Need circuit for to detect if 24v goes below certain level - Electronic Circuits Projects Diagrams Free

eld101 · 28th April 2008, 03:51 PM

Hi all,

I am looking to see if anyone has a circuit to detect if a 24vdc power wire falls below lets say, 22 volts. The circuit must run on the same 24 volts, but a regulator can be used to bring the voltage down if needed. It also needs to be pretty effecient since it will be running off of a super capacitor. The output of this circuit should be sinking.

Any help is apreciated.

Thanks,
Eric

crutschow · 28th April 2008, 04:17 PM

Quote:

Originally Posted by eld101

The output of this circuit should be sinking.

Does that mean it needs to sink a load when below 22V? What is the load it has to sink?

on1aag · 28th April 2008, 04:34 PM

Hi Eld101,

A simple circuit with a TL431 might do the trick.

on1aag.

eld101 · 28th April 2008, 04:35 PM

Quote:

Originally Posted by crutschow

Does that mean it needs to sink a load when below 22V? What is the load it has to sink?

It is connected to an input card going back to a plc. It just needs to sink enough current to turn on the input.

Thanks!

crutschow · 28th April 2008, 07:32 PM

A way to minimize current would be to use N-Chanel MOSFETS. They have a high input impedance so you can use resistors in the megohm range for bias.

For example, you could connect a 1 megohm pot (pot wiper connected to CCW end of pot) from the transistor gate to common (transistor source to common). Also connect a 1 megohm resistor from the 24V to the gate. Connect the transistor drain to the plc input.

Adjust the pot to give the desired 22V threshold.

The current draw from the 24V would be less than 20uA.

The output of this circuit is low (sinking) when the voltage is above 22V and high otherwise. Hopefully, you can adjust the signal requirements for the plc to accept this. If not, then you would need to add another transistor to provide the inversion. Just add a 1 megohm resistor from the drain of the first transistor to the 24V. Connect the drain to the gate of the second transistor.

The circuit does not have high stability with temperature but may be sufficient for your needs.

Ubergeek63 · 28th April 2008, 07:38 PM

why not just a 22V zener and a bipolar with an emitter resistor? not very accurate, but effective

crutschow · 28th April 2008, 09:24 PM

The design to use will depend, in large part, on the current draw for the cricuit you can tolerate from the cap.

Ubergeek63 · 29th April 2008, 12:06 AM

and a simple zener circuit would be among the lowest current draws, as well as the least expensive.

eld101 · 29th April 2008, 11:08 AM

Quote:

Originally Posted by Ubergeek63

and a simple zener circuit would be among the lowest current draws, as well as the least expensive.

Do you have a diagram of this circuit by any chance?

Thanks,
Eric

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28th April 2008, 03:51 PM	(permalink)
eld101	Need circuit for to detect if 24v goes below certain level Hi all, I am looking to see if anyone has a circuit to detect if a 24vdc power wire falls below lets say, 22 volts. The circuit must run on the same 24 volts, but a regulator can be used to bring the voltage down if needed. It also needs to be pretty effecient since it will be running off of a super capacitor. The output of this circuit should be sinking. Any help is apreciated. Thanks, Eric

28th April 2008, 07:32 PM	(permalink)
crutschow	A way to minimize current would be to use N-Chanel MOSFETS. They have a high input impedance so you can use resistors in the megohm range for bias. For example, you could connect a 1 megohm pot (pot wiper connected to CCW end of pot) from the transistor gate to common (transistor source to common). Also connect a 1 megohm resistor from the 24V to the gate. Connect the transistor drain to the plc input. Adjust the pot to give the desired 22V threshold. The current draw from the 24V would be less than 20uA. The output of this circuit is low (sinking) when the voltage is above 22V and high otherwise. Hopefully, you can adjust the signal requirements for the plc to accept this. If not, then you would need to add another transistor to provide the inversion. Just add a 1 megohm resistor from the drain of the first transistor to the 24V. Connect the drain to the gate of the second transistor. The circuit does not have high stability with temperature but may be sufficient for your needs.

28th April 2008, 07:38 PM	(permalink)
Ubergeek63	why not just a 22V zener and a bipolar with an emitter resistor? not very accurate, but effective

28th April 2008, 09:24 PM	(permalink)
crutschow	The design to use will depend, in large part, on the current draw for the cricuit you can tolerate from the cap.

29th April 2008, 12:06 AM	(permalink)
Ubergeek63	and a simple zener circuit would be among the lowest current draws, as well as the least expensive.