120V PTC thermistor to raise temp from ~65F to ~82F

Thread Starter


Joined Nov 26, 2017
Hi all-

Novice here, trying to pursue an idea I've had burning for a couple of years now. Tried to sum it up in the title. I'm trying to come up with a cheap way to bring the temperature of a small (~5 cm diameter, 2 mm thick) heating plate from room temperature (roughly 62 - 72° F) up to about 82° F, +/- 5° F. It is important that the temperature not rise much about ~87° F. It would be strongly preferable for the device to run on 110 VAC.

I was thinking that a PTC thermistor would be the most cost effective way to do this. I've torn apart quite a few coffee-cup warmers, mini-crock-pots, etc., and I seem to see the vast majority of them use a length of resistance wire (~.8 - 1.1 kΩ). I've yet to get my hands on one that uses a PTC thermistor, but "bigclivedotcom" did disassemble one in this video. I like that it regulates the current based on the temperature, providing less heat as the temperature rises. I'd really rather not have this heating plate running at full tilt when the desired ΔT is only a few degrees, as it would potentially overheat the plate.

I estimate that about 3 - 10 watts will be the maximum power draw do achieve this ΔT. This is based on a rough approximation, where I used a fixed resistance 17 watt coffee-cup warmer to do a similar task. I had to add several layers of insulation in order to bring the temperature down from ~ 100° F to 80° F.

I'd really appreciate any direction from this community as to possible approaches to this task. I'm happy to do lots of research on my own, but I'm in that nasty situation where, "I don't know what I don't know", and I could use a bit of guidance. I might be barking entirely up the wrong tree.

Is a PTC thermistor the correct way to approach this?


Joined Jun 22, 2012
You're better using a comparator circuit for temperature regulation using either a Ptc or Ntc thermistor, if the element is mains powered, then using a relay or triac to pulse it.

There are dedicated chips by ON Semi that use mains power directly to control temperature for chicken incubators, to +/- 1C.. Here is one i made using the U217b chip with a Ntc thermistor for 20 to 40C.





Last edited: