Hello,

I'm getting from a Hall Effect magnetic sensor a variable voltage between 0.8 and 3V. When the voltage is around 0.8-0.9V, I want to activate a solenoid valve. If the voltage goes more than 1V, the valve should be off. So I thought of an inverting comparator like shown below. For the voltage threshold, I will put a precision pot to carefully adjust it to let's say 1V. The valve is 24V, and less than 1A. I don't have the exact current rating now. My question is: Is this the circuit idea yo would choose for this application? I mean, is there a better way? I'm actually looking for a fast prototype. No worth spending much time on this. So this is the basic idea I came up.

 

You need to ensure your 24v supply is well regulated.
Max.

 

You need to ensure your 24v supply is well regulated.
Max.

I think it's better to step down the 24V with a 5 or 12V regulator for the comparator and the reference and let the 24V only for the valve.

 

When the voltage is around 0.8-0.9V, I want to activate a solenoid valve. If the voltage goes more than 1V, the valve should be off.

I would divide the voltage down accurately and think about a window comparator with hysteresis circuit. This allows you a window of operation and hysteresis so when right at a limit your solenoid is not banging away. Keep in mind comparators like the LM339 are open collector out. Anyway based on your description I would think about using two comparators in a window configuration. Your solenoid will only be on when your signal is in the window.

Ron

 

Decouple the MOSFET Gate C from comparator output with something like 50
ohms or so resistor.

https://www.edn.com/design/analog/4368612/Comparator-directly-controls-power-MOSFET-gate

Also use low esr bypass caps on power rails -

Regards, Dana.

 

I'm thinking of using a small signal BJT as my current less than 200mA. The 2N4401 is rated at 600 mA.

 

You will need a pullup resistor on the gate as the 393 is open collector as mentioned above. And maybe a zener or resistor to gnd from the gate as 24V is too high for the gate voltage. The gate voltage problem will go away when you run the circuit on 12V. And a 12V reg will give you a more stable voltage reference.
Adding hysteresis to the comparator will be a good idea too. Have a search for that.
And I think the 1n4148 diode is too small for the coil. Go for a larger current.

 

Hello,

Here's a window comparator. You can use npn drive transistor if you want but I recommend a properly sized mosfet.

See attached.

 

Hello,

Here's a window comparator. You can use npn drive transistor if you want but I recommend a properly sized mosfet.

See attached.

View attachment 161946

Many thanks. Very useful information. I think I could modify it to operate it at 12V. The BJT would also do the job.

 

This little project worked perfectly. I added a hysteresis with a resistor between non-inverting input pin and output pin of the comparator. Below I attach the schematic and the valve I wanted to control.