Easiest overvoltage protector.

[goodpickles]

I need a component/ set of components that totally cutoff the current if the voltage goes over 11.6 volts. Any ideas?

 

[ante]

Search for ”crowbar circuit” and you will find a simple circuit.

Ante :roll:

 

[samcheetah]

yes i agree with ante. a crowbar circuit is an easy to built overvoltage protector. there are a few variations of the crowbar. one in which a zener is used to keep a reference. one with an NPN transistor and one with an opamp. using a zener is the easiest way but it is also the slowest one while an opamp solution will be the fastest. well that depends on what your application is. do u need the speed or not? just surf around for crowbar circuits and see what goes right for u.

 

[goodpickles]

speed is no issue. will try to look around a little.

 

[Nigel Goodwin]

speed is no issue. will try to look around a little.

Try giving more details, there are various ways of doing this, but a lot depends WHY you want to do it.

 

[goodpickles]

Ok this is what I want to do with it. It is to protect a lead acid battery from being discharged below it's threshold point of 11.8 volts. Once the voltage drops below 11.8 it sends voltage to the rest of my circuit. Am am thinking of using 2 zener diodes connceted in series. (3.3, and 8.2 volts makes 11.5 volts which is close enough.

 

[Optikon]

Ok this is what I want to do with it. It is to protect a lead acid battery from being discharged below it's threshold point of 11.8 volts. Once the voltage drops below 11.8 it sends voltage to the rest of my circuit. Am am thinking of using 2 zener diodes connceted in series. (3.3, and 8.2 volts makes 11.5 volts which is close enough.

And how is this over voltage protection? Consider showing us a diagram of what you are thinking of doing.

 

[Styx]

dont you want an undervoltage detection circuit?

Use a comparator with a fixed reference

 

[Nigel Goodwin]

dont you want an undervoltage detection circuit?

Yes, rather different to what he asked!.

Use a comparator with a fixed reference

I agree, use a comparator with a fixed reference, preferably a precision one - something like a TL341, so it will be nice and stable. Use the output of the comparator to switch 'something' OFF when the voltage falls below the required threashold (which could be made adjustable with a preset).

It hard to be more specific about 'something', because there are no details again?. It's rather dependent on the load involved!.

 

[goodpickles]

Yes an undervoltage protector would be a much better name. that specific that I will be hooking up will be a 555 timer that makes a buzzer chirp every five minutes.

 

[Nigel Goodwin]

Yes an undervoltage protector would be a much better name. that specific that I will be hooking up will be a 555 timer that makes a buzzer chirp every five minutes.

Right, so you don't want to turn the load OFF, just give an audible warning?.

 

[goodpickles]

yes exactly

 

[Optikon]

yes exactly

"It is not enough to know what to say. One must know how to say it."

-Aristotle

And it can be a non trivial problem!

 

[goodpickles]

Would a zener diode work? The problem that i have with a comparator is that I am monitering the voltage source

 

[goodpickles]

Can you tell me why I get 2.143 volts...should I not get 11.8 volts.

 

[Styx]

You might want to concider doing it this way

Use a lower voltage reference ZENER (say 5.1V) going to one of hte pins of the comparator. On the other setup a voltage divider from the 13V battery. This way it will operate as the voltage drops.

You will need to find a comparator that can work from say 5V upto a voltage higher than your battery. That way you can ensure that the comparator IC will operate as teh supply drops.

Also for the 5.1V zener choose the the series resistor to be say 1k. This will ensure that with the supply voltage at 11.8V there is enough reverse current through the zener to ensure that the zener holds its voltage

At the positive comparator input use a voltage divider using 5k1 and 6k8. Dont forget a 100k positive feedback resistor. You might want to juggle the resistor values but that should give a trip point of abt 11.8 11.9V. This will cause the output of the coparator to go from HIGH to LOW

 

[goodpickles]

you mean like this

 

[Styx]

almost

1) turn the Zener the other way up. In its present arrangement then negative pin will have a reference of 0.6V and not the 5v6 that you want.

2) you might want to drop the 10k series burn resistor to unless the zener you have can hold a stable voltage at 640uA - most zeners like around 5mA so a 1k should do.

3) between pin 7 and pin 3 of the comparator put a 100kOhm resistor (to provide positive feedback). This will provide some hysteresis on tripping so you dont get lots of high freq triggers.

4) concider putting a 10nF capacitor across the zener to smooth out the voltage from teh zener and help keep is stable.

5) depending on how fast you want it to respond concider putting a capacitor between pin 3 of the IC and the negative rail. say round 10nF again.

6) the burn resistor to do with the LED seems a bit low. As it stands it will provide the LED with 30mA when the comparator's output is LOW. That sounds a bit high for a LED. IT will be bright but will burn out. Try doubling it ir even trippling it.

but yep that should do. The pot then sets the trip voltage