# voltage cut off tl431

#### barg

Joined Dec 23, 2015
55
Hi,

i would like to build a battery voltage cut off when the battery reaches tsh of 11v by using tl431 like the one of MikeMl circuite as in the links below:

Appreciate if anyone can asist concerning a few questions for this scheme/simulation as the below link:

1. What is the meaning of U1 (Rtot=5k) in this circuite?

2. how it is possible to add a hysteresis to this circuite so it will not chatter?

3. how can i make this circuite independent to reset it self?

4. is it possible to add to this circuite an independent Over votage cuttof when it reaches 14.4v using another tl431?

Thanks,

#### Lestraveled

Joined May 19, 2014
1,946
@barg
Here is a circuit I built up about a year ago. I used it to discharge a battery down to a selected voltage and then cut off. The basic circuit worked well but the IR losses through the relay, socket, battery clips made it loose accuracy. I intended to make some changes but lost interest.
It has Zero chatter. You connect it to the battery and nothing happens until you press SW. This closes the relay and U1, Q1, and Q2 keep it closed until the voltage drops to a predetermined level. The load is connected between BP1 and BP2.

Also, I used an LM385-adj which is a lot like the TL431.

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#### ronv

Joined Nov 12, 2008
3,770
Hi,

i would like to build a battery voltage cut off when the battery reaches tsh of 11v by using tl431 like the one of MikeMl circuite as in the links below:

Appreciate if anyone can asist concerning a few questions for this scheme/simulation as the below link:

1. What is the meaning of U1 (Rtot=5k) in this circuite?

2. how it is possible to add a hysteresis to this circuite so it will not chatter?

3. how can i make this circuite independent to reset it self?

4. is it possible to add to this circuite an independent Over votage cuttof when it reaches 14.4v using another tl431?

Thanks,
And a different one:
I'm guessing it is a different circuit (charger) that you want to disconnect at the high voltage?

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#### Lestraveled

Joined May 19, 2014
1,946
Going back and looking at @MikeML design, I kind of like it.

Having built and tested the circuit in post #2, I learned that IR losses will screw up a good/great paper design. To make this a viable circuit to me, I would have to separate the high current path and the sense electronics. I see this being a problem with both MikeML and @ronv design.

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#### ronv

Joined Nov 12, 2008
3,770
Going back and looking at @MikeML design, I kind of like it.

Having built and tested the circuit in post #2, I learned that IR losses will screw a good/great paper design. To make this a viable circuit to me, I would have to separate the high current path and the sense electronics. I see this being a problem with both MikeML and @ronv design.
Les, can you elaborate? The curent drain is only a few micro amps in the cut off condition.

#### Lestraveled

Joined May 19, 2014
1,946
Les, can you elaborate? The current drain is only a few micro amps in the cut off condition.
It is not the voltage in the cut off condition, it is when the battery is under load. The cut off occurs too early because the IR losses make the TL431/LM385 see a reduced battery voltage. Also the IR loss changes per the load placed on the battery, so it can not be compensated for. I realized that I had to make a Kelvin connection to the battery to get rid of the effect of the IR loss.

#### barg

Joined Dec 23, 2015
55
Hi Les,

i wonder if you change the relay to mosfet, will the IR loss for this circuite will be minimal even uA when its in load mode (disgarding from the outside connection cables)?

in addition, i didnt understand why you need a manual switch in your circuit? the circuite i am looking needs to alow the battery to charge after it reached its overload v cuttoff automatic without intervention, so i dont understand purpose of adding a manual switch?

thanks,

#### barg

Joined Dec 23, 2015
55
Hi Ron,

i think this is what i am looking like the one you noted, but i need to mention that this is a secondary ptc, so its connected only to the battery in the same two wires, i will upload a circuit to ilustaret it.

regards,

#### dannyf

Joined Sep 13, 2015
2,197
would like to build a battery voltage cut off
I think you will find it much easier to implement those functions through a mcu. A 8pdip pic or avr will do it with lots to spare.

edit: just as a demo -> here is a 12f675 acting as an on/off controller -> it has a complimentary output pair, GP1/GP2, to control relays or switches.

The kick-in is set at 2700mv (=VBAT_HIGH), and kick-out at 2300mv (=VBAT_LOW) -> user adjustable.

All for maybe 5 min of work.

the logic is as follows:
Code:
        tmp = adc_read(VBAT_ANC);        //read the output

if ((_output==0) && (tmp > mv2ADC(VBAT_HIGH))) _output = 1;    //low to high transistion
else     if ((_output==1) && (tmp < mv2ADC(VBAT_LOW))) _output = 0;    //high to low transistion

VBAT_OUT(_output);                //output the status
Implementation on other mcu would be almost identical. 20% of flash used (unoptimized), and three pins unused -> you can add tons of functions, like user-calibrated VBAT_HIGH and VBAT_LOW.

With a divider + a couple of caps, you are in business.

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#### ronv

Joined Nov 12, 2008
3,770
I think you will find it much easier to implement those functions through a mcu. A 8pdip pic or avr will do it with lots to spare.
Hey Danny,
Could you draw something up?

#### Lestraveled

Joined May 19, 2014
1,946
I hope the block diagram below clarifies what I am talking about.
In the top example the IR losses infect the sense electronics. In the lower example the IR losses do not infect the sense electronics.

It won't matter if you use a relay, a mosfet, analog or a PIC chip, if you wire it as in the top example, it will not be accurate. Wire it as in the bottom example and you will be way ahead.

I hope this makes sense. (Pun intended)

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#### slackguy

Joined Feb 11, 2016
67
he asked what U1 means did you say?

on other schematics i see it means for ex "(parts list) U640: microcircuit, di: quad 2-input nand gate"

it looks like symbol for zener (though i'm likely wrong check that)

#### barg

Joined Dec 23, 2015
55
I think you will find it much easier to implement those functions through a mcu. A 8pdip pic or avr will do it with lots to spare.

All for maybe 5 min of work.

Hi Danny,

Exactly the point i am in, wether to do this with pic or analog ic's, my prefered is with the pic but i am not sure about the reliablity vs analog.

Thanks,