LM431 as over discharge protection

Thread Starter

gkeep

Joined Oct 21, 2017
48
Hi Guys,

I'm a bit stuck on building an over discharge protection circuit for on of my projects. Its basically running an RC lipo battery (2S). I want to protect it against over-discharge. Can someone tell me if this circuit is on the right track? Its my first attempt building a circuit with an LM431.

over discharge protection.jpg
 

DickCappels

Joined Aug 21, 2008
7,427
Unfortunately the anode will act like a voltage follower and very slowly thurn off the mosfet. I think you will be much better off keeping the anode grounded and taking the signal from the cathode to turn off the mosfet.

A PNP in the cathode curcuit with appropriate resistors can switch the mosfet on and off. (One possible approach).
 

Alec_t

Joined Sep 17, 2013
12,065
Ideally each cell of the battery should have its own over-discharge protection circuit, Otherwise, if the cells were unbalanced, one could discharge below a safe level before your 2S circuit kicked in.
 

Thread Starter

gkeep

Joined Oct 21, 2017
48
So, something more like this?

I might be way off. but I think this looks kind of right, I'm also not 100% sure if this is the right choice of pnp transistor.

Thanks for all your help!

What do you think?


over discharge protection.jpg
 

Thread Starter

gkeep

Joined Oct 21, 2017
48
Thanks Alec_t, I did think of that. The cells will be balanced every time the battery is charged so, with this voltage divider will trigger the zener at 6.5V (instead of 6) to be safe.
 

Thread Starter

gkeep

Joined Oct 21, 2017
48
Oh yeah, sorry that pnp should be flipped on its horizontal axis.

Its okay. I think its probably wrong anyway. I don't know how to get all that info on LT spice.
 

ericgibbs

Joined Jan 29, 2010
12,902
hi gk,
Still not working as you expect.
At what voltage do expect low level cut off.?
E

BTW:
Remember that when the load is removed from the battery by the cut off circuit, the battery voltage will most likely rise.
You need hysteresis in the switch

E
 

Attachments

Thread Starter

gkeep

Joined Oct 21, 2017
48
hi gk,
Still not working as you expect.
At what voltage do expect low level cut off.?
E

BTW:
Remember that when the load is removed from the battery by the cut off circuit, the battery voltage will most likely rise.
You need hysteresis in the switch

E
regarding hysteresis in the switch, Do you mean like a flyback diode? or do you mean because of the internal resistance of the battery will raise the voltage once the load is removed?


hi gk,
Unzip this zip file.
E
I found your thread on the TL431 and imported it into LT spice that doesnt seem to be the problem.
 

Thread Starter

gkeep

Joined Oct 21, 2017
48
I'm probably going to give up.

I'll just use a ZY12PDN and a PD power bank. Much more expensive but over-discharge protection is too hard for me.

Thanks for your help.
 
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