Thanks for all the helpful and thought-provoking responses!
Such a question depends upon the detail of the power supplies you are modifying, being 'similar' is not sufficient to provide an answer. If you can find an exact schematic for the supply you are purchasing then some advice might be possible.
By "similar" I mean almost "exactly", with only component values changed to meet the specs for each model. That's what I read somewhere while digging into this. In any case, I'd like us to assume the schematic is correct for the purpose of my questions. Obviously I'll be checking that when I get my specific units. But in the areas of interest indicated, any variations will hopefully be minor, and can be allowed for. I think my questions are general enough to proceed on that basis.
I once tried to use a digital potentiometer in the feedback loop of a step-up-converter - unfortunately it locked up
due to EMI or any disturbance and the DC-DC went havoc, destroying its diode, the digital potentiometer and the
arduino.
Sounds not too good. I'll still get some DP's and play around with this. But I'll use an isolator so that Arduino should be safe.
But what worked was a NPN-transistor parallel to the adjustment potentiometer. The regulation loop of the converter was filtered so well that I could directly feed the NPN with a 10kHz-PWM signal from the microcontroller. If you want to I can sketch up the schematics.
Your description is clear enough. This is a neat solution, the NPN could even be an optocoupler's transistor. The replacement adjustment schematic shown was to get a more linear response to potentionmeter angle, but that's not so important when a micro is controlling it. I'll definately play around with that.
I think you are aware that an analogous regulation loop (the OpAmp) needs its supply voltage before its too late. A digital pre-regulation can only work if the signal gradients are either quite slow or predictable.
That was a concern, so maybe automatic adjustment is not such a good idea. With fans and a modified heatsink on the opamp, one could have a larger headroom between PSU output voltage and the opamp's needs. Anything less than the full 36v output would be a help.
And I do envisage that mostly it would be a slow and predictable signal. And in that case, no automation is needed, as the PSU voltage can just be programmed accordingly.
One example usage would be as a programmable "variac", by driving the opamp into the secondary of a step down transformer.
If the transformer does provide a split (3-terminal) output then yes I could see you modifying the highlighted bit of circuit to give +/- outputs. The two half rails would be weaker however (half the current limit?)
I do not expect all power supplies to be wired that way or even have split-output transformers.
The PSUs I'm getting are 15A units, so current won't be a problem. I'm hoping that they are wired like that. I should know in a few days...
I believe this is called "group regulation", lots of ATX computer power supplies do it.
Duh! Of course, I hadn't thought of that. Many SMPSs only use one output for feedback, so that's a good sign.
An audio amp shouldn't present loads on the rails during normal operation, so I suspect it would be fine.
The amp is DC-coupled, and will be used to extend the capabilities of a function generator. I imagine there will be cases where the signal will be asymmetrical, or have a DC offset. So something to watch out for.
Controller ICs need feedback with certain parameters. Sometimes using too large or too small resistors can upset the feedback loops (as per Phil1977) causing things to misbehave and/or blow up.
Small tweaks are OK, but installing pots may or may not work. Also if the pot is dirty then the PSU might blow up whilst you are turning it. It would be safer to used fixed voltage dividers, but again be wary of switching between them with a switch (depending how you implement it).
Yes, I'll need to experiment carefully. Some of the reading I've done suggested that there should be resistors across the pot to set a safe output should the wiper get "scratchy".
Digital control of voltage dividers for PSU feedback probably has gotchas too EDIT: Do digital pots have capacitance/bandwidth issues, or do they work OK?
I shall find out...
When you're switching/messing around with the feedback using digital control the best solution is to usually feed the midpoint of the voltage divider with a resistor and DAC output. There's a calculator built into Ti's Power Stage designer tool that lets you figure out what values to use for whatever resistances you have in place in circuit. Bonus is too is this method also doesn't mess with your circuits loop/gain characteristics.
Minimum destabilizing influence on standard operation seems like a worthwhile goal. I'll look into experimenting with what you've described as well.
Sounds incredibly complicated.
- What voltage do you need, is +/-36V and 5A necessary?
- Why do the two opamps need to be isolated?
Doesn't bother me if it gets complicated, I'm quite happy for this to become a substantial project if I can justify it.
It will be nice to have a general purpose box with two 36V/15A programmable PSU's, as well as the power opamps to suit.
The "variac" example function mentioned above can make use of all the power it can get.
In the general purpose piece of equipment that I'm thinking about, two isolated halves would always be a good thing as it's less restricted and more flexible.
Is the psu quality any better then few years ago?.
What i measured 12v 30A psu:s ab. 5yrs ago psu:s make lots of noise.
12v 40A black ones was so bad i cant plug it to mains when im listening HF radio.
S9 +30db noise all over the bands.
I don't know, I hope not, I'll find out!
...why so complicated?
You could connect + to + and - to - and create a "virtual ground" with a resistive divider
(maybe with an OPAmp to buffer).
Good luck
Thanks. But creating a "virtual ground" for a high power supply? Wouldn't that require another power opamp, just to hold the virtual ground still? (and connecting the two PSUs together would only be required if I can't get a negative output from each)