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I have been trying to make a temperature controller based on a closed-loop control utilizing a Wheatstone bridge configuration and an op-amp. The temperature setpoint is adjusted by a trimpot. The controller seems to be working. When it is powered up, it draws about 40 mA of current.

However, sometimes it behaves strangely. Sometimes after connecting it to a DC power source, the circuit doesn't draw any current. If I disconnect the power lead and reconnect it, then it starts drawing current again! This is happening to me quite frequently. As a newbie I'm not sure what is going on. Why does unplugging/plugging back the power lead make the circuit work? What am I missing here?

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There are two points when the circuit is in equilibrium and that is a problem because one of those points is when the transistor supplies no current to the bridge and, when this happens, the inputs to the op-amp are perfectly equal resulting in zero output voltage from the op-amp. This is self-sustaining and, is a point of equilibrium.

You need to prevent this from happening.

Maybe by ensuring the base of the transistor is always above (say) 1 volt. Or maybe by placing a resistor in parallel with the collector and emitter of the BJT.

The other time when this happens is at your normal running point (also a point of equilibrium).

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First guess... Looks like you might need a pull up on the output of the LT1364 to bias the BC547 (or something to bias it). If everything is 0V at startup or if the two terminals ever reach 0V, then the BC547 will never turn on and the terminals of the opamp will also be 0V and the output will be 0V.

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  • \$\begingroup\$ Should I pull up the output of the op-amp to the power rail through a resistor? \$\endgroup\$
    – TAK
    Dec 14, 2022 at 18:22

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