Need a simple small low voltage cutoff circuit

[flinx]

I need a low voltage circuit that will take the input from a 3 cell LIPO battery and when the voltage gets down to 9 volts it will cutoff. The circuit needs to handle about 2 amps.

I have googled and searched and found a few commercial products that I can buy, BUT the amp ratings are way too low.

I have read that this can be accomplished with just a single zener diode, but have not found anything that says exactly how to wire it. Also where would one get a Zener diode that can handle 2 amps?

Everything including the LIPO battery needs to fit in a space the size of this:
http://outdoorpowertoolsaccessories-...9000-9-6v.html

I would like to convert my old Makita to run on a LIPO. I have seen other people do this, but they all just connect the battery directly to the Makita. this means that I have to guess when the battery is low and stop using it. Since LIPO's do not like to be discharged below 2.7 volts per cell, I need a cut off that is 9 volts or 3 volts per cell.

No I will not be using the old battery cartridge to hold the LIPO.
No I will not be using the NICAD charger to charge the battery.

All the circuit needs to do is cause the drill to drastically drop power so I know when to trade out the battery.

[Th3_uN1Qu3]

A voltage comparator, a TL431 reference, a few resistors and a high current mosfet, 40A or so for minimal loss. You can pull one from a motherboard. I can draft you a quick circuit tomorrow.

[flinx]

After more research a low voltage alarm would work good too.

[Th3_uN1Qu3]

Originally Posted by flinx

After more research a low voltage alarm would work good too.

As in, simply light up a LED or something? Then you can get rid of the MOSFET as well. See attachment. As soon as the battery (represented here by V1) drops to 9v, the LED lights up.

R2 and R3 must be 1% tolerance. Other than that, no restriction on the parts used, you can use any comparator.

[flinx]

Originally Posted by Th3_uN1Qu3

As in, simply light up a LED or something? Then you can get rid of the MOSFET as well. See attachment. As soon as the battery (represented here by V1) drops to 9v, the LED lights up.

R2 and R3 must be 1% tolerance. Other than that, no restriction on the parts used, you can use any comparator.

what are the pin numbers for the lm393?

[tom66]

http://www.google.co.uk/url?sa=t&rct...b0M270drQsQeYw

Page 3.

[flinx]

Originally Posted by tom66

http://www.google.co.uk/url?sa=t&rct...b0M270drQsQeYw

Page 3.

I mean what pin numbers relate to what plus and minus on the diagram for lm393? I have the diagram from that document already.

[tom66]

Originally Posted by flinx

I mean what pin numbers relate to what plus and minus on the diagram for lm393? I have the diagram from that document already.

The plus and minus on the top of the triangle refer to Vcc(V+) and GND.

The plus and minus on the left refer to the non-inverting and inverting inputs respectively.

Output is on the right.

The chip has two comparators, A or B. You can use either. If you don't use a comparator, tie its inverting and non-inverting inputs both to ground to prevent possible noise problems.

[flinx]

Like this?

[flinx]

Originally Posted by tom66

The plus and minus on the top of the triangle refer to Vcc(V+) and GND.

The plus and minus on the left refer to the non-inverting and inverting inputs respectively.

Output is on the right.

The chip has two comparators, A or B. You can use either. If you don't use a comparator, tie its inverting and non-inverting inputs both to ground to prevent possible noise problems.

or is this correct?

[flinx]

I built it and it worked first try.... Sort of. The LED comes on at about 6 volts.

[Th3_uN1Qu3]

That shouldn't happen. What voltage do you get at pins 1 and 3 of the 431?

[flinx]

Originally Posted by Th3_uN1Qu3

That shouldn't happen. What voltage do you get at pins 1 and 3 of the 431?

with my variable power supply set to 9 volts I get about 5.3 on pins 1 and 3. I am going out to dinner with my wife now and cannot work on this for another 1 to 2 hours.

Is there a typo in the original picture? R1 says 4k7 and R2 says 47k.

[flinx]

Originally Posted by Th3_uN1Qu3

That shouldn't happen. What voltage do you get at pins 1 and 3 of the 431?

Misread that. I will check 431 when I get home

[Th3_uN1Qu3]

No, there is no typo, the resistor values are right. On the other hand, the current is quite low and could be prone to interference. You can try scaling down the resistors in the voltage divider 10x, making R2 4k7 and R3 1k8, see if it works better.

But at the currents required to toggle the inputs of a comparator, the shown values should be more than enough, i'm betting more on a bad 431.

[flinx]

Originally Posted by Th3_uN1Qu3

No, there is no typo, the resistor values are right. On the other hand, the current is quite low and could be prone to interference. You can try scaling down the resistors in the voltage divider 10x, making R2 4k7 and R3 1k8, see if it works better.

But at the currents required to toggle the inputs of a comparator, the shown values should be more than enough, i'm betting more on a bad 431.

now I am completely confused what is 4k7?
the drawing shows 18K, not 1k8.

I could not find a tl431 but did find a ka431 and the voltage across 1 and 3 of that is zero. I am not sure I have it wired right. If I am reading the drawing right it shows pin 1 and 3 connected together. It would help if the drawing said what pin number of the tl431 goes to what pin number of the lm393. I looked up the specs on the tl431 and ka431 and based on the drawing and the specs that is how I wired it.

per the specs pin 1 is cathode, 2 is anode, and 3 is reference.

[flinx]

after googleing 4k7 I found that in places not where I am from people use that to mean 4.7k so I replaced r1 with a 4.7k resistor and now it works right. Sorry I have never seen that before.

[flinx]

Well....
Here it is! the worlds ugliest low Voltage alarm.
I will be encasing it in epoxy before wiring it in to my drill.
I also have a piezo buzzer to go along with the LED.

It works so....

[Th3_uN1Qu3]

Originally Posted by flinx

after googleing 4k7 I found that in places not where I am from people use that to mean 4.7k so I replaced r1 with a 4.7k resistor and now it works right. Sorry I have never seen that before.

Good job.

This type of notation was invented before the introduction of the color code - resistor makers figured out that the dot would easily get erased, so, to prevent people replacing with the wrong type of resistor, they decided to write 4.7k as 4k7, 0.22 ohms as 0R22, 1.5Meg as 1M5 and so on.

[flinx]

finished product.
http://youtu.be/Qy1e3GnIKBQ