Is the R supposed to be in the 15 -- or 28 -- volt or is this the control voltage ?

Can you tell me more about your application ?

<nbucska@pcperipherals.com>

please tell me more about the appplication..

dydt

Hi, you can use a fet as a variable resistance but its resistance changes with the drain source voltage as well as the gate source voltage.You could try using a cds photo detector but you will not get down to 0 ohms.

http://freespace.virgin.net/ljmayes.mal/comp/vcr.htm

http://www.vishay.com/docs/70598/70598.pdf

http://www.semiconductors.philips.com/acrobat/various/SC07_98_IDEAS_1.pdf

These sites may be of assistance.

The aplication is to replace the thermostat.
Simulate the circuit to act as thermostat, and for the sensor to read resitance of circuit(in replace of thermostat). The resitance of thermostat goes higher as the temperature goes down. The sensor reads resistance of thermostat as it varies and gives out its temperature reading.

Control voltage is varied from 0 to 15vdc and in return should produce variable resistance.
I know it won't output exact 0 ohms. it should produce 3k ohms-1M ohms versus 15VDC-0VDC or 28vdc - 0vdc.

Have anyone used Phillips Semiconductors N-channel field-effect transistor(PMBF4416)?

Could you tell me what circuit the thermistor operates
in ? Do you have FAX # ? mine is (303) 439-8651 ATT: NICK

<nbucska@pcperipherals.com>

i think you should be able to use a wheatstone bridge circuit...you just have to work around the values of the voltages and resistance you need...

dydt

What about a digital pot? Never used them so not sure they have that wide of a span.