I’m designing a submersible heater that is controlled by an SSR and I need to add a mechanical safety relay. I would like this relay to latch off once triggered, so that it requires a power cycle to turn on again. I am using an op-amp comparator as the controller and simply have a diode feeding back into the non-inverting input to serve as a latch. My question is, will the circuit even turn on? Doesn’t the initial low state of the op-amp turn itself off immediately? This seems to work in my simulator.
Port 1 is state output to the microcontrolle.
Port 2 is a control line from microcontroller.
The op amp is an L272AM, which is good for 700ma.

Are there any glaring issues with the design?

 

The diode between the input and output of the op amp is the wrong way round. I think you need a capacitor in a suitable place to ensure the circuit powers up in the correct state. If I was doing this I would use a signal level SCR (Such as a 2N5060) to drive the relay or use a set of contacts on the relay to make it latch.

Les.

 

I am fairly certain that the diode is in the correct orientation. Remember that I am trying to latch it in the off position, once the temperature goes too high (NTC thermistor resistance drops too much).

Output goes low, which clamps the non-inverting input to ground, preventing the op-amp from turning on again. This does work in my simulator (iCircuit on iOS).

So would an RC circuit to the right of the diode do it?

 

My mistake. I thought you wanted to latch the relay in the on condition. To latch it on at power up I would add add a resistor in series with the non inverting input. Add a capacitor as follows. Positive of the capacitor to the positive rail. (Assuming an electrolytic.) Add a resistor between the capacitor negative to the negative rail. I would suggest the time constant of this resistor and capacitor to be about one second. Add a diode between the capacitor negative and the non inverting input with the diode anode to the capacitor. When the circuit is first switched on the capacitor will be discharged so it will pull the non inverting positive until the capacitor charges.

Les.

 

I’m designing a submersible heater that is controlled by an SSR and I need to add a mechanical safety relay. I would like this relay to latch off once triggered, so that it requires a power cycle to turn on again. I am using an op-amp comparator as the controller and simply have a diode feeding back into the non-inverting input to serve as a latch. My question is, will the circuit even turn on? Doesn’t the initial low state of the op-amp turn itself off immediately? This seems to work in my simulator.
Port 1 is state output to the microcontrolle.
Port 2 is a control line from microcontroller.
The op amp is an L272AM, which is good for 700ma.

Are there any glaring issues with the design?View attachment 140390

Yes it will work, the output of the op amp will be high as long as the temperature is low enough to make the voltage on the Inverting pin less than 6V, you can put an electrolytic capacitor across the preset resistor to ensure the op amp kicks up positive.