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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?
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    \$\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
    Commented Jun 7, 2010 at 12:47

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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!

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  • \$\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\$
    – John Burton
    Commented Jun 7, 2010 at 8:43
  • \$\begingroup\$ I wasn't worried about you... just the component. Not to sound unconcerned! \$\endgroup\$
    – Scott Murphy
    Commented Jun 7, 2010 at 11:07

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