[SOLVED] What u think about this reverse polarity design? - Page 1
Forum section: electronics / projects

author: Chriss
author_rank: Frequent Contributor
timestamp: July 09, 2019, 08:32:28 pm
content: Hi!I'm repairing a tool which has this type of reverse polarity protection from the picture I draw.I'm curious what you think about the circuit?Especially what I'm interested is:This tool under repair has also an USB connector to connect to the PC and the tool can be powered from the USB too.But sometime it need to use the external 12VDC power source to be connected too.I'm a bit confused with the GND line of the external power source and the GND of the USB port on my pc when they are together connected.Actually, the two GND lines, 1. from the ext. pwr source and 2. from the PC USB port are somehow separated cos of thediodes on the ext. pwr source connector.The complete GND line is actually feed from the 5v regulator middle pin to the whole electronic when it is powered from the ext. pwr source.From my point of view, the two GND's (ext. pwr source & the PC USB ) should be connected directly, without any diodes in between.But in that case there would be no protection anymore if the ext. pwr. source is reversed, especially on the PC USB port.How bad or dangerous can this be actually?Should I pay attention to this type of protection circuit?Thank you.

author: TimNJ
author_rank: Super Contributor
timestamp: July 09, 2019, 09:38:34 pm
content: In general, I don't see anything "wrong" with this type of protection. It's just a bridge rectifier. In this case, it works to maintain one polarity at the input of the voltage regulator. The main issue I see with this kind of design is there are 3 diode drops. Better off using a low Rds-on MOSFET.That said, is your question more related to protectiveearthon the external supply vs. earth of the PC/USB port? Is your 12V supply earth referenced?

author: strawberry
author_rank: Super Contributor
timestamp: July 09, 2019, 09:47:31 pm
content: what D3 do?external SMPS will short circuit diode D4 or D5 . what will happen? (nothing)

author: Gyro
author_rank: Super Contributor
timestamp: July 09, 2019, 09:50:00 pm
content: It looks as if they're dumping some voltage in diode drops so that the 7805 runs cooler.

author: coromonadalix
author_rank: Super Contributor
timestamp: July 09, 2019, 10:58:57 pm
content: why d3 ?? anti reverse voltage going into the rectifier bridge ?voltage drop ?I suggest this pictureThe diode rectifier bridge will never isolate any supply, or protect your pc from any 12vdc problems, you have to use an dc - dc converterIe 12 vdc in / 5vdc out, there is some models with 5vdc in / 5vdc outPersonally i would not play with usb ports, sure some have protection, but old pcs can supply little current amounts, and newest can go up to 1 amp in some cases, maybe more.

author: Chriss
author_rank: Frequent Contributor
timestamp: July 09, 2019, 11:22:50 pm
content: What I also not understand is, when the ext.pwr.s. is connected, the voltage regulator goes hot. Not extremly hot but hot. I have also to say does theoriginal VR. on the pcb is the ST KF50 which is a 5V 400mA reg.When I connect my amp meter to the input of the VR I can measure a current of 60mA.For me, 60mA should not warm up a VR which max current delivery rate is 400mA.I interchanged the KF50 with a regular 1A 7805 VR and the result is the same.It goes also hot.The voltage is stable but I don't like the temp. The temp could be maybe around 60-70C.On the device is only one green LED on and the uC is running but.No other parts are goes warm on the pcb, except the VR.Could this be related to the revers voltage protection?Sent from my SM-J500F using Tapatalk

author: coromonadalix
author_rank: Super Contributor
timestamp: July 09, 2019, 11:41:24 pm
content: if you drop 12 vdc into 5vdc, thats 7 volts dropThat 7 volts drop is dissipated in heat, and if you add current consumption over this it will get hot rapidly7 volts x 0.4 amps = 2.8 watts, you should use an lm7805 1 amp regulator with an small / medium to-220 heatsinkand i you use 1 amp at the output, this equal at 7 watts of dissipation lossbut the first circuit and my (your) modified one doesn't have any current limitation.... you should add an polyfuse (resettable) or a standard fuse for added protection

author: Chriss
author_rank: Frequent Contributor
timestamp: July 10, 2019, 08:02:29 am
content: Thanks guys for your help.I got all the answers I needed and I got a clear picture about the heat and linear voltage regulators.I appreciate your help.