# thermal cycler circuit interpretation

Discussion in 'The Projects Forum' started by justtrying, Sep 28, 2012.

1. ### justtrying Thread Starter Active Member

Mar 9, 2011
330
836
I am trying to brush up on my circuit reading skills. I am at a loss with four mosfets. I also have never seen "peltier", did some research on it, it is temperature controller which makes sense given the application.

I really can only go so far as if motor+ goes above 5V, U1 should go high and "close" 3904 (T1). If we assume that the other motor results in T2 being open, would current then flow through M1 because gate is now tied to ground via short? Then I think T4 and M4 would be active too and other transistors will be off. I really am quite lost...

I will try to go through this again but will appreciate any help.

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2. ### wayneh Expert

Sep 9, 2010
15,233
5,573
What are you trying to do?

That circuit appears to be an "H-bridge" motor controller (which allows for reversing the motor) adapted to a TEC (or peltier). It can send current thru the TEC in either direction, for heating or cooling, based on the control signal it receives.

3. ### justtrying Thread Starter Active Member

Mar 9, 2011
330
836
I am trying to understand figure out the current flow for different conditions. I've worked with H-bridge circuit once, but transistors are not my strong point. This circuit has p and n-channel mosfets and I want to be able to trace out current given different conditions (say as above) .

I am guessing that the current should flow, depending on conditions, through the peltier, meaning that pairs of mosfets that are on at the time time are M1/M4 and M2/M3? Were my assumptions above wrong?

This is to brush up on some weak points and to prepare for something that I have coming up...

4. ### wayneh Expert

Sep 9, 2010
15,233
5,573
T1 and T4 are on together (base voltage higher than emitter), as are T2 and T3 (on with base voltage below collector).

When T3 or T4 are ON, the gates of M3 and M4 go high and they conduct (in either direction).

T1 and T2 ON pulls the gates of M1 and M2 low, which causes them to conduct.

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