LiPo low voltage indicator

[j8m8l]

Hi Guys

I am planning on getting some LiPo's in the summerso i and am going need to get a Low Voltage indicator so i dont kill them :(.

I thought that i would try and design one (for fun).

Attached is what i have come up with, do you think that it will work? I want the LED to come on, if the voltage is below 6.5, so is the pnp the right one?

I guess that the transistor could also be replaced by a not gate too.

Thanks

Josh

[Robbie d]

the transistor isn't required. Using the reverse bias curve of the zener diode, it will stop conducting below it's cutoff voltage. I didn't draw this diagram but it illustrates it. All i did was solder the resistor to one leg of the led, the zener to the other, with cable to plug them in. neat and small. I used hookup wire so you can change the angle of the led to point forwards or whatever direction you want the light

[j8m8l]

Thanks

I wanted it to go on when the volagte when below 6v, and not vice versa.

I suppose i could use a NOT gate, but i have to get some transistors anyway.

Thanks

Josh

[Mr.RC-CAM]

J8m8l, the circuit will probably not work as shown. The PNP will not conduct if the emitter is on the negative rail, as shown. Also the base's bias is incomplete and LED turn on may prove unreliable.

FWIW, I suspect that {with a simple implementation like this} the LED will not "snap" on/off. It will probably grow from full off to full on as the battery discharge curve knee is passed. A simple OpAmp comparitor circuit would be easy to use and may be a more predictable method. An LM358 and some resistors are cheap. ;)

RC-CAM

[j8m8l]

Hi Mr-Cam

Thanks for the input, this is the first thing that i have really "designed".

Could you give me some detials of the OpAmp, i need to make some indicators soon (just about to get them LiPo's)

Unfortunatley i do not know much about electronics, i am only 17, but i plan on taking elec engineering at uni.

Thanks

Josh

[Mr.RC-CAM]

Since you are new to electronics, I think we should stick with your transistor solution. So, attached is a two transistor circuit that was just prototyped on the bench. I assumed that you wanted it to detect something between ~5.5 and ~7.5V, so that is how the components were chosen.

The circuit has sufficient gain to ensure bistable operation of the LED. It will snap on/off (or nearly so) as the low voltage threshold is crossed. The LED comes on when the voltage is low. Pot R2 sets the low voltage threshold.

The circuit is very sensitive to component choices. Do not sub D1, Q1, or Q2. Temperature will affect things too, but hopefully not enough to be a big concern. These sort of issues would not have been a concern with the OpAmp based solution.

You will want to have access to a well filtered variable bench supply to test/adj the circuit. It will be painful to have to use a discharging battery to do so.

Lastly, you must ensure that the battery is removed/disconnected when not in use. Otherwise, the continued current draw will ruin your LiPo pack if it is allowed to discharge too low.

Regards,
Mr. RC-CAM

[j8m8l]

Thanks

You da man, RC- Cam (hey that rhymes) LOL

I have a supply at school, so i will make some of these after the exams.

Cheers

Josh