voltage to resistance

Hi All,
Ive recently come into possession of some nice solid state relays. They're proportional control relays that will control a 240vac, 25A load; 0-500k ohms across the control terminals equates to 0-100% across the output terminals.

I'd like to control these relays with a 0-10v signal from a PLC analog card. Since my op-amp design skills are a little (a lot) rusty, I'd really appreciate it if I could get some pointers on how convert from voltage to resistance.

I'm happy building the circuits, as this is for a personal project; I'd rather not spend the money on commercial signal converters, if they even exist.

Thanks for any help or pointers.


Sincerely,

rpoet

You sure it's 500K? That would make it quite sensitive to noise. Without knowing the internal circuitry of the SSR, an Op-Amp circuit would be difficult.
A variation. Motorized pot. See model 931.
Another method is a Digital Pot from Maxim. None of them have an analog input. None of them have a 500K output. Search "digital volume control" for circuit ideas.

Sounds like it wants a pot to control it, with a maximum resistance of 500K. If you provide the mfg and part number we can give better advice.

Keith,
It says 500k ohm right on the relay. I'll post a link to a data sheet when I can find one.

rpoet

Are there 3 terminals for the pot?

If so chances are the pot sources a voltage at one of the terminals. Hook one up and see what it it. Then you can probably connect an analog signal with matching range to the other two terminals (wiper and com).

TConnolly said:

Are there 3 terminals for the pot?

If so chances are the pot sources a voltage at one of the terminals. Hook one up and see what it it. Then you can probably connect an analog signal with matching range to the other two terminals (wiper and com).

Click to expand...

There are actually only two control terminals. It's one of those hockey puck style solid state relays.

Here's the link to the data sheet.

http://us.100y.com.tw/pdf_file/KR20XXAX.pdf

I have the KR2025AX relays.



-rpoet

Look here: http://www.kyotto.com/eng_examples.htm
and here: http://www.kyotto.com/PDF/PDF/KR2010AX.pdf

The application examples are a good place to start with your circuit. Connecting a pot between the terminals changes the phase angle of where the UJT fires in the module, giving proportional control of the output. You could make some current measurements there & make your op-amp circuit drive a transistor in it's linear mode to mimic the pot.

Best is to use the resistor with a small motor. you are sure thius way.
however looking at the datasheet, i see 24 volts and 10 mA as input.
measure the inputs with a voltmeter and the pot in place.
If it is DC and not connected to the mains you can do it otherwise you will need converter.
Use a simple transistor and a C resistor of 24kiloohm
a base resistor of 10 kohm. quick and very dirty but it will work.