Q: Are Switching Controllers Tolerant of Reverse Voltage? - Page 1
Forum section: electronics / projects

author: TheRuler8510
author_rank: Regular Contributor
timestamp: May 02, 2014, 01:51:56 am
content: Are switching controllers (in general) tolerant of reverse voltage?I can't tell by looking at datasheets, in particular the LT1303, but others too.A typical battery application has to be tolerant of the user putting batteries in backwards.I'd hate to burn up a $4.77 chip trying it.Thanks,

author: georges80
author_rank: Frequent Contributor
timestamp: May 02, 2014, 02:30:42 am
content: Put external reverse polarity circuity in place if mechanical reverse battery prevention scheme is not possible.FETs work great and have low voltage drop versus a schottky diode.cheers,george.

author: Rerouter
author_rank: Super Contributor
timestamp: May 02, 2014, 02:31:59 am
content: in general no, you can use a mosfet as a polarity switch if the voltage is above 2.5V so that you get near no voltage drop

author: TheRuler8510
author_rank: Regular Contributor
timestamp: May 02, 2014, 02:37:02 am
content: Quote from: Rerouter on May 02, 2014, 02:31:59 amDo you mean as a manual or automatic mosfet switch? I'm not sure what you mean.It seems silly to burn 0.5V on series diode protection when two AAA batteries are supplying 2-3V total--what a waste...

author: georges80
author_rank: Frequent Contributor
timestamp: May 02, 2014, 02:50:39 am
content: Google fet reverse polarity. Obviously you need a FET that will fully turn on at whatever battery voltage you are supplying.cheers,george.

author: TheRuler8510
author_rank: Regular Contributor
timestamp: May 02, 2014, 03:03:28 am
content: Quote from: georges80 on May 02, 2014, 02:50:39 amAwesome!--I didn't know about that trick.Thanks,Tom

author: TheRuler8510
author_rank: Regular Contributor
timestamp: May 02, 2014, 03:48:00 am
content: Quote from: georges80 on May 02, 2014, 02:50:39 amGeorge,Oh...one more thing. What about ESD sensitivity of the mosfet in this circuit. Wouldn't an ESD discharge on the battery terminals zap the mosfet? Or is a 10K resistor in the gate enough to save it?Thanks,

author: georges80
author_rank: Frequent Contributor
timestamp: May 02, 2014, 04:05:22 am
content: I use a 100k resistor on the gate. Shipped 1000's of boards using the scheme and no calamities yetcheers,george.

author: Niklas
author_rank: Frequent Contributor
timestamp: May 02, 2014, 05:33:59 am
content: Reverse polarity protection?If it is not specifically mentioned in the datasheet, then don't assume it is able to withstand it. Either if it is datasheets, politicians, marketing people, always check what they are saying and what they leave out. Sometimes it is more revealing to see what is not mentioned, because that is usually the weak points.ESD?That depends on the application. Will the PCB be fitted inside some package, with only the battery terminals exposed, or will it be a "naked" board? The naked board option would be very difficult to protect, if even possible.

author: miceuz
author_rank: Frequent Contributor
timestamp: May 02, 2014, 10:38:00 am
content: You can always add a "Fools diode" - reverse biased diode which will short the battery when inserted wrongly. You have to size the diode appropriately, works me for 3V coin cell battery -https://github.com/Miceuz/PlantWateringAlarm/blob/master/schematics.png

author: Niklas
author_rank: Frequent Contributor
timestamp: May 02, 2014, 10:49:51 am
content: Quote from: miceuz on May 02, 2014, 10:38:00 amNot a very clever idea unless you have a fuse in series with the battery. Depending on the type of battery, this setup could cause a fire or leaking battery.At 10:13 in this teardown video by Dave, both the diode and the fuse are visible:http://www.eevblog.com/2012/08/08/eevblog-330-medela-swing-teardown-repair/