Reverse voltage protection both for main PCB as external devices - Page 1
Forum section: electronics / projects

author: Jeroenvdv
author_rank: Newbie
timestamp: May 06, 2025, 09:27:24 am
content: Hi,I was looking to improve the reverse voltage protection on my PCB. I currently use a zener in the 24V in on my main pcb.This works well if there is nothing connected to my main pcb, but the moment there is a sensor or a programmer connected the 24V placed on the GND pin this will fry either my programmer or the sensor (speaking from experience :-( ).I have looked at the different reverse protection circuits using an IC or a mosfet but I think these also do not protect the external circuits...What would be the best way to protect both the Main PCB as the external components?thx,Jeroen

author: madires
author_rank: Super Contributor
timestamp: May 06, 2025, 10:29:53 am
content: Please add the Zener diode (Zener voltage?) to the drawing to help us understanding your current setup. Are the 24V PSU, 5V PSU and laptop powered by mains?

author: Jeroenvdv
author_rank: Newbie
timestamp: May 06, 2025, 11:12:57 am
content: Hi Madires,Thx for the quick reply.I made a pseudo schematic as the complete design is much larger and I tried to simplify the question.I added some components of the main pcb including the positioning of the zener diode.Yes laptop, 24V and 5V are powered by mains.Hope this clarifies it enough. Let me know if this pseudo design does not work for you, then I could detail it out completely...

author: madires
author_rank: Super Contributor
timestamp: May 06, 2025, 12:36:33 pm
content: What's the reason for using a Zener diode instead of a standard diode? And what is the Zener's voltage?

author: Jeroenvdv
author_rank: Newbie
timestamp: May 06, 2025, 01:07:06 pm
content: apologies, it's a Schottky not a zener

author: Doctorandus_P
author_rank: Super Contributor
timestamp: May 06, 2025, 01:30:46 pm
content: Another way is to put a diode between the power supply and GND, but behind the fuse. In that case, when the power connection is reversed, the diode shorts out the power, so the PCB does not see a high voltage, and because the diode is behind the fuse, the fuse will also blow, and this protects both the diode, and the power supply.

author: 4lfre1re
author_rank: Newbie
timestamp: May 06, 2025, 01:31:13 pm
content: Hi... something like this, perhaps...?Regards... ;-)

author: madires
author_rank: Super Contributor
timestamp: May 06, 2025, 01:35:02 pm
content: My guess is that the leakage current by the EMI suppression caps in the wall-warts cause the issue you're seeing when reversing the polarity of the 24V PSU. And the separate 5V PSU for the sensor board could cause problems too (AC ground loop via mains).

author: ledtester
author_rank: Super Contributor
timestamp: May 06, 2025, 01:35:35 pm
content: Quote from: Jeroenvdv on May 06, 2025, 01:07:06 pmWhat is the part number of the Schottky?

author: Jeroenvdv
author_rank: Newbie
timestamp: May 06, 2025, 01:40:16 pm
content: @4lfre1reIf I reverse the 12v connection there will be 12V on the gnd, which (I expect) will flow to the programmer which cannot cope with 24V on the ground?

author: Jeroenvdv
author_rank: Newbie
timestamp: May 06, 2025, 01:44:14 pm
content: Schottky is a SSB44-E3/52T

author: Jeroenvdv
author_rank: Newbie
timestamp: May 06, 2025, 01:51:09 pm
content: Quote from: Doctorandus_P on May 06, 2025, 01:30:46 pmAfter the fuse blows there will still be 24V on the ground flowing to the programmer and the sensor (possibly via capacitor or pulldown)

author: 4lfre1re
author_rank: Newbie
timestamp: May 06, 2025, 02:19:13 pm
content: Quote from: Jeroenvdv on May 06, 2025, 01:40:16 pm

author: Jeroenvdv
author_rank: Newbie
timestamp: May 06, 2025, 03:02:11 pm
content: I understand the simulations, they will work for the main pcb.But the programmer has it's own ground and Vin, these are not drawn in your simulations.

author: 4lfre1re
author_rank: Newbie
timestamp: May 06, 2025, 03:15:32 pm
content: You can use one protection circuit in each power input, or evaluate other options...