low voltage cut-off?

Javalin

All,

Is there an easy way to make a low voltage cut-off?· I.e. to protect lipo's from over discharge (3v per cell).

I have low drop out reg's with ENable pins - I just cannot find an easy way to drive high or low when the voltage is low.

Thanks

James

Phil Pilgrim (PhiPi)

A three-pin voltage detector, such as one of the Microhip Technology TCP-series offerings may do the job for you. One problem you may encounter, though, is that when the detector turns off your regulator, the battery voltage may "snap back", since there's no longer a voltage drop through its internal resistance. To keep the protection circuit from continuously cycling, you may want to connect the voltage detector's output to a latch that holds the regulator off until manually reset.

-Phil

Javalin

Thanks (again) Phil!

Javalin

Phil,

Would a thyristor with the gate connected to the TC54 Voltage detector, the Anode to the EN on the reg and the cathode to ground do it?· What do I look for to spec the thyristor?

I.e - the attached diagram!

thanks,

James

Phil Pilgrim (PhiPi)

Your use of an SCR here is interesting! As it stands, I think it's going to latch during power up, so you need a way to prevent that. Voltage detectors come in both positive- and negative-acting forms; also with and without a turn-on delay. I'm thinking that maybe a second detector with a much lower voltage and a delay could be used on the now-grounded side of the SCR to hold if off until the main voltage detector asserts itself. Then when the voltage drops between the two thresholds, the SCR will latch, turning off the regulator. You will have to recycle the power to get it to come back on. This will take some breadboard work to get it right, I'm sure.

BTW, you will need a pullup on the EN pin.

-Phil

Phil Pilgrim (PhiPi)

Here's another thought: a 555 includes two comparators and a latch. There might be a way to combine a CMOS 555 with a reference diode and some delay circuitry to do what you want.

-Phil

Javalin

Hey Phil,

Thanks for the info again - i'll get some bits and let you know.

J

Tracy Allen

Most of the reset circuits do have some hysteresis, and it is possible to add more by placement of a couple of resistors, so that the battery voltage has to rise substantially before the circuit switches back on. Here is a battery cutoff switch based on a reset IC + mosfet (scroll down).

One thing about SCRs is that they need a quite a bit of anode current to "hold" and the SCR in your circuit would need a low value pullup resistor in the anode circuit. But current is what you are trying to avoid. I don't think that circuit would work as intended.

If there is a micro like the Stamp in the circuit, maybe it can monitor the battery and turn itself off. When the "Start" button is pressed to apply power, the Stamp immediately makes a certain pin a high output which connects to the regulator enable pin to hold the power on. The Stamp reverts that pin low or input when it wants to turn itself off. There may have to be a pulldown R on the regulator enable.

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Tracy Allen
www.emesystems.com

ChrisP

If your using more than one lipo giving you a min voltage of 6V total, then why not just voltage divide and read with the micro, or use your SCR and a regular comparator with a zener refference?

Javalin

Hi Tracy,

Thats a good idea! I like! There will be a propeller in-circuit so I can put an ADC in and then I get the option to do a "safe" shutdown before switching off.

cheers,

James