low voltage cut off module

[calin666]

Hello again. I have gotten some great help from this site and its memebers in the last few weeks, and I just wanted to say thank you.
I need one more thing for my project and was wondering if anybody could help.
I need a low voltage, hard cut off module, that cuts power at about 3.0 - 3.1 volts. When voltage drops that low, it simply shuts off, and stops the entire project from working.
Does anybody know where I can get something like that? It cant be very big, maybe the size of a US quarter, or smaller.
Thanks for any help.

 

[(*steve*)]

There are devices that do essentially this, although you may need to pair them with a power mosfet in order to switch a significant load.

Here is a datasheet for a representative sample of 1

This and a mosfet could possibly fit in the area of a dime (and be not much thicker).

 

[calin666]

Hmmm.... Cant seem to get that link to work.
I was also kind of hoping that somebody might know of a place I could buy something like the cut off, pre made, since I need to get them in bulk.
If not, then I guess I could look at making them, if it is not too difficult. I am still very much a beginner with making stuff.
Do you know maybe where I could get an exact diagram of how to build it, and the parts I need?
I am more than willing to give it a try.
Oh, and max input voltage will be no greater than 8.3 volts, if that helps.

 

[(*steve*)]

If you click on the link you'll probably find it downloads a pdf called bd48xxg-e.pdf

You may be able to right click and save as or open.

 

[calin666]

I finally got it.
If I am reading this right, it looks like a BD4831 is the one for my application (I know I said a 3.0v cutoff, but to be on the safe side, 3.1v might be better).
It looks like I can just wire it in with my circuit, and I will be fine.
Am I right in my thinking?

 

[(*steve*)]

Am I right in my thinking?

You may be.

The maximum supply voltage is higher than your expected max, so that's fine.

The only issue is that these devices have a limited output current capability (actually none for an open collector variant) so you may need to use the output to switch a mosfet if you are going to use this to apply/remove power from your circuit. If all you need is a logic level then you're fine.

 

[calin666]

I need to run the power through it, so that it monitors the voltage, and allows the power to flow through it, until the voltage level reaches 3.1v, at which point it shuts off power.
I am using a li-po battery, and the battery will be permanently damaged if the voltage goes bellow 3.0v, hence the cut at 3.1v for safety sake. These batteries can be expensive. Lol
Thanks for all your help Steve. I really appriciate the time.

 

[daGenie]

try using an op amp

 

[calin666]

I am a bit of a noob so, what do you mean by "op amp"?
I always want to learn more.

 

[(*steve*)]

OK, so you need the voltage monitor to be permanently connected to the battery and the output connected to a logic level mosfet which will, in turn, provide power to your circuit.

Another alternative is to purchase something which has been manufactured for this purpose.

Here and here are two examples and here is the chip they reportedly use (it will download, not open)

 

[(*steve*)]

try using an op amp

That suggestion is like someone asking "How can I get from London to Paris?" and you answering "with a tyre".

Sure a tyre may be used, but it needs to be coupled with a rim, and the rest of a vehicle.

I have been reading your posts daGenie. Please don't post unless you can actually help. Many of your posts are less than helpful. I note with some relief that your stuck "." key seems to have resolved itself.

 

[calin666]

Outstanding! This just shows how much of a noob I am. I totally forgot about looking at PCB's.
The two you linked look good, and the price is great. But, I can't seem to find at what voltage they cut at. Lipo batts are very touchy, and if the go below 3 volts, they are longer recognized by chargers as lipo packs, and can not be chargee any more (you can actually, but damage can and most likely will occur. Not recomended).

 

[(*steve*)]

Those protection circuits kick in at 2.5V and 4.35V respectively (check the datasheet I linked to).

They're really designed to prevent damage if things go pear shaped. I probably wouldn't recommend them for everyday low power shutdown.

They're essentially the same as what I suggest, except the chip does more than detect low voltage, it detects low and high voltage and high current. It's a shame the low voltage isn't variable

 

[calin666]

Cool. I would like thank you once again for your help. It has been very informative.
I think I found one that will work perfect, but before I order it, I would just like to get your opinion one last time.
I have an account, and order with these guys already (so it makes things a bit easier), and they have this: http://www.all-battery.com/protecti...ndfor37vli-polymerbattery35alimit-pcb1sr.aspx
It looks like it would be perfect.

 

[(*steve*)]

It seems identical to the DealExtreme one.

 

[calin666]

Steve, you da man.
I can't thank you enough for all your help. You have solved a serious problem for me.
THANK YOU.