Pwm controlled by 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
    Ok. I know have a problem. I have a pwm that needs to control a fan motor in the roof of house. Temperature in the roof sits between 30 and 60 deg celcius during the day.

    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 pot 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 motor.

    Application: Want to replace pot with thermistor that will operate the motor 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.

    Voltage to duty cycle : 5V → 0%, 0V → 100%.
    Temperature range : 30℃ ~ 60℃ ?
    thermistor :
    25°c ≈ 114Ω
    30°c ≈ 74Ω
    40°c ≈ 52Ω
    50°c ≈ 20Ω
    60°c ≈ 14Ω

    Pwm voltage : 2.5V - 0V.

    On my previous, unrelated thread about using a pwm to control vehicle accessories, someone suggested using an Op Amp to get the results.
    Any help is appreciated.
  2. Dodgydave

    Distinguished Member

    Jun 22, 2012
    We need to see your circuit, then modify it possibly with an op amp.
  3. LDC3

    Active Member

    Apr 27, 2013
  4. mad_mat222

    Thread Starter New Member

    Aug 14, 2013
    How do I put up a picture? The circuit looks good. If I chopped everything including the capacitor and diode out, would I then use voltage at R4 as the controller voltage ie would it read between 2.5-0v.
  5. LDC3

    Active Member

    Apr 27, 2013
    With tweaking, yes.
    I need to go, so I'll get back to you later.
  6. bertus


    Apr 5, 2008

    You do not have to hide your automotive need.
    Pwm controlled b thermistor

    The last thread was closed because of it.

    If you try to post the same question again, a ban will be given.

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