Bench Power Supply

[goom]

I am considering building a new bench power supply (my old one has bit the dust). It strikes me that a Picaxe may be ideally suited by using ADC to monitor the DC supply and PWM (driving a MOSFET) to regulate it to a pre-set value. With a voltage divide/pot arrangement on the ADC input, this would then permit the controlled voltage to be varied, or perhaps discrete voltages (3.8V, 5V, 12V etc) could be programmed via, say, 3 of the digital inputs.
Current limiting, digital current/voltage display (LCD module?), +/- tracking voltages etc. also come to mind as possibilities.
Am I "out to lunch", or does this sound feasible and practical?
I would welcome any thoughts and/or experience in this field.



 

 

[flyingnunrt]

Have you thought about using a PSU from an old junked computer.
They have +5v at several amps
+12v at a few amps
-5v and _12v at 500 mA
although the regulation on the 12v rails is a bit sus.

 

[andrewpro]

The regulation on all voltages will be suspect unless you provide a load for the PC power supply. ~500ma on each voltage should suffice, sometimes you can get away with just the 5 volt to stabilize the entire system, but not always. Depends on the supply.

--Andy P

 

[manuka]

"Out to lunch" - haven't heard that phrase since I lived in BC in the 70s! Welcome to THE friendly Forum, but suggest you do a past topic search too, as this PSU idea has emerged before. An issue may be electrical noise, as Picaxes are VERY sensitive to electrical hash- a major reason I recommend batteries for starter projects.

I'm a classic PSU builder myself & have also pondered Picaxe versions. However - having recently costed out lab. PSUs- I'd have to say that general purpose PSUs ( 0-30V @ say 5A) & specialised PSU ICs are so cost effective that it's hardly worth DIY now. Even ex PC PSUs can be pushed to work for peanuts.

You may have special needs however, so let us know ! Stan

 

[Michael 2727]

Not knocking the picaxe, but why not just use a plain old LM317/LM350 T or K. They are
cheap and almost available at the corner shops.
If it ever blows up, a few bucks and your on your way again.

Mount it on a good heatsink and it should
last for years, you can almost throw bricks
at them in the punishment department.

Edited by - Michael 2727 on 1/25/2006 6:24:05 AM

 

[BeanieBots]

I'd be cautious about using the PICAXE for doing the active control because it is not too quick. If for example, you suddenly switched on a load, how quickly could the PICAXE respond? or worse, how high would the supply go when the load was removed?
You could however get the PICAXE to generate a demand voltage that further circuitry would then use to regulate the supply.

 

[goom]

Many thanks for the suggestions and comments. This forum is really awesome with the quality and speed of responses.
I really want a variable power supply, both for voltage and current limiting. I found that my old (now dead) homebrew supply was extremely versatile since it could be used for anything from powering Picaxes to charging batteries (constant current and/or voltage).
Linear regulators are certainly simple to use, but when used as variable voltage or current devices do not easily adapt to zero levels. I test all of my circuits with the current limit on, so any unanticipated short circuits (=errors) simply result in the PSU pulling down to 0 volts, so avoiding any damage.
Furthermore, using a Picaxe PWM control of a Mosfet would effectively be a switching power suplly and avoid the heat dissipation issue associated with linear regulators.
I cannot really rationalise why speed would be an issue. The code to measure the voltage and current and adjust the PWM output would be quite short (I think), and the response of the Mosfet very fast.
Electrical noise should not be a problem with a bit of sensible power supply filtering to the Picaxe. I have built a reversing speed controller (DPDT relay and single Mosfet) with no more than a 0.1 micro F on the power supply with no apparant noise problems.
I guess the only way is to build and test it. If I get round to it I'll post my experiences.

 

 

[premelec]

You can use one LM317 for voltage regulation and another LM317 wired as a current regulator in series [see LM317 spec sheet] for current limiting - that's pretty cheap but not very energy efficient - however at low power levels that may not be a needed consideration. For tighter current regulation with less voltage drop you can use an op amp to sense current and influence the LM317 voltage regulator appropriately. There is some hazard of self oscillation with all these designs but usually not too hard to kill if you don't need super fast response.

On the 0v levels - just add a -1.3v bias in the control leg and then the output can be taken to 0 - I think this is in the LM317 data sheet too...

Edited by - premelec on 1/26/2006 3:58:20 AM

 

[frankeltham]

As premelec says, you could use an OP Amp to sense current.
In fact with a bit of tinkering you can make an op-amp act as a variable overload-switcher-off. Stuff in a capacitor or two and you can create a brief delay so that initial power surges of your subsequent devices don't shut it down too quick - a bit like a user-variable anti-surge fuse.
Have done it myself- worked a treat.

If you want to go the SM route (with the inherent noise) wouldn't it be better to get the PICAXE just to control the voltage sensing pin of a commercial SM Regulator? i.e. let the 'proper' chip do its stuff with your PICAXE in overall control? This could leave more time for other things - perhaps a software controlled overload switch. Anyway, I'm just thinking aloud. It sounds like an interesting project.

 

[BeanieBots]

I'm with frankeltham as hinted at earlier.
If the bulk of the excersise is to explore power supplies, then a PICAXE controlled switch mode would be really good education in many respects.

 

[vk6bgn]

Hey Goom,

Here is my home brew power supply. It's only adjustable from about 10 - 16 volts DC. Not sure if that is what you need. It will also do about 35 amps too. Not sure if you need that kind of power? Possibly by adjusting the values of R6, R7 & R12 in the schematic you could get the output voltage to adjust over a much wider range?

http://58.6.1.31/ham_radio.htm

I hope the link works....

Steve VK6HV