Reverse polarity protection dual line non isolated power supply - Page 1
Forum section: electronics / beginners

author: dylan0806
author_rank: Newbie
timestamp: April 12, 2017, 03:51:31 pm
content: I'm working on a school project and i thought it was smart to include a reverse polarity protection.I need +20 and -20 Volt and these are not isolated. Here is what i come up with.http://imgur.com/a/QtefgI use the following MOSFETS:http://www.infineon.com/dgdl/irfi4410zpbf.pdf?fileId=5546d462533600a4015356242aab1f86http://www.vishay.com/docs/71291/si4825dy.pdfCan anyone confirm if i choose the right MOSFETS?

author: Zero999
author_rank: Super Contributor
timestamp: April 12, 2017, 04:53:53 pm
content: That will work but I'd say it's overkill.Why not just use ordinary diodes?The MOSFET approach is useful in situations where very little voltage drop is desirable, but in this case you're using linear regulators which will just have to dissipate more power. The MOSFETs just add expense and complexity to the circuit.

author: dylan0806
author_rank: Newbie
timestamp: April 12, 2017, 05:03:10 pm
content: We also need to +20 and -20V so we want as little as no drop over the diode/mosfet that's why I want to go with mosfets.I also thought about going with a diode and fuse like they did herehttps://www.eevblog.com/forum/beginners/circuit-protection/?action=dlattach;attach=20420;image

author: TheUnnamedNewbie
author_rank: Super Contributor
timestamp: April 12, 2017, 05:19:21 pm
content: Using diodes you can just use them in series, no need to include the fuse.Depending on how much current you are drawing, you won't lose a lot of power doing this.

author: Zero999
author_rank: Super Contributor
timestamp: April 12, 2017, 05:29:02 pm
content: Quote from: dylan0806 on April 12, 2017, 05:03:10 pmBut cant you not see, that doesn't make any sense? The LM7815 and LM7915 drop the voltage from 20V to 15V, a total of 5V!Put the diodes in series, so the linear regulators don't have to drop as much voltage and will run slightly cooler.

author: dylan0806
author_rank: Newbie
timestamp: April 12, 2017, 05:37:00 pm
content: QuoteI've only drawn the required parts for the simulation, it is not near the complete circuit.We need +20 Volt if we have 19.5 Volt our circuit will have clipping, and there is no way to increase the input voltage.The current used is around 8 ampere total.

author: Zero999
author_rank: Super Contributor
timestamp: April 12, 2017, 06:22:00 pm
content: Quote from: dylan0806 on April 12, 2017, 05:37:00 pmNow that makes more sense.This is why it's important to give as much information as you possibly can when asking questions on the Internet.Yes those MOSFETs will do the job. Is there any reason why one is through hole and the other is surface mount?Another option is a Schottky diode. For example, the MBR2045 will drop less than 500mV@8A if both of the diodes in the package are connected in parallel. Normally paralleling diodes is not good, as current sharing will be poor but in this case, both diodes are on the same die and will current share quite well.

author: dylan0806
author_rank: Newbie
timestamp: April 12, 2017, 10:56:06 pm
content: Quote from: Hero999 on April 12, 2017, 06:22:00 pmI think I'll go for the mosfets because 0.5 volt is still a lot more then 0.093 volt.I choose the through hole one because it was the only one i could find with a sufficient Vgs.

author: Zero999
author_rank: Super Contributor
timestamp: April 12, 2017, 11:08:18 pm
content: Quote from: dylan0806 on April 12, 2017, 10:56:06 pmWhy not use a potential divider or zener to limit the VGSbelow the rate value?

author: Damianos
author_rank: Frequent Contributor
timestamp: April 13, 2017, 05:25:29 pm
content: If I understood correctly: there is a load that demands 8 Amps from a well voltage regulated power supply. But these are not permanently connected to each other!Are there any voltage sensing wires from the source to to the load? If not, then how the voltage remains the same using another cable? If yes, how is achieved the correctness of this connection?If always is used the same cable why not to use a proper power connector (with an appropriate cable)?An 8 Amp current is not very low to have cables connected and disconnected and assuming that the voltage remains stable.Perhaps there are other issues to be solved first and the last is to find a solution for the polarity, if necessary.