basic power supply

from Google search...

Dave

You will need some rectifiers as well. The capacitors are connected after the rectifiers.

Find a datasheet for the regulator ICs you intend to use - it should have some sample circuits.

See

for some useful information (found by googling for "dc power supply circuit")

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

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.

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

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.

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.

Hehe. I had worried a little.

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?

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.

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.

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

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):

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.

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).

--
Cats, coffee, chocolate...vices to live by

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 as

to try to get an idea of the probably VA rating of your transformer. (compare sizes or weights)

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.

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.)

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

e

th

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.

d e

ur

he

r

Ok. So max voltage out is 48V + or -, from the 7824 and 7924.

tty

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).

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.

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.

e

th

Oh, also I already have my regulators. Maybe later, if I find one I like.

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.

Oh, and

as

to try to get an idea of the

I prefer to design things using the most appropriate part - and L317s are really cheap. (But it is your power supply...)

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

wrote in message news: snipped-for-privacy@p20g2000yqi.googlegroups.com...

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

==============================================================================

Version 4 SHEET 1 976 680 WIRE -256 -192 -512 -192 WIRE 240 -192 -256 -192 WIRE -656 -128 -688 -128 WIRE -512 -128 -512 -192 WIRE -512 -128 -592 -128 WIRE -256 -128 -256 -192 WIRE -192 -128 -256 -128 WIRE -32 -128 -112 -128 WIRE 272 -128 -32 -128 WIRE 464 -128 368 -128 WIRE -576 -16 -624 -16 WIRE -512 -16 -512 -128 WIRE -32 16 -32 -128 WIRE 32 16 -32 16 WIRE 320 16 320 -64 WIRE 320 16 128 16 WIRE 464 16 464 -128 WIRE 464 16 368 16 WIRE 528 16 464 16 WIRE 592 16 528 16 WIRE -688 80 -688 -128 WIRE -576 80 -688 80 WIRE -256 96 -256 -128 WIRE -208 96 -256 96 WIRE -96 96 -144 96 WIRE 80 96 80 80 WIRE 80 96 -32 96 WIRE 80 112 80 96 WIRE 240 112 240 -192 WIRE 368 112 368 16 WIRE -688 128 -688 80 WIRE 528 160 528 16 WIRE -256 176 -256 96 WIRE -128 240 -160 240 WIRE -80 240 -128 240 WIRE 16 240 0 240 WIRE 368 240 368 192 WIRE 368 240 304 240 WIRE -688 272 -688 208 WIRE -624 272 -624 -16 WIRE -624 272 -688 272 WIRE -160 272 -160 240 WIRE 160 288 80 288 WIRE 240 288 160 288 WIRE 368 288 368 240 WIRE -576 304 -576 80 WIRE -464 304 -512 304 WIRE -688 400 -688 272 WIRE -640 400 -688 400 WIRE -464 400 -464 304 WIRE -464 400 -576 400 WIRE -256 400 -256 240 WIRE -256 400 -464 400 WIRE -160 400 -160 352 WIRE -160 400 -256 400 WIRE 160 400 160 368 WIRE 160 400 -160 400 WIRE 368 400 368 368 WIRE 368 400 160 400 WIRE 528 400 528 240 WIRE 528 400 368 400 FLAG 592 16 Vout FLAG -128 240 Vin FLAG 368 400 0 SYMBOL npn 16 192 R0 SYMATTR InstName Q1 SYMATTR Value 2N3904 SYMBOL npn 304 192 M0 SYMATTR InstName Q2 SYMATTR Value 2N3904 SYMBOL pnp 128 80 M270 SYMATTR InstName Q3 SYMATTR Value 2N2907 SYMBOL res 64 96 R0 SYMATTR InstName R1 SYMATTR Value 30 SYMBOL res 224 96 R0 SYMATTR InstName R2 SYMATTR Value 50 SYMBOL res 144 272 R0 SYMATTR InstName R3 SYMATTR Value 50 SYMBOL res 352 96 R0 SYMATTR InstName R6 SYMATTR Value 500 SYMBOL res 352 272 R0 SYMATTR InstName R7 SYMATTR Value 1k SYMBOL voltage -688 112 R0 WINDOW 123 0 0 Left 0 WINDOW 39 24 132 Left 0 SYMATTR SpiceLine Rser=.1 SYMATTR InstName V1 SYMATTR Value SINE(0 22 60 0 0 0 100) SYMBOL voltage -160 256 R0 WINDOW 123 0 0 Left 0 WINDOW 39 0 0 Left 0 WINDOW 3 -91 198 Left 0 SYMATTR Value PWL(0 .1 5m .1 100m 1 200m 1 300m 2 400m 2 500m 4 600m 4 700m

8 800m 8 900m 10 1000m 10) SYMATTR InstName V2 SYMBOL res 512 144 R0 SYMATTR InstName R8 SYMATTR Value 10 SYMBOL npn 272 -64 R270 SYMATTR InstName Q4 SYMATTR Value 2N3055 SYMBOL res -96 -144 R90 WINDOW 0 0 56 VBottom 0 WINDOW 3 32 56 VTop 0 SYMATTR InstName R4 SYMATTR Value 0.2 SYMBOL diode -208 112 R270 WINDOW 0 32 32 VTop 0 WINDOW 3 0 32 VBottom 0 SYMATTR InstName D1 SYMATTR Value 1N4148 SYMBOL diode -96 112 R270 WINDOW 0 32 32 VTop 0 WINDOW 3 0 32 VBottom 0 SYMATTR InstName D2 SYMATTR Value 1N4148 SYMBOL diode -656 -112 R270 WINDOW 0 32 32 VTop 0 WINDOW 3 0 32 VBottom 0 SYMATTR InstName D3 SYMATTR Value MURS320 SYMBOL diode -576 0 R270 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

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.

When copying into LTspice, fix the wrapped lines ;-)

...Jim Thompson

--
| James E.Thompson, P.E.                           |    mens     |
| Analog Innovations, Inc.                         |     et      |
| Analog/Mixed-Signal ASIC\'s and Discrete Systems  |    manus    |
| Phoenix, Arizona  85048    Skype: Contacts Only  |             |
| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  |
| E-mail Icon at http://www.analog-innovations.com |    1962     |
             
 I love to cook with wine     Sometimes I even put it in the food

If you can get a copy of "How to Design Your Own Power Supplies" by Huffman, published in the June 1975 Popular Electronics, it will probably be a lot of help. If I can find my copy, I'll offer to scan and email it to you. Or you can go to the local library and get it from the Referrence Desk, where the have the Reader's Guide to Periodical Liturature (which is how I found it.) It is pages 36-39 of the June 1975 Pupular Electronics, by Huffman. The librarian will probably help you find it if you give them this info.