Isolated variable resistor function?

Any chance you can PWM a resistor?

-Lasse

Does it have to be four-quadrant? Floating? There are some cute fake resistor circuits around but the ones I've tried using need one end of the pot at "ground". That and an isolated DAC (DAC on the far side of an isolator, even) will solve your tolerance problem. You probably won't like the cost, though. ;-)

On a sunny day (Sat, 25 Aug 2012 10:34:56 -0700) it happened Joerg wrote in :

PIC with ADC on one end with RS232 out one opto PIC on other end with PWM DAC out (assuming you need some control voltage).

Would be nice but the part where the resulting signal goes into is blazingly fast (has to be). So the PWM would cause an undesired modulation.

But it is an idea, maybe I can slow something down in there. And go in with several meggeehoitzes. The uC we have would moan and groan though.

That's similar to what John suggested. Thing is, this variable resistor is going to be the bottom resistor of a voltage divider and has to be varied between zero and 10k or so.

Actually single quadrant is ok and one side of it can be grounded. Cost is a factor but not like in most of my other design. Meaning a buck is acceptable but two bucks would be a stretch.

You really live your PICs :-)

I don't need a control voltage but a control resistance. That's the challenge. Getting a voltage across is way easier: You'd just send PWM out of the uC that we already have, onto an optocoupler, and then RC-filter this into a DC voltage one the other side. Could be done for

On a sunny day (Sat, 25 Aug 2012 12:14:05 -0700) it happened Joerg wrote in :

How good of a resistor do you need? Is an opamp fast enough?

something like this:

-Lasse

even)

Yes, it is fast enough. But it has to behave like a real resistor, up to several kHz.

That would be method "b". The AD5274 is over a buck and needs optocouplers so it'll be quite painful financially. 15% tolerance would be better than the 20% from the MCP4011 though but the premium for that is rather steep.

On a sunny day (Sat, 25 Aug 2012 12:34:43 -0700) it happened Joerg wrote in :

No no, it saves a micro. This assumes that the equipment is used by a human. Why the electronics? he / she can twiddle the pot directly.

On a sunny day (Sat, 25 Aug 2012 12:33:45 -0700) it happened Joerg wrote in :

I have made very nice 'resistors' with MOSFET with feedback resistor network to gate.

certain

barrier

the

most

there

power

of

two-channel

resistance

Done

estate.

modulation.

Some of the MSP430's have Timer D, which will run PWM at

Jon

I should have said "clocked at 256MHz," but you already know that the count per period would divide this down for the PWM frequency. Sorry about not being precise in writing that.

Jon

to gate.

So have I, but tough to achieve a sustained 5% linearity over production runs. I've done it in servo fashion but with all that you quickly reach a point where a 30-40c digital potmeter is simply the better solution.