I would like some opinions on these power distribution PCBs - Page 1
Forum section: electronics / projects

author: physx
author_rank: Newbie
timestamp: June 21, 2025, 04:31:06 pm
content: I'm building some power distibution boards that turn a relay and thus the power on or off on a 3.3v signal, coming from pin3 of CN3.The power inputs are two 8-pin EPS connectors and a JST-style connector, wherein these may also be used as an output to daisy-chain these boards.They also include headers for one or two NID100-5 DCDC converter and a 556 timer based fan controller.Could anyone just look at the schematics and PCBs and give me an opinion on whether there are some obvious issues with the layouts?Thank you!

author: PGPG
author_rank: Frequent Contributor
timestamp: June 21, 2025, 07:45:47 pm
content: What is the relay coil current that you drive Q1 base with 10mA and as D1 use 1A diode?

author: Benta
author_rank: Super Contributor
timestamp: June 21, 2025, 08:22:44 pm
content: Which schematics? I just see a number of parts on a sheet.

author: mariush
author_rank: Super Contributor
timestamp: June 21, 2025, 09:44:02 pm
content: my thoughts....I don't like the surface mount electrolytic/polymer capacitors... you have lots of through holes, so adding a few more holes for capacitors is not a big deal.Using a relay is kinda dumb... you don't use it for isolation so what's the point?Use some beefy p-channel mosfets as on/off switches ... a p-channel mosfet is on by default ... keep the gate pulled down to ground using a resistor and turn off the mosfet by putting 12v on the gate.An AONS21309C is good for 70A and it's under a dollar and has under 4 mOhm rds(on) resistance ....https://www.digikey.com/en/products/detail/alpha-omega-semiconductor-inc/AONS21309C/12166062Or something like AONR21307 or AON21321 for more reasonable currents and resistance and stocked in big amounts :https://www.digikey.com/en/products/detail/alpha-omega-semiconductor-inc/AONR21307/9664181orhttps://www.digikey.com/en/products/detail/alpha-omega-semiconductor-inc/AONR21321/9664182If both 12v switched outputs are powered by same relay" then you could just have a big fat copper fill on the pcb in the shape of an L or something like that instead of having two thick traces go to each connector. You don't seem to control the connectors independently.

author: physx
author_rank: Newbie
timestamp: June 22, 2025, 11:49:30 am
content: The relay is a 12V 0.2W part, so that would put it at 16.7mA.I do like the idea with the mosfets, but I'm not too familiar with them in general. I don't need high frequency switching, which is why I thought of using a relay, but you're right, I do not need the rails to be isolated, so a mosfet could work just as well. The only reason why I'm a bit afraid of using them is because I have no idea how much heat they would dissipate with 10-15A of D-S current. And as a side-effect, the lower profile would also be useful because these would go into a somewhat space-constrained enclosure.My only question regarding the mosfets would be how to control them using the 3.3V input? I would think that there are mosfets that can be triggered using a lower voltage, but if not, I guess I can keep the transistor I use for the relay and wire it up to the mosfet instead. Or just keep it anyway for isolation? I don't really know what the best practice is. The 3.3V input can supply 500mA, so current isn't really an issue.The SMD caps are there because it keeps the bottom of the board cleaner and I will order the boards with assembly anyway, so I wouldn't really benefit from using TH caps. Unless they are better in some other way I'm not aware of.And yes, all the above may sound a bit pedantic, but I am by no means an electrical engineer and this is just a project I'm working on in my free time.Of course, I can make the traces bigger too, which would be benefiical, although I don't really expect a single output connector to draw more than 5A on either the 5V or 12V rail. But I know that the bigger cross-section is better, if there is enough space for it on the PCB.

author: PGPG
author_rank: Frequent Contributor
timestamp: June 22, 2025, 01:40:26 pm
content: Quote from: physx on June 22, 2025, 11:49:30 amFor typical transistor you can assume beta being about 100.In such switching application you don't want to be 'at border', but you can safely put into base 1/20 of collector current so about 1mA is enough.In 10k you loose 0.7/10 = 0.07mA so through R2 you need 1.07mA.Now (3.3-0.7)/1.07 = 2.4k. Practically for R2 any resistor between 1k and 3k3 is OK.Quote from: physx on June 22, 2025, 11:49:30 amSelect PMOSFET based on its Rds on (and P=I²R) when gate-source driven by -12V.Quote from: physx on June 22, 2025, 11:49:30 amYou need P-MOS with its source connected to +12V. MOSFETs are driven by gate-source voltage. So you have to drive it with voltage referenced to +12V. If your controll circuit have VCC of 3V3 or 5V it doesn't matter. You have to transfer the signal to +12V reference and yes your Q1 (with resistor instead of relay coil) is doing just this (you get -12V at gate against source).I have never used MOSFETs with such high currents so I only know that the important can be how fast it is switched because during switching much, much higher power will be dissipated in it and the time of this pulse can be critical to MOSFET withstanding it.