Thermisistor!!!!!

bomber

Recently, I was mesing around with a computer power supply, and after tripping a circuit breaker by accidentally touching the chopper transistor's heatsink, I thought that I should just use that baby for parts. I started looking around and on the heatsink (that was NOT the chopper's heatsink) I found a part that looked like a small blue capacitor. It was a thermesistor!!!!!! I hooked up the circuit (the circuit uses a 555 running so that a resistance between pins 2 and 3 wil change the output frequency) , and used the code below to measure the outcoming pulses. After doing some math in the pogram, I was ready to go.
Code:
' {$STAMP BS2}
' {$PBASIC 2.5}
temp VAR Word ' variable to hold temperature.
DO
COUNT 0, 250 ,temp ' count the number of pulses in 1/4 of a second
temp = temp / 4 'calibration: divide by 4
temp = temp - 5 'calibration: subtract 5
DEBUG CLS,? temp ' display temp
LOOP

erco

Sweet when things go your way, huh? You could also measure the thermistor resistance directly (without a 555) using the RCTime command.

Unsoundcode

Hi, just a note for future reference. A thermistor develops a very small voltage across its terminals, small changes in voltage relate very accurately to temperature. So temperature is measured using a voltage measurement device when a thermistor is involved. On the other hand a RTD (resistance temperature detector) has a changing resistance that relates to a certain temperature depending on the value of resistance. If you have a device controlling the frequency of a 555 with temperature changes it is more than likely it is a RTD.

hope this helps
Jeff T.

bomber

these are both intresting suggestions. the reason that I didn't use the RCTIME command was because the 555 circuit was already on the breadboard from expirament six.

erco

@UnsoundC: You may be thinking about a thermocouple, which is a metal junction (iron/constantan, for example) that gives off a millivoltage proportional to the temperature. Thermistors are basically resistors that change with temperature, they come in positive and negative coefficient types.

Unsoundcode

erco, yes I was thanks for the correction. Similar devices in many ways, this should help me distinguish the difference in the future.

thanks
Jeff T.