The TL431 is a programmable Zener Diode, perfect if you don't have the exact part handy. The resistors program the zener foldover voltage. You can look it up in the data sheet for details.

I now understand its operation from http://www.reuk.co.uk/TL431-Battery-Voltage-Monitor.htm

Thanks everyone

 

I now understand its operation from http://www.reuk.co.uk/TL431-Battery-Voltage-Monitor.htm

Thanks everyone

One of the prime requirements of battery operated equipment is to keep power usage down to a minimum. Though I like this circuit for its simplicity, it does not meet that criteria. This because the adjustable Zener (U1) is constantly shunting current that would (otherwise) be routed to the LED. U1 does this until low bat threshold is reached. This isn't a criticism of k7elp60's circuit, it's just the nature of that circuit.
http://forum.allaboutcircuits.com/attachment.php?attachmentid=26570&d=1295139211

With two logic level FETs you can build a slightly more complicated circuit that would draw < 50uA while the LED is off.

 

One of the prime requirements of battery operated equipment is to keep power usage down to a minimum. Though I like this circuit for its simplicity, it does not meet that criteria. This because the adjustable Zener (U1) is constantly shunting current that would (otherwise) be routed to the LED. U1 does this until low bat threshold is reached. This isn't a criticism of k7elp60's circuit, it's just the nature of that circuit.
http://forum.allaboutcircuits.com/attachment.php?attachmentid=26570&d=1295139211

With two logic level FETs you can build a slightly more complicated circuit that would draw < 50uA while the LED is off.

Why did you put @k7elp60's schematic again, or you meant to put your circuit instead?

Thanks for your advice.

 

Why did you put @k7elp60's schematic again, or you meant to put your circuit instead?

Thanks for your advice.

No, I wanted to show the circuit I was referencing. What's the AH rating of your bat? If it's beefy then my comments (regarding current draw) won't matter that much.

Do you want me to post a 2 FET alternative?

 

No, I wanted to show the circuit I was referencing. What's the AH rating of your bat? If it's beefy then my comments (regarding current draw) won't matter that much.

Do you want me to post a 2 FET alternative?

Sure I want
My rating is 1.2Ah. Thanks in advanced

 

One of the prime requirements of battery operated equipment is to keep power usage down to a minimum. Though I like this circuit for its simplicity, it does not meet that criteria. This because the adjustable Zener (U1) is constantly shunting current that would (otherwise) be routed to the LED. U1 does this until low bat threshold is reached. This isn't a criticism of k7elp60's circuit, it's just the nature of that circuit.
http://forum.allaboutcircuits.com/attachment.php?attachmentid=26570&d=1295139211

With two logic level FETs you can build a slightly more complicated circuit that would draw < 50uA while the LED is off.

I agree with you about current consumpton for battery systems. Off course the value of R3 in my circuit determines the major current consumption. I would really appreciate your circuit using two logic level FET's.
Up till now I have been using a more complicated circuit(voltage reference and a comparator) for voltage level of 12V SLA batteries so my latest circuit with the TL431 was a breakthrough.

 

I agree with you about current consumpton for battery systems. Off course the value of R3 in my circuit determines the major current consumption. I would really appreciate your circuit using two logic level FET's.
Up till now I have been using a more complicated circuit(voltage reference and a comparator) for voltage level of 12V SLA batteries so my latest circuit with the TL431 was a breakthrough.

Regarding the comparator circuit, I wish to ask that, is it we need 1 more voltage supply(Vcc) for the comparator?

 

Schematic attached.

VR1 can be changed to 1Meg, if you like, but it will make adjusting the threshold a bit trickier, while the 500K yields better vernier adjustment.

R3 can be reduced (practical values) down to 1K, but the lower R3 is, the higher the quiescent battery current will be. I wanted a 1Meg but it was pushing the limits of the FET's Vds value when in it's off state.

NOTE: Lowering R3 to 1K defeats the minimum current draw goals of this circuit. Also note that I didn't do any temperature analysis on this circuit.

 

Regarding the comparator circuit, I wish to ask that, is it we need 1 more voltage supply(Vcc) for the comparator?

No I generally use on source(Vcc and ground), a voltage reference and a single voltage comparator.

 

No I generally use on source(Vcc and ground), a voltage reference and a single voltage comparator.

What is actually the "voltage reference"? Is it a source? sorry for my stupid question.

 

A "voltage reference" is simply a fixed voltage that doesn't change (within tolerances, of course.)

A Zener diode with a current limiting resistor in series with a source of current is one such example. The voltage across the resistor might change due to changes in current, but the voltage across the Zener diode will be relatively stable.

 

Which is what the schematic I showed early in this thread has.