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Bench PSU kit comments?

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arnoldus:
I stumbled across this kit (30V-2A):
http://tuxgraphics.org/electronics/201005/bench-power-supply-v3.shtml#3lfindex9

It's quite cheap, even when counting the costs of the additional needed parts.

I wanted to know 1) if you think there are better readybuilt or kit alternatives for this price
2) If the method of using pulse modulation to generate the voltages is a proper way? Does it have drawbacks?

alm:
I dislike this method of control, it looks almost the same as his previous version. My main issue with this design is that the ADC of the AVR is pretty slow, max 10kS/s or so. Plus the base current will only be increased/decreased by one DAC unit (of the ~3600 units between fully on and fully off). This means that it takes about .4s to go from no base current to fully saturated. It will probably have a hard time tracking even simple 50/60Hz ripple! There is (at least in the previous version) a separate regulation that will shut it down in case of a dead short, but in any other case it's really slow. Even a crappy opamp like the ua741 will easily have thousand times the slew rate. In my opinion the right way to design a programmable power supply is to use DAC's to generate a reference voltage, but have op-amps do the actual feedback loop. Op-amps are much better at this type of work than a micro controller, this seems just doing it digitally for the sake of digital, almost as bad as using an Arduino to blink a LED ;). Plus I dislike having safety features like current limits implemented in software (cf. Therac-25). Something like an LM723, L200 or double LM317 will have a much better regulation.

EEVblog:

--- Quote from: alm on June 13, 2010, 07:30:05 pm ---I dislike this method of control, it looks almost the same as his previous version. My main issue with this design is that the ADC of the AVR is pretty slow, max 10kS/s or so. Plus the base current will only be increased/decreased by one DAC unit (of the ~3600 units between fully on and fully off). This means that it takes about .4s to go from no base current to fully saturated. It will probably have a hard time tracking even simple 50/60Hz ripple! There is (at least in the previous version) a separate regulation that will shut it down in case of a dead short, but in any other case it's really slow. Even a crappy opamp like the ua741 will easily have thousand times the slew rate. In my opinion the right way to design a programmable power supply is to use DAC's to generate a reference voltage, but have op-amps do the actual feedback loop. Op-amps are much better at this type of work than a micro controller, this seems just doing it digitally for the sake of digital, almost as bad as using an Arduino to blink a LED ;)

--- End quote ---

I agree. Much better to do it with opamps. I'm working on just such a neat PSU design at the moment actually.

Dave.

Anders:
When do we get to see this PSU Dave?  :)

I also agree on the OP-Amp solution even though you (if you must use a micro) could choose a PIC which has faster ADC than the AVR.

alm:
Sure, and a less brain-dead control algorithm could also improve things. But that's entirely missing the point. The point is that it's stupid to implement an analog feature with dedicated IC's for a few cents available in software. It's not like there's some complex DSP taking place, it's just 'if input > threshold + hysteresis than dac_output += 1; elseif input < threshold - hysteresis than dac_output -= 1'. That's exactly what an op-amp does, and even the really crappy ones will have a gain-bandwidth product easily into the hundreds of kilohertz.

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