# 24Volt Low battery monitor using LM339 and output relay

#### imani9009

Joined Nov 8, 2013
56
HI,
I have made a circuit using LM339 comparator Ic. I am having an issue. When I supply comparable power at 6 7 terminals with 24 volt Power supply my circuit does not work. But if I apply power using 9 volt battery it works Ok.
It also works if I use a 10k resistor in series with the positive terminal of 24v power supply system. Guide me how can i use 24 volt for this system. As i am going to activate a relay at 23.9 coming from a 24 volt battery system or less volt at one side and other at reference 24 volt by power supply.

#### ericgibbs

Joined Jan 29, 2010
18,845
Hi imani,
The LM339 is latching up, check the d/s for input voltage limits.
E

Power the LM339 from the 24V supply.

#### ericgibbs

Joined Jan 29, 2010
18,845
Hi imani,
Look at this edited version.
E

Last edited:

#### Ian0

Joined Aug 7, 2020
9,804
You need to compare a fraction of the battery voltage with a fixed reference (as in @ericgibbs 's circuit)
If you compare a fraction of the battery voltage with a different fraction of the battery voltage you will always get the same answer!

#### BobTPH

Joined Jun 5, 2013
8,945
Just to explain, eric’s circuit uses a 12V zener diode to produce a voltage of 12V for any battery voltage above 12V. You can then compare a fraction of your battery voltage to that fixed voltage.

#### crutschow

Joined Mar 14, 2008
34,412
Your posted circuit is a perfect example of those found on the web that obviously was never built or even simulated by the author, since it can't work as shown.
It needs a fixed reference voltage, as eric's circuit in post #3.
i am going to activate a relay at 23.9 coming from a 24 volt battery system or less volt at one side and other at reference 24 volt
But from the above, you apparently want the relay actuated by a voltage below 24V.
For that the two inputs to the LM339 need to be reversed.

#### eetech00

Joined Jun 8, 2013
3,942
That's alot of circuitry just to light an LED.
Are you going to operate something else with the relay besides the LED?

#### imani9009

Joined Nov 8, 2013
56
That's alot of circuitry just to light an LED.
Are you going to operate something else with the relay besides the LED?
LED IS JUST AN EXAMPLE, I want to add heavy and light load option in my solar inverter to break heavy load for example air conditioner when it goes on my battery when solar is not available.

#### ericgibbs

Joined Jan 29, 2010
18,845
hi imani,
Can you conform the conditions when the relay operates and releases. ?
E

#### imani9009

Joined Nov 8, 2013
56
hi imani,
Can you conform the conditions when the relay operates and releases. ?
E
Relay need to operate on voltage less than 24. Less than 24 means heavy load is being used on battery, which should be cut off.
When mains supply is back charging is started at boost voltages of 28V this will again trigger the relay to turn off and attach the heavy load to inverter.

#### ericgibbs

Joined Jan 29, 2010
18,845
i imani,
Just drawing up a circuit, you are asking for 4Volt Hysteresis level, between ON/OFF

E

#### ericgibbs

Joined Jan 29, 2010
18,845
hi imani,
This is one possible option.
E

#### Attachments

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#### imani9009

Joined Nov 8, 2013
56

#### ericgibbs

Joined Jan 29, 2010
18,845
So what is the usage of 33K resistor with diode and 2 different capacitor at v28 and 1 with vref
hi,
The caps on V28 are for the simulation only.
The one on the Vref , is to prevent electrical noise on the Vref from the V28 line.
Also note it is a 9V Zener diode.

E

#### crutschow

Joined Mar 14, 2008
34,412
So what is the usage of 33K resistor with diode
That provides a small amount of positive feedback (hysteresis) to provide the 4V difference between where the turn-on and turn-off voltages of the relay.

#### imani9009

Joined Nov 8, 2013
56
hi imani,
This is one possible option.
E
View attachment 275051
So what is the usage of 33K resistor with diode and 2 different capacitor at v28 and 1 with vref
hi,
The caps on V28 are for the simulation only.
The one on the Vref , is to prevent electrical noise on the Vref from the V28 line.
Also note it is a 9V Zener diode.

E
Can you tell me which program you have used to simulate
Also could I use 12 volt zener diode instead of 9 volt any small modifications

#### ericgibbs

Joined Jan 29, 2010
18,845
hi,
Modified circuit.
To compensate for component tolerances, you need the two Pots to adjust the switching points.
Note: they are interactive, I would use a bench power supply with a Vout of 22V through 30V for setting up.
E

So what is the usage of 33K resistor with diode and 2 different capacitor at v28 and 1 with vre
My colleague Carl in Post #15 has explained this part of the circuit.

That provides a small amount of positive feedback (hysteresis) to provide the 4V difference between where the turn-on and turn-off voltages of the relay.

#### Attachments

• 2.7 KB Views: 3

#### imani9009

Joined Nov 8, 2013
56
hi,
Modified circuit.
To compensate for component tolerances, you need the two Pots to adjust the switching points.
Note: they are interactive, I would use a bench power supply with a Vout of 22V through 30V for setting up.
E

My colleague Carl in Post #15 has explained this part of the circuit.

View attachment 275099
I want this relay to get on for less than 24 volt. Will it work

#### ericgibbs

Joined Jan 29, 2010
18,845
As I do not know what type of relay you have, how can, I possibly answer that question.?

And also, what is less than 24V ? How much less.?

#### crutschow

Joined Mar 14, 2008
34,412
Can you tell me which program you have used to simulate
Eric, myself, and several others here use the free LTspice from Analog Devices.
Also could I use 12 volt zener diode instead of 9 volt any small modifications
Yes.
That's what's shown in Eric's post #17 circuit.