Third party circuit protection Standalone PCB? - Page 1
Forum section: electronics / beginners

author: ElectricPower
author_rank: Frequent Contributor
timestamp: October 22, 2021, 07:37:35 pm
content: I want to build an general random circuit protection PCB for circuits between 0-24V. I mean an own Standalone PCB that all power goes through. That way i can just add that board to any circuit for reverse voltage protection.Som ideas how i can do that in the best way?

author: ElectricPower
author_rank: Frequent Contributor
timestamp: October 22, 2021, 10:35:58 pm
content: Will this circuit work?IRF3708 is an N-Channel Logic Level Mosfet. Max voltage rating for Gate-Source is 8V, so i added an 6V Zener.

author: Manul
author_rank: Super Contributor
timestamp: October 23, 2021, 12:34:43 am
content: Your circuit would almost work. The main issue is that there is no current limiting resistor for the zenner. People typically use PMOS and switch the positive side, but negative can also be used.Down is your schematic with some improvements. First, there is R1 which is 10k and limits the current to the zenner (very important). Second, for additional improvement I added diode D2 which will clamp negative voltage in case something goes wrong or if there will be short glitches due to mosfet capacitance or an event when input voltage switches polarity very quickly. Small capacitor C1 is also for glitches / ringing. Last, I added bidirectional TVS diode on the input, to protect against voltage transients exceeding maximum mosfet voltage. If you want to use this on 24V, you should maybe look at slightly higher voltage mosfet then IRF3708 and choose TVS accordingly.

author: ElectricPower
author_rank: Frequent Contributor
timestamp: October 23, 2021, 05:46:42 am
content: Thanks for good explanation!But i'm just wonder...I want the third party protection board to work with voltages from 3.3 to around 24V and around 10A max current. Will your circuit do that? And what type of TVS diode do i choose?

author: ElectricPower
author_rank: Frequent Contributor
timestamp: October 23, 2021, 07:36:54 am
content: Here is my new chematic, without the TVS diode. I do not have this is my sortment.Is that better?This is an 4 layer board (yes, i know it's overkill, but it is convenient and does not cost mutch).I have made an own layer for GND, GND2 and VCC. Therfore i have placed vias. Also for the mosfet. My plan is to bend the mosfet against the board for making more heat transfer.

author: ElectricPower
author_rank: Frequent Contributor
timestamp: October 23, 2021, 09:21:12 am
content: I have done som tests now, and here is the exact circuit i tested with:When i place the probes between ground and Gate i can mesure an voltage spike to the gate. Almost 2V spike before zener do the job. Is there any solution to this?Under you can see my connections and a picture from oscilloscope spike.

author: Manul
author_rank: Super Contributor
timestamp: October 23, 2021, 11:39:20 am
content: Voltage spike can not be true, so I guess you measured wrong. Sorry, I can't see clearly what is going on on your breadboard. Gate voltage is measured between source and gate.

author: ElectricPower
author_rank: Frequent Contributor
timestamp: October 23, 2021, 11:45:51 am
content: Okay. Then i have mesured it wrong i think. I measured between input ground and gate, not source and gate. I will try to measure again an give feedback. Thanks

author: ElectricPower
author_rank: Frequent Contributor
timestamp: October 23, 2021, 02:19:26 pm
content: I tested it now an you are right. No voltage spikes to the gate pin. I think the circuit will work fine without the TVS diode. Thanks for all helpBought my self an cheap incodecent lamp for 5 dollar for random load testing and som fun too

author: Manul
author_rank: Super Contributor
timestamp: October 23, 2021, 02:46:56 pm
content: You can use it without TVS, it just depends how serious you want to be. Diode on the output is recommended though. And capacitor should be electrolytic, not ceramic. Or none at all.Minimum voltage for such circuit depends on mosfet gate threshold. But threshold is not enough, gate voltage needs to go a bit above that to fully turn on the mosfet. At 3.3V IRF3708 should be alright, but not lower.

author: ElectricPower
author_rank: Frequent Contributor
timestamp: October 23, 2021, 03:50:25 pm
content: Quote from: Manul on October 23, 2021, 02:46:56 pmThanks for good advices!Can you explain to me what good an Diode will do? I know what an diode does, but what is the point in this spesific circuit?

author: Manul
author_rank: Super Contributor
timestamp: October 23, 2021, 05:06:02 pm
content: Quote from: ElectricPower on October 23, 2021, 03:50:25 pmPlease read again my first post.

author: ElectricPower
author_rank: Frequent Contributor
timestamp: October 24, 2021, 08:52:40 am
content: Think i understand now, thanks.Here is my last circuit and PCB layout:So my last question is, just for clarifying:If i connect 12V to the input (with right polarity) and reverse polarity on the output (load) i am protected by that circuit? Also if i feed the board with 3.3V and connect an ESP8266 with wrong polarity on the output?

author: Manul
author_rank: Super Contributor
timestamp: October 24, 2021, 10:22:59 am
content: Quote from: ElectricPower on October 24, 2021, 08:52:40 amNo.Quote from: ElectricPower on October 24, 2021, 08:52:40 amNo. There is no protection against connecting wrong way to the output.

author: ElectricPower
author_rank: Frequent Contributor
timestamp: October 24, 2021, 11:05:33 am
content: Okay. Then i have missunderstood everything. The circuit will then not help me at all.Thanks for clarifying that.

author: mc172
author_rank: Frequent Contributor
timestamp: October 24, 2021, 12:20:26 pm
content: By "reverse polarity at the output" do you mean preventing someone from connecting the load the wrong way around?

author: ElectricPower
author_rank: Frequent Contributor
timestamp: October 24, 2021, 12:21:36 pm
content: Quote from: mc172 on October 24, 2021, 12:20:26 pmYes, thats correct. But i see that this maybe was an stupid thought...?