Diode value for repair of reverse polarity protection? - Page 1
Forum section: electronics / beginners

author: Bug2k17
author_rank: Contributor
timestamp: July 27, 2017, 02:41:10 pm
content: I need to replace the reverse polarity protection diode after I stupidly didn't double check the battery was installed the correct way round!After a bright display the diode across the input blew. I have attached a picture of the circuit before this happened (the one with the blue ring). As I am a beginner with regards to electronic design, could any of you guys point me in the right direction for a replacement diode type/value?The device is an old SRM cycle computer which is powered by a 3.7v LiOn battery. I'm not sure on it's current draw, but the original 110mAh battery was rated for at least 30hours of use (discounting standby). I've since 3d printed a new case to house a 2500mAh battery so hopefully a little more now.The circuit still works fine with the battery connected correctly, but it's probably best I replace the diode just in case I do the same again.Thanks.

author: helius
author_rank: Super Contributor
timestamp: July 27, 2017, 03:03:48 pm
content: Do you find it ironic that reversing polarity destroyed a reverse-polarity-protection diode?Presumably it really only protects against theinputvoltage being reversed. That diode is a DO-213 / MELF package, which comes in a few different sizes. The thing to do is measure the diameter and length, to match against the datasheet of a replacement candidate.On second thought, the picture shows a diode directly across the battery leads: this doesn't protect against reverse polarity at all, but can be used to limit charge voltage if it is a Zener diode. It would also explain why it blew, as the polarity is actually never intended to be reversed. If it is, the Zener will conduct forward all the current the battery can supply until it overheats. So now you have a different problem: find the voltage at which the battery pack is fully charged, and select a Zener with that same voltage as its reverse breakdown value (Vbr).

author: schmitt trigger
author_rank: Super Contributor
timestamp: July 27, 2017, 03:15:34 pm
content: I'm also with Heliu; thisis nota reverse-polarity protection diode.Most likely it is a clamping device. Probably something like a 4v7 zener if the battery is a 3v7 LiPo.Having said this, its energy clamping capacity will be low and thus the usefulness will be very limited. Good clamping diodes like the P6KE series are hefty devices.

author: Bug2k17
author_rank: Contributor
timestamp: July 27, 2017, 03:33:08 pm
content: Thanks for the replies.I'm not sure if this would have anything to do with the charging circuit. The original layout has a diode in line with the wire from the +ve terminal on the input connector. This then attaches to the charging circuit which was glued to the battery. As this was continually running flat, even with a new battery, I just replaced this charging circuit and battery with a replaceable 18650 and disconnected the +ve from the input connector as I don't need to charge these in the case. I just replace the battery and charge them in my bench charger.This is the original configuration:(edit: wrong url!)Thanks.

author: Bug2k17
author_rank: Contributor
timestamp: July 28, 2017, 01:06:03 pm
content: Apologies in advance for the stupid questions, but is this just a matter of finding a diode with a similar package to fit on the board with a voltage rating above the battery voltage? What would be the disadvantage if I picked one with a voltage rating far higher than needed, I presume there is going to be some leakage across this, is this likely to have a significant effect on battery life?Thanks.