Suggestions on how to limit current from LiPo battery?

I’m designing a board which a user may power from a LiPo battery. There are pins for input and pins for output, and the output pins have mosfets on them to allow for high current devices to be attached.

I’m concerned about the user destroying the board if they accidentally plug a switch into the output pins and turn it on. But my mosfets can handle 15A peak and 2.7A continuous and I’d prefer not to limit their output too much. I don’t anticipate needing more than 1A on each output, but I’d like to keep the option there.

I don’t want to resort to a constant current regulator. Too costly, too complex… And I want to make this board as small as possible. Also, if it’s gonna generate a lot of heat, then it’s probably not gonna do for this application.

I’ve been looking at polyfuses on Digikey, and I found these:

But while these can handle higher constant levels of current (1A-3A) they can take over a second to trip at current levels as high as 10A. That’s a long time to be putting 10A through a small switch and 28awg wire.

And again, I need to point out, these boards may be powered by LiPo batteries designed for RC cars which have no overcurrent protection circuity built in and which can definitely put out that much juice.

I’m not dead set against the idea of using some kind of current regulation circuit, but I have no experience with them, and like I said, the board is small. If the regulator put out a lot of heat it’s a no go, so I suspect any simple linear one would be no good. And I know a 3A 5V switching regulator for an RC car is pretty large, and I suspect a constant current version of the same would be as well., so that may not be an option. And putting the regulator on a separate board isn’t an option either, I’ve been doing that with my previous board design and my users hate it. So this board needs to have all the regulation built into it.

So, any suggestions?

Fast-blow fuse - those are the ones with a little spring that pre-tensions the fuse
wire so it breaks quickly.

Just remember, sometimes the circuit protects the fuse. :wink:

You can buy "battery protection PCBs" (intended for incorporation directly into the cell or battery pack) with a very wide range of overcurrent and undervoltage cutouts quite cheaply from all-battery.com

http://www.all-battery.com/protectivepcbsforli-ionandli-polymerbatterypacks.aspx

Have you looked into using an N channel JFET as a current limiter? Very small solution: http://www.allaboutcircuits.com/vol_6/chpt_5/15.html

jremington:
You can buy "battery protection PCBs" (intended for incorporation directly into the cell or battery pack) with a very wide range of overcurrent and undervoltage cutouts quite cheaply from all-battery.com

Yeah, I've seen those, but two problems:

  1. They don't list how many they have in stock. I will need at least a couple hundred of these a year, and I have been waiting forever (literally, like over a year) for them to get more of their high capacity 11.1v LiPos in stock, so I have no faith in their ability to keep these boards in stock either. (Too bad for them; I had 200 customers last year I would have recommended buy their batteries because the crappy China made CCTV ones on ebay are often not of the advertised capacity and take forever to charge. And the one time they did have a battery in stock, they told the customer in New Zealand that they couldn't ship it to him because apparently the USPS won't allow it, and they couldn't be bothered to ship via FedEx.)

  2. They're $8-$10 a pop. I could recommend my customers buy them, but if I include them with my product I have to eat that cost because I can't raise the price of my product by $10 for a battery protection circuit. It's $195 already.

Actually, I think I’ll go with the polyfuses after all.

I got excited about specs and wasn’t thinking straight. The pin headers are only good to 2A or so, so I wouldn’t want to exceed that anyway, and my traces are gonna be relatively thin.

I found some polyfuses which take 0.5s to trip at around 2A, but at 10A will blow in 1/100th of a second. They can handle a constant current of 1A. That seems like a decent option. There’s also one which can handle 1.5A and trips in 0.8s with 3A. If I use that one I could put one fuse for every two outputs and still have an impressive 4.5A total constant current handling capability across my 6 outputs. Far more than I ever expect will be needed.