Mains powered variable resistance

How about a pair of "power" wires going into a box and being ignored, while you connect a 3K NTC thermistor to the "output" side?

Depending on what hooks up to it, you may do a lot better with a heavily filtered switched capacitor (or switched resistor) for the resistive element on the output end -- trying to accurately control the resistance of a FET is going to be problematic.

As long as the thing connected to the outside end can stand a capacitively loaded "thermistor" then a switched capacitor should be accurate enough to control open-loop, with duty cycle or whatever.

Irregardless, if you need to work over a large temperature range you're going to be challenged: thermistors can present an enormous range of resistances.

I've asked for a sample of the kit that this unit has to drive so I can check how it responds to different inputs.

Maybe I wasn't clear.

I am simulating the response of an NTC thermistor for a humidity sensor. So 0% RH gives the same resistance as 0C, 25% RH has the same resistance as 25C, etc.

Temperature itself isn't a big issue.

use an NTCs control temperature with another NTC + FET as heater ?

might want a few NTCs to get above ambient

-Lasse

Hahahaha

You were perfectly clear -- I just forgot that part.

Still, you'll want to make sure you work over the whole resistance range that would be present from 0 to 100C.

Except I missed the part about wanting humidity to read as temperature.

So the joke's on me :/

The power requirements certainly seem low, so why not a capacitive dropper, running a single-chip switcher driving a tiny transformer? ...Jim Thompson

I don't really get the joke. You need enough power to heat up the NTC and read it's resistance by something else. Of course, it's overkill or overspec. They are probably too lazy to redo the device reading the NTC.

Alternatively, mount a 12V auto battery behind the wall to power it. Perhaps, they will get the joke of how ridiculous the spec is.

Not a bad idea at all. Have two identical NTCs, one on each side of the PCB (for isolation). Say top = primary side, bottom = secondary side.

Have a heating transistor on the top/primary side along with the sense NTC, while your "simulated" resistor lies on the back side.

Have a small U or Omega shaped cut into the PCB so as to limit heat conduction to other regions of the PCB, and get as identical as possible temperatures on both sides.

Except you'll have either to figure how to cool the PCB to 0°C or you'll need to use a higher resistance NTC so that at your max ambient, the NTC resistance is above your targeted 0°C resistance. Of course, if you need to reach the 100°C resistance at reasonable temperatures, you'll want a higher beta NTC.

How much do you know about what is connected to the resistor?

"Emulate". Not "Implement".

What RH sensor type are you required to use? ...Jim Thompson

I'm just a fan of dummy parts, and stuff like that.

microchip makes weirdo parts like the OP wants

That would be an easy way- the "wiper" resistance will limit the high RH end and the resolution will also be crude at the high end (but RH measurements are pretty cruddy usually anyway) Probably want 1024 steps .. eg. AD5161. The voltage of both of the rheostat ends has to lie within the power supply voltage limits of the digital pot. It would be easy enough to opto-isolate the SPI signals, but it might be easier to just use a tiny SMPS to generate a single isolated supply, since you'll need power on the "thermistor" side of the barrier anyway, and presumably the isolation from the mains will have to comply with safety standards.

RL

The only way I can see that happening in low part count is a photo resistor opto-isolator being PWM with a SIN to give you the log output you're looking for. THis would offer the isolated R you need.

As for the cap supply, there was a thread here not to long ago on that and I came up with a couple of ideas that seemed to me, to be a viable application. If you're interested I can post that spice on that one.

Jamie

The customer says it is a thermostat. I'm waiting for a sample.

I've not chosen the RH sensor yet. I already use a Sensirion SHT11 on another product. Easy to use and doesn't need calibrating.