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Which Switching Transistor?

Discussion in 'General Electronics Discussion' started by woody119, Nov 3, 2013.

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  1. woody119


    Nov 3, 2013
    I use to do a bit of electronics over 15 years ago so a bit rusty and need some help.

    I want to make a Chiller for my aquarium and the low trigger voltage and high current needed to power the cooler is causing issues.

    I have a LM35 temp sensor which will output 0.3V when I want the system to turn on.
    I have a Themo Peltier Cooler that I will use to chill the water. It has an input of 12V 8A.

    All I need is a switch so when the base voltage is above 0.3V the 12V 10A will turn on and power the cooler module.

    Thanks for any help
  2. Laplace


    Apr 4, 2010
    The LM35 is a Centigrade temperature sensor with a scale of 10 mV per degree. So an output of 300 mV equates to a temperature reading of 30 deg C, while 301 mV is 30.1 deg C. So if you want full-current turn-on within 0.1 deg C, then use a voltage comparator and a band-gap voltage reference to set the 300 mV comparison voltage, and add 1 mV of hysteresis to keep the comparator from oscillating. Use the output of the comparator to drive a high-current bipolar transistor or power MOSFET for switching current to the Peltier cooler.
  3. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

    Nov 28, 2011
    Where does the 12V for the Peltier cooler come from?

    If it's a switching power supply, it's probably simplest to shut the power supply down when the temperature is low, and enable it when the temperature is high.

    As Laplace says, you'll need a comparator with hysteresis. The hysteresis is needed to prevent oscillation and hunting, and to slow down the on/off transitions. I would suggest at least 1 degree Celsius of hysteresis. The physical relationship between the cooler and the temperature sensor is also significant.

    A typical comparator would be the LM311 (single) or LM393 (dual). You can generate a reference voltage using something like a TL431. You can also get comparators that have voltage references built-in.

    Edit: MAX921 looks like a very suitable device. It contains a comparator with adjustable hysteresis and a 1.182V reference with 1% tolerance. It's available in an 8-pin through-hole package in 1-up quantity from Digikey for around USD 5. Not cheap but a nice chip.

    If you're using an ATX power supply from a PC to power your Peltier cooler, things are even simpler - you have a permanently enabled 5V power rail, and a signal that your circuit can drive low to power up the other rails of the power supply.

    You may need to load the 5V and/or 3.3V rails to get proper regulation. Actually I would look into changing the feedback circuit in the power supply so that it regulates the 12V rail voltage, but that requires modifying the power supply.

    You may also need to adjust the power supply's output voltage to get the correct amount of current flow and therefore the proper cooling effect.
    Last edited: Nov 4, 2013
  4. woody119


    Nov 3, 2013
    Thanks, im using an old ATX PSU. I will look into those options and see what suits. I didn't think about the oscillating issue when the temperature is around the turn on. Manual may just be the best for now.
  5. Brianj_92505


    Nov 2, 2013
    ATX Power supplies are the Bomb. So powerful and yet so plentiful. I’ve got three of them in my garage waiting for some worthy project to power. I used to enjoy designing my own supplies with the transformers and 78xx chips but to get 20 amps required a transformer the size of a suitcase. The first time I saw what an ATX put out I about flipped.
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