WARNING: 30V was a mistake -- better to keep voltage down 12V as a beginner! I tested it over a very short time. Got the things hot over a long time with much less voltage -- apparently not enough energy was dissipated to destroy the components.

Thermistors are a type of resistors, according to Wikipedia. Two thermistors black-white (18p written on it) and gold-orange-gold-yellow (no text). If the colour coding is the same to resistors and reading up-to-bottom, the former is 8 Ohms and the last one is 4k Ohms. I tested both with 30V: the former warm, the last not warm, the current 0,0A (not measurable with my equipment).

Sizes are 0.2cm times 0.3cm for both of the components.

Accepted Power Estimate by size(tip): 3.14*(0.025m)^2 = 0.00196... so P ~=10^-3

  • How to estimate current as it was below 10^-2?
  • How to read thermistors' colour coding and what does 18p mean, 18 pico Ohms or some change?
  • 1
    \$\begingroup\$ 30V is too much for a thermistor unless the thermistor resistance is very large > 10k Ohms. If the thermistor resistance is very large you will still experience a significant amount of self-heating with the thermistor. Instead of applying a fixed voltage and measuring current have you just tried measuring resistance directly at a known temperature? \$\endgroup\$
    – mjh2007
    Jun 7, 2010 at 12:47

1 Answer 1


Thermistors are only a type of resistor in a limited sense. Within a given temp. range they behave linearly in current. The packaging of Thermistors varies quite a lot, Glass beads to plastic packs. As far as I know there is no color coding that is common. I did a check on Omega and Honeywell's site. No mention of a color coding there either. I would think the 18p is a manufacturing code and not an electrical spec.

Also, testing with 30V!! How big are these things!

  • \$\begingroup\$ If they really were 4 ohms, that would give a current of 30/4 amps and therefore power of 225 watts which instantly destroy most components. \$\endgroup\$ Jun 7, 2010 at 8:43
  • \$\begingroup\$ I wasn't worried about you... just the component. Not to sound unconcerned! \$\endgroup\$ Jun 7, 2010 at 11:07

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge that you have read and understand our privacy policy and code of conduct.

Not the answer you're looking for? Browse other questions tagged or ask your own question.