Cheap Variable Power Supply

SgtWookie

Joined Jul 17, 2007
22,230
I should have read that first and saved time.

It's impossible to meet those specs.
And you heard that from a bonafide expert on the topic.

I threw together a couple of switching supplies using the LT1171; schematic & simulations shown here:
http://forum.allaboutcircuits.com/showpost.php?p=408856&postcount=7
Efficiency can top 90%, depending on Vout & load. The expensive part would be finding a suitable 30v-36v input supply capable of cranking out the amps. The nice thing about the two is from a single input, you'd wind up with both a positive and a negative supply from 1.2v-30v and -1.2v to -30v; you could use an LT1170, LT1171 or LT1172 depending on how much current you wanted out.

On your variable supply - you will run into heating problems when supplying moderate to high current at low voltages; your heat sink is small, your project box is sealed, plastic, and there is no fan & vent holes to get rid of the heat that will build up. It's hard to kill an LM317, but you will probably find that it will cycle on and off due to it's built-in protection features.
 

Thread Starter

T.Jackson

Joined Nov 22, 2011
328
On your variable supply - you will run into heating problems when supplying moderate to high current at low voltages; your heat sink is small, your project box is sealed, plastic, and there is no fan & vent holes to get rid of the heat that will build up. It's hard to kill an LM317, but you will probably find that it will cycle on and off due to it's built-in protection features.
I use this power supply for powering circuits @ few hundred mA. I do have some holes at the rear as well. The transformer would prevent the supply from ever trying to deliver any significant amount of power, as it is only rated at 5VA. It is also thermally and electrically fused too.
 

thatoneguy

Joined Feb 19, 2009
6,359
$3.84 for 1mH@3.1A

The caps total that much for filtering as well, so the 78xx would be about the cheapest part in the whole setup, outside of resistors.

--ETA: Seems kind of wrong through, after all these years of choosing caps to avoid oscillation, here they are embracing it.
 

SgtWookie

Joined Jul 17, 2007
22,230
Just for grins, I threw the 78xx switcher together and had to fix an error just to get it to run.

They showed a junction at C2/D2/R2/R3; it should have been a crossing.

If you use a 1mH inductor with no parasitic resistance and a 20v input with a 7805, you wind up with about 17v out until your load can pull it back down. :eek:

Adding 1 Ohm series resistance in the inductor got it down to 9v peak during startup. :rolleyes:

I1 is a load that goes from 10mA to 1A every 50mS, so that you can see the response.

Frequency is about 1.2kHz under load, and ~200Hz @ idle current.

Efficiency turns out to be about 35%. :(

All in all, a rather dismal showing. But, that's what I expected.

Next lesson: installing screws with a hammer. :)
 

Attachments

Last edited:

bountyhunter

Joined Sep 7, 2009
2,512
Ever heard the phrase, "putting lipstick on a pig"?

;)

Try pricing out a 1mH inductor capable of handling 4-5A.
You know, application engineers at national lead lonely lives...... some of the only fun we had was coming up with the most absurd uses we could for the parts we released.

It was like the chefs in a restaurant who get bored and start putting bizarre things into the food and watching the people eat...

"HEY! Look! He's actually eating it!"
 

Wendy

Joined Mar 24, 2008
23,464
Just remember these guys are trying to help. I have a feeling you are going to have trouble finding what you want mainly due to current, this stuff does not come cheap. I probably have the sweetest setup of most guys here and I would have a lot of trouble doing it on the cheap.

I will enforce the peace dagnabit! People will play nice or else! or else being I will or else again!
 
Last edited:

Thread Starter

T.Jackson

Joined Nov 22, 2011
328
Just remember these guys are trying to help. I have a feeling you are going to have trouble finding what you want mainly due to current, this stuff does not come cheap. I probably have the sweetest setup of most guys here and I would have a lot of trouble doing it on the cheap.
Well it does have to use a plug pack with no mains wiring; it will of course be a "toy", but good enough for some applications.
 

Thread Starter

T.Jackson

Joined Nov 22, 2011
328
You see how I am doing this piece by piece? In the end it all goes together like click, click clack. No simulator.

Block >> schematic >> detailed schematic (specific parts and values etc ...) >> proto build >> repeat steps if problems >> PCB >> build >> document.

I am not new to this. I have done some big projects before.
 

Wendy

Joined Mar 24, 2008
23,464
Problem is the 78XX / 79XX series series is limited to one amp.

About 6 months ago there were 24VDC 6.5A switchers for $15 ea all over the place, but the supply has dried up. I bought 4 for a similar project, though I wasn't going for tracking, just dual ± supplies with independent settings.
 

Thread Starter

T.Jackson

Joined Nov 22, 2011
328
Usually, I start with the case after I get a working build. The artwork for the case. People only really care what it looks like on the outside. The way I design the outside appearance defines how the insides are put.
 

Thread Starter

T.Jackson

Joined Nov 22, 2011
328
About 6 months ago there were 24VDC 6.5A switchers for $15 ea all over the place, but the supply has dried up. I bought 4 for a similar project, though I wasn't going for tracking, just dual ± supplies with independent settings.
These supplies are arguably unsafe for an experimental supply powering circuit boards on your desk, because they're not isolated.

The power isn't as clean either. Not suitable for audio, and after all this will be a dual rail supply.
 

Thread Starter

T.Jackson

Joined Nov 22, 2011
328
A common bench mark for a decent supply is 5mV ripple PP, and +/- 0.1% regulation.

You won't get this with a switcher.
 
Top