Latching "OFF" circuit, LiPo cutoff. - Page 1
Forum section: electronics / beginners

author: peter.mitchell
author_rank: Super Contributor
timestamp: October 15, 2012, 10:51:46 pm
content: I have been looking around the internet for some LiPo over-discharge protection boards, but most of them protect from massive overdischarge, EG, running the cell significantly below its empty voltage, as opposed to a safe cut off level between 0 and 5% charge, this is very bad for the cells as one would assume. Additionally, they generally don't handle high current.I decided i'd make a circuit to do what I want, as it should be pretty cheap and easy but I'm getting stuck on a part of it;You see, when you cut the load off of the cells, the voltage in the cells climbs back up, the cut off turns off, the load is re-applied, voltage drops, load gets cut off, ect and the circuit oscillates.I am trying to think of a way to get my circuit to latch OFF when it turns off, so when the voltage rises back up it stays off, I wouldn't mind having to press a push button to turn it back on.The circuit is very basic but does the aside from this small issue, switching the load off when the cell voltages reach 3.2v (12.8v on a 4S LiPo config)Some ideas would be greatly appreciated.

author: peter.mitchell
author_rank: Super Contributor
timestamp: October 17, 2012, 08:25:44 am
content: The other option is to add a hysteresis, BUT, I would prefer do it this way as the hysteresis level would have to be pretty big, making the accuracy suffer.The reason why I am making this is to be able to use lighter, less expensive, readily available LiPo batteries for projects instead of SLA batteries, as they have a number of advantages; including faster charge and discharge rate, lighter weight/smaller size.Code:The particular batteries I would be using are these; http://www.hobbyking.com/hobbyking/store/__15521__Turnigy_5000mAh_4S1P_14_8v_20C_hardcase_pack.htmlThey are incredible value for money; 4x 5000mAh (74Wh, compared to a much heavier and larger 12v 7Ah SLAs 84Wh) cells in series, with a main high current, series lead, as well as a balancing header, with a tap at every battery.I do also realize that I need appropriate charging circuitry and balancing circuitry, which is surprisingly easier to find off the shelf compared to this :S

author: PA0PBZ
author_rank: Super Contributor
timestamp: October 17, 2012, 11:45:09 am
content: Looking at your circuit, if you move the top of R2 to the PMOS output (load), and then bridge the PMOS with a 'start' switch, wouldn't that do what you want?

author: peter.mitchell
author_rank: Super Contributor
timestamp: October 17, 2012, 01:20:03 pm
content: Yes, that is excellent sometimes the solution is staring you right in the face, however, instead of bridging the mosfet, i'll just add a switch to the gate, so the switch doesn't have to carry a large current if a load is connected, unless this is exactly what you meant, then GENIUS!So, it should look something like this;Does this work? I try it in LT spice but the output seems to be on regardless of the battery voltage, i'm aiming for 12.8v switch off.http://www.fairchildsemi.com/ds/FQ/FQP47P06.pdfseems like a pretty good mosfet for my application, low Rds on, reasonably high current, high Vgs too

author: peter.mitchell
author_rank: Super Contributor
timestamp: October 18, 2012, 11:17:27 am
content: No matter what I do in LTspice I can't get LTSpice to work with the P-channel mosfet, It works fine with a PNP transistor though, im probably missing something blaringly obvious with this too...

author: AndyC_772
author_rank: Super Contributor
timestamp: October 18, 2012, 12:58:10 pm
content: It won't work if you simply leave it called "PMOS" - you have to choose a specific device, then it should be OK.Doesn't your solution involve quite a bit of wasted current, though? R3,R4,R5 form a path that'll draw over 1mA continuously, even when the circuit is supposed to be off.

author: peter.mitchell
author_rank: Super Contributor
timestamp: October 18, 2012, 02:16:57 pm
content: Heh, weird, without specifying any particular type of pnp device (just putting in the place holder "PNP") it works as expected, but yes, as you stated, you do have to specify the type of MOSFET.Power consumption isn't really an issue when it's turned off, I will be buying the batteries in internal resistance matched sets and running them in a 4S6P configuration to give about 30Ah at a nominal 14.8v, using this circuit to disconnect the load once the batteries reach about 10% capacity, so that is still a significant amount of power remaining. However, if I do use this with single packs it may be worth while reducing the "off" current...