Power supply protection with a diode. Is this the easiest solution? - Page 1
Forum section: electronics / beginners

author: castingflame
author_rank: Regular Contributor
timestamp: November 26, 2020, 09:36:49 am
content: I have a device that can either be powered from one of 2 sources but I want to provide electrical protection against plugging in 2 sources. Additionally, one of the power inputs acts as a power output.RequirementsEliminate the potential that voltage from one supply does not back power the other.Close as possible to be near 9V @0-2.4Amp depending on load.This is the standard setup proposal.Scenario 1: Supply 1 is in use and outputting to the regulator and the output. The problem is that there is a 2 diode drop. What is the best I can hope for at a maximum of 2.5 amp using Schottky diodes?Scenario 2: Supply 2 is in use and outputting to regulator.What are the best diodes recommended for this type of application? Just a correctly rated Schottky diode? I need to be as close as possible to 9V.I have looked at other solutions too like the LM66100 but parts qty and cost need to be as close to zero as possible (don't ask - beyond my control) which is why diodes seem the most reasonable.Thanks for any help you can give.

author: Doctorandus_P
author_rank: Super Contributor
timestamp: November 26, 2020, 10:39:17 am
content: It would be simpler to just short the 2nd diode with the switch.

author: Gyro
author_rank: Super Contributor
timestamp: November 26, 2020, 12:45:11 pm
content: A double pole switch would save you a diode drop on output 2, and would make D2 and D3 redundant.If the second signal is both input and output then it's not possible to provide reverse polarity protection.

author: castingflame
author_rank: Regular Contributor
timestamp: November 26, 2020, 02:43:12 pm
content: Quote from: Gyro on November 26, 2020, 12:45:11 pmThat is not an option. It has to be as is.QuoteI am not trying to provide reverse polarity protection. I am trying to provide Power isolation so that the voltage from one supply does not back power the other. I will edit my original post to make this clearer.

author: Gyro
author_rank: Super Contributor
timestamp: November 26, 2020, 03:30:33 pm
content: Ah, ok. I was probably reading too much into the thread title and assuming reverse polarity protection.In that case, the logic of your schematic is probably as close as you can get. Schottkys are probably as close as you can reasonably get for voltage drop, unless you want to go to the added complexity of Mosfet 'Ideal diode circuits (it sounds not).Choose diodes that are generously rated for current to minimise voltage drop as much as possibly. Schottkys tend to have a higher reverse leakage current that standard ones, but from the currents that you're taking about, I doubt that's an issue.

author: castingflame
author_rank: Regular Contributor
timestamp: November 28, 2020, 09:29:35 am
content: Thanks Gyro for taking the time

author: tkamiya
author_rank: Super Contributor
timestamp: November 29, 2020, 05:22:59 am
content: Can you just put forward transistor for each supply and tie the other end? This will prevent current back-flowing. Which-ever the source that higher voltage wins.

author: gcewing
author_rank: Regular Contributor
timestamp: November 29, 2020, 05:50:47 am
content: If there are two supplies plugged in, and the voltage of the one at input 2 is more than 2 diode drops below input 1, and the switch is turned on, input 1 will power input 2. Is that acceptable?