Pwm controlled b thermistor

Discussion in 'General Electronics Chat' started by mad_mat222, Aug 14, 2013.

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  1. mad_mat222

    Thread Starter New Member

    Aug 14, 2013
    Hi all,
    Been bashing my head trying to figure out how to control a pwm using a thermistor. At present it is controlled by a pot. Some details:
    Pwm is controlled by pot. Voltage running through it is 5v. 5v results in 0% duty cycle and 0 volts 100% duty cycle or wot which then allows the full 12v to flow to the pump.

    Application: want to use the pwm to control 12v water pump in an intercooler system in my 4x4. Want to replace pot with thermistor that will operate the pump at 1/2 speed when @ 40°c (or perhaps 30, have to run further tests) and increase speed to wot @60°c. The actual temperature settings at this point in time are irrelevant as its just a matter of playing with resesitors.

    Problem: tried replacing in the thermistor. I could only get a ntc type which are non linear which is not ideal but will do. Made more difficult because resistance decreases with temperature.

    Initially created a dividing circuit resulting in about 5v going to the pwm which means its 0% duty cycle. To control the pwm the way I want I'm looking at an input voltage (to the pwm) of between 2.5 - 0 volts. I have a bread board which I have been playing with resistors etc but I am no good at the maths which would make it a lot easier than guessing and i have not come close at getting the outputs I need.

    I hope it's clear as mud. Any help appreciated. I would draw a picture i knew how to add it.
    Last edited: Aug 14, 2013
  2. Dodgydave

    Distinguished Member

    Jun 22, 2012
    What circuit are you using, if you can post it would be helpful, you could add an op amp for the temp setting.
  3. LDC3

    Active Member

    Apr 27, 2013
    All thermisters are NTC; that is their fame. Probably the best way to get your voltage levels is to have the thermister as one side of a bridge and feed the result into an op-amp that outputs your control.
  4. ScottWang


    Aug 23, 2012
    Voltage to duty cycle : 5V → 0%, 0V → 100%.
    Temperature range : 30℃ ~ 60℃ ?
    thermistor : what's the range of resistors?
    Pwm voltage : 2.5V - 0V.

    Are these correct?
    You have to figure out a values of range comparing to the others -- the proportion, and using Op Amp to get the results.
  5. mad_mat222

    Thread Starter New Member

    Aug 14, 2013
    Thanks ScottWang. That is a lot prettier than my waffle.

    The resistance of the thermistor at room temperature is as below and then rapidly ( non linearly) decreases in resistance as temperature increases

    25°c ≈ 114Ω
    30°c ≈ 74Ω
    40°c ≈ 52Ω
    50°c ≈ 20Ω
    60°c ≈ 14Ω

    Using a cheap multimeter and a glass of hot water so these value are ≈ at best but the values themselves are easily changed its getting the flaming circuit done that is the hard part.

    To previous comment. I initially was look at ptc thermistors as this seemed to be exactly what I was looking for but can't get them here. Anyway I'm sure you genius' will be able to guide
  6. nsaspook

    AAC Fanatic!

    Aug 27, 2009
  7. Wendy


    Mar 24, 2008
    The owners of All About Circuits has elected not to host discussions of automotive electrical system modifications/enhancements due to safety concerns, the potential of legal ramifications and the possible circumvention of vehicle regulations at the state and federal level.

    This thread is against the AAC forum rules, Chapter 6, as seen here:

    This can be found in our Terms of Service (ToS)

    Automotive modifications of any kind are strictly forbidden. Therefore, this thread will be closed.

    Please try to understand the reasons behind this action, and feel free to browse and use the forums.

    You might find answers to your questions in one of these forums:

    Another good forum that shares many of the same membership is . Good luck.
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