwal

Jan 1, 1970
0
Ah.. yes it is VAC, not VDC. sorry. And yes, I do have some bridge
rectifiers (left over from a project using relays, where the relays
only allowed switching in 1 direction, but I needed to be able to use
2). 15.5VAC is written on the transformer, but it's center-tapped and
the markings seem to indicate that is between two adjacent taps (and
not the end taps). And this isn't really something that I'm doing for

So figure out your connections for a proper center tap, and hook up your
rectifiers. Go buy (or scrounge) some much larger electrolytic caps -
you want (generically, without getting into the details of selecting
them) 1000+ uF sort of size, not 30 uF sort of size - ie, those are not
even in the right ballpark, as power supply filter caps go, IMHO.

Here's a handy site (couple of page links - more are there, poke around)
found per quick websearch (there may be better, but it seems OK for the
basic idea):

http://www.play-hookey.com/ac_theory/ps_rectifiers.html

http://www.play-hookey.com/ac_theory/ps_filters.html

A real book might also serve well - having had some exposure to terrible
books, I'm fond of The_Art_of_Electronics as being a very good one. If
you have a college bookstore nearby, look for a used copy.
I want variable voltage from the regulators, which I understand can be
done by either putting a pot on the output (relatively obvious) to
lower it, or floating the regulator above ground with a resistor
between ground and what the reg "thinks" is ground.

Eww. Yuck-poo and merely yuck. There are much better ways to make
variable voltage supplies. Old tech, but works well and still available
(variable [+] regulator datasheet, variable [-] regulator datasheet).

http://www.national.com/ds/LM/LM117.pdf

http://www.national.com/ds/LM/LM137.pdf
 
On Sun, 1 Mar 2009 14:21:25 -0800 (PST), [email protected] wrote:
I want to built a basic power supply with 6 7[89]?? regulators. I have
a line transformer capable of supplying 31VDC (15.5 + center-tap,
scrap transformer).
You probably mean VAC.  Transformers on their own don't work well with
DC.  Where did you find the term 31VDC written?
By the way, I love the idea of learning some electronics by trying a
hand at building a power supply.  So this is a great project to work
on.  You can start by ignoring some details that would probably be
more confusing that helpful and later get better at it by bringing in
those details later on when you are ready for them.
Ah.. yes it is VAC, not VDC. sorry.

Hehe.  I had worried a little.
And yes, I do have some bridge
rectifiers (left over from a project using relays, where the relays
only allowed switching in 1 direction, but I needed to be able to use
2).

Okay.  I'm still not clear on the amps available.  Do you have a way
of finding out?  Or, alternately, can you say what you want to be able
to support?

Amps available: 0.75 amps is marked on the secondary.
Ok, I lied. That's the FIRST secondary (the 15.5V one).
The SECOND secondary gives 30 volts (I think - I'm at school and don't
have it with me) but only 0.25A (I'm sure about that), so not enough
amps for what I want.
It might help to measure from the ends, as well.  In other words, work
out the details.  I gather you have a meter, so use it.  Doesn't take
that much to hook it up to a snipped off AC cord (I have those laying
around from stuff I throw away.)  Alligator clip it, if you have to.


Well, I'll do what I can.  I learn as I go, too, being a hobbyist. The
"big boys" will jump in when they feel I screw up badly, I'm sure.





Yes, but really, really terrible, too.  In the sense of -- "it just
ain't done" except in VERY low current situations (your usual
'divider' situation.)  Certainly, not for a power supply.

Ok. So max voltage out is 48V + or -, from the 7824 and 7924.
Yeah.  I've hooked them up to AC power outlets back as a kid just to
watch them blow up.  Bad gases, I heard later.  But the smell isn't
the kind of thing you stick your nose into, anyway, so I mostly
avoided getting too much into my system back then.  And I didn't blow
up more than a small number, even then.  Gets boring.


For some of the IC regulators, perhaps.  What bothers me is that you
may need something larger than 30uF to keep the ripple down leading to
your regulator if the current is much of anything.  The dV is easily
seen from the I=C dV/dt equation, as dV = (I/C) dt.  Your dt is pretty
much determined by your AC Hz and the choice you make for the diode
topology.  With 60Hz and a bridge, you are talking about 120Hz bumps
coming out.  That's 8.33ms per.  And the capacitor may need to supply
current for a substantial part of that time.  Assume for a moment that
it is 72% of the time or about 6ms.  Then you have a ripple dV of some
200*I.  If you can accept a 2V ripple, that means your load current
needs to be 10mA or less.  Basically, a heavy load suggests bigger
caps to keep the ripple down to something manageable.  And if you
accept a lot of input ripple, just to keep the caps smaller, then your
power supply regulator needs to be better at ignoring the ripple.  If
it is a linear one besides, you still need to make sure that the
lowest point in the ripple is still large enough that it provides the
output voltage required plus whatever overhead is required by the
regulation system.

So it's important to know what you plan for the regulator (switcher vs
linear), what you hope to get out as a maximum load current, and over
what range of output voltages, plus some idea of what you are willing
to accept as ripple on your regulated output, too.  Among other things
I'm sure I will be told about.

It's a linear (I think - that's what I'm told) regulator, but I would
think that a 2V ripple would be fine (I'm probably wrong, of course).
Well, that sounds great.  You have some decent equipment and hopefully
have reasonable understanding about applying them (I learn myself over
time to use what I have better.)


Okay.

Are you interested in working through the design thinking?  Or just
want a circuit and don't care to understand why?

And in the meantime, can you measure that transformer and let us know
what range of output voltages are okay with you (both the low end as
well as the high end -- does it really need to be able to go down to
0.2V or to 0.00V, for example?)  Also, what compliance you want for
the current?  (Which gets back to maybe telling us just how heavy that
transformer is or if it has a rating on it, somewhere, or part
number.)

Jon

I mostly just want to build it, but I could easily learn some design
principles...

And for the low and high ends, I would probably like each regulator to
be able to go down to half the rated voltage (though that is obviously
not feasible, except for low current, as you said) and up to double
the rated voltage.

They're a TO-220 package, so I should be able to get a nice amp out of
them... of course, the transformer doesn't handle that, but it's tiny,
and I can always upgrade later.
 
Ah.. yes it is VAC, not VDC. sorry. And yes, I do have some bridge
rectifiers (left over from a project using relays, where the relays
only allowed switching in 1 direction, but I needed to be able to use
2). 15.5VAC is written on the transformer, but it's center-tapped and
the markings seem to indicate that is between two adjacent taps (and
not the end taps). And this isn't really something that I'm doing for

So figure out your connections for a proper center tap, and hook up your
rectifiers. Go buy (or scrounge) some much larger electrolytic caps -
you want (generically, without getting into the details of selecting
them) 1000+ uF sort of size, not 30 uF sort of size - ie, those are not
even in the right ballpark, as power supply filter caps go, IMHO.

Here's a handy site (couple of page links - more are there, poke around)
found per quick websearch (there may be better, but it seems OK for the
basic idea):

http://www.play-hookey.com/ac_theory/ps_rectifiers.html

http://www.play-hookey.com/ac_theory/ps_filters.html

A real book might also serve well - having had some exposure to terrible
books, I'm fond of The_Art_of_Electronics as being a very good one. If
you have a college bookstore nearby, look for a used copy.
I want variable voltage from the regulators, which I understand can be
done by either putting a pot on the output (relatively obvious) to
lower it, or floating the regulator above ground with a resistor
between ground and what the reg "thinks" is ground.

Eww. Yuck-poo and merely yuck. There are much better ways to make
variable voltage supplies. Old tech, but works well and still available
(variable [+] regulator datasheet, variable [-] regulator datasheet).

I know someone who recommends this method. And he's very smart. And I
know someone else who sees no problem, so I will simply ignore you by
the democratic process. Remember, this is America.
 
On Sun, 1 Mar 2009 14:21:25 -0800 (PST), [email protected] wrote:
I want to built a basic power supply with 6 7[89]?? regulators. I have
a line transformer capable of supplying 31VDC (15.5 + center-tap,
scrap transformer).
You probably mean VAC.  Transformers on their own don't work well with
DC.  Where did you find the term 31VDC written?
By the way, I love the idea of learning some electronics by trying a
hand at building a power supply.  So this is a great project to work
on.  You can start by ignoring some details that would probably be
more confusing that helpful and later get better at it by bringing in
those details later on when you are ready for them.
Ah.. yes it is VAC, not VDC. sorry. And yes, I do have some bridge
rectifiers (left over from a project using relays, where the relays
only allowed switching in 1 direction, but I needed to be able to use
2). 15.5VAC is written on the transformer, but it's center-tapped and
the markings seem to indicate that is between two adjacent taps (and
not the end taps). And this isn't really something that I'm doing for
one of my first projects - I'm building an 8-bit TTL-based calculator
right now. I just posted it in basics because it's a basic power
supply - nothing fancy. So gimme the details, if you want.

In my experience, transformers are usually labelled with the full
secondary voltage.  You should apply power to the primary, and measure
between the various secondary terminals to confirm the secondary
voltage.  If you have no ratings for the transformer, you might have a
look at some transformer manufacturer's websites  (such ashttp://www.hammondmfg.com/5cindex.htm) to try to get an idea of the
probably VA rating of your transformer. (compare sizes or weights)

I want variable voltage from the regulators, which I understand can be
done by either putting a pot on the output (relatively obvious) to
lower it, or floating the regulator above ground with a resistor
between ground and what the reg "thinks" is ground. I know that
electrolytic caps have a polarity (though I believe I have blown at
least one by hooking up a not-so-well-marked car radio up backwards),
so that's not an issue.

I would suggest that you use LM317 (positive) and LM337 (negative)
variable voltage regulators rather than the 78xx/79xx family, if you
want variable voltage, as the 317/337 are designed for that
application.


I found two pairs lying around which are 30uF and 1uf. Will that be
good enough?

You will want a much larger primary filter capacitor - 1000uF or more,
depending on the current you expect to draw from the supply.  You will
also want a 1 - 10 uF capacitor on the output of the regulator (check
the regulator data sheets for recommended circuits and component
values.)
I have plenty of resources - true-RMS multimeter with transistor
checker and (!) capacitance measurement, 100MHz oscilloscope (it cost
about 1/4 of the online price at a place called Stewart Smith's (I
live in Syracuse, New York)), home-built parallel port logic analyzer
(might even be powered off of this supply!), and of course the
mandatory connection to the net.
I have an old HP 524C counter which I have gotten numerous golden-
years-of-electronics-no-clue-what-semiconductors-are-age components,
including the above caps. And there are two electronics stores in
town, so no problem getting parts.

--
Peter Bennett, VE7CEI  
peterbb4 (at) interchange.ubc.ca  
GPS and NMEA info:http://vancouver-webpages.com/peter
Vancouver Power Squadron:http://vancouver.powersquadron.ca

Oh, also I already have my regulators. Maybe later, if I find one I
like.
 
Eww. Yuck-poo and merely yuck. There are much better ways to make
variable voltage supplies. Old tech, but works well and still available
(variable [+] regulator datasheet, variable [-] regulator datasheet).

I know someone who recommends this method. And he's very smart. And I
know someone else who sees no problem, so I will simply ignore you by

This is why democracy only works in theory. Your friends may be smart
in their fields, but this is just stupid. Those are the two worst ways
of doing it, horrific.
 
P

Paul E. Schoen

Jan 1, 1970
0
Oh, also I already have my regulators. Maybe later, if I find
one I like.

I came up with this design in LTspice of a regulated power supply with
current limiting that uses simple bipolar transistors. It would probably be
a good learning project. The PWL voltage source should be replaced with a
pot across a zener for an adjustable output. I used it only for simulation
purposes. There are probably many ways this could be improved, but you
could learn much more from this than a supply with a fixed integrated
voltage regulator.

Paul

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WINDOW 0 32 32 VTop 0
WINDOW 3 0 32 VBottom 0
SYMATTR InstName D4
SYMATTR Value MURS320
SYMBOL diode -512 320 M270
WINDOW 0 32 32 VTop 0
WINDOW 3 0 32 VBottom 0
SYMATTR InstName D5
SYMATTR Value MURS320
SYMBOL diode -576 416 M270
WINDOW 0 32 32 VTop 0
WINDOW 3 0 32 VBottom 0
SYMATTR InstName D6
SYMATTR Value MURS320
SYMBOL polcap -272 176 R0
WINDOW 3 24 64 Left 0
SYMATTR Value 6800µ
SYMATTR InstName C1
SYMATTR Description Capacitor
SYMATTR Type cap
SYMATTR SpiceLine V=25 Irms=2.07 Rser=0.03 MTBF=2000 Lser=0 mfg="Nichicon"
pn="UPR1E682MRH" type="Al electrolytic" ppPkg=1
SYMBOL res 16 224 R90
WINDOW 0 0 56 VBottom 0
WINDOW 3 32 56 VTop 0
SYMATTR InstName R5
SYMATTR Value 1k
TEXT -256 424 Left 0 !.tran 0 1 10u
 
Oh, andhttp://en.wikipedia.org/wiki/Appeal_to_authority

And I suppose you're going to say, with your stupid e-mail address and
"knowledge" of Wikipedia, that you're smarter than... let's see,
what's appr - Ah. You think you're smarter than GWB? Interesting. I'll
keep that in mind.
 
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