Wiring and connector question - Page 1
Forum section: electronics / beginners

author: redgear
author_rank: Frequent Contributor
timestamp: August 30, 2020, 11:16:54 am
content: Hi everyone,I need to connect the mains supply to my PCB board. I read that it is recommended to connect a fuse asa the power enters the product.Thepower supplyon the PCB converts the power to a 5v for MCU and 7-seg displays. The powered is also wired through the relay on PCB to an external load.The external load draws a current of 2A and inrush current peaks to a value of 21A for 185 micro seconds. The power supply's inrush currents peak to 40A but I am not sure how long. I used online wire size calculators to calculate the size of the wire and the result was 8 AWG(i assumed the peak current of 40A. Is that correct? Should I use nominal current or inrush current ratings while selecting a wire?What's the best way to connect these wires to the PCB, I was thinking to connect them using spade connectors. Would soldering them directly on board be a safe and reliable option?Thanks

author: Vovk_Z
author_rank: Super Contributor
timestamp: August 30, 2020, 03:24:15 pm
content: We don't use inrush current to choose a wire crossection. It depend's on rated current mostly and possible short circuit current value, and mecanical reliability. And if we have chosen wires right they will withstand inrush currents too (they are short in time).Mechanical reliability tells us to have wires crossection that starts from 0.5 mm2 or 0.75 mm2 - that is 24-22-20 AWG.Then you can look for AWG table and see what current value 24-20 AWG can withstand. For example 24 AWG can withstand up to 3-4 A in chassic wiring. So it is ok for yours 2 A. I prefer to go a bit wider if there is no reasons to save money.

author: redgear
author_rank: Frequent Contributor
timestamp: August 30, 2020, 04:35:06 pm
content: Quote from: Vovk_Z on August 30, 2020, 03:24:15 pmThank You.Do you suggest soldering them directly to the board or using some connectors like JST, Molex or quickonnects, etc?

author: bob91343
author_rank: Super Contributor
timestamp: August 30, 2020, 09:33:39 pm
content: The inrush current will determine the wire size for long runs due to insufficient voltage at the load for small gauges. The only reasons to use heavier wire are mechanical strength and voltage drop. If inrush current lasts for too much time, wire heating may be a concern but how long is inrush?

author: Vovk_Z
author_rank: Super Contributor
timestamp: August 30, 2020, 10:02:40 pm
content: Quote from: redgear on August 30, 2020, 04:35:06 pmI prefer to use some suited connectors. Look what is used in professional devices. It is a large variaty. It can be any type whith enough clearance between a neutral and phase line wires.

author: redgear
author_rank: Frequent Contributor
timestamp: August 31, 2020, 04:21:58 am
content: Quote from: bob91343 on August 30, 2020, 09:33:39 pmThe inrush for the load is 21A for 185 microseconds. The powersupply draws a 40A inrush but I am not sure about how long it lasts.Quote from: Vovk_Z on August 30, 2020, 10:02:40 pmOk, I will search for them

author: Vovk_Z
author_rank: Super Contributor
timestamp: August 31, 2020, 09:13:32 am
content: I still don't get what is "Load" and what is "power supply" but anyway still inrush current doesn't matter.

author: Vovk_Z
author_rank: Super Contributor
timestamp: August 31, 2020, 10:02:47 am
content: I get to my work, and now I can list some connectors which are suited and are wide used (by me or somebodyelse):1. A3960H-03P (3 pin, 3,96 mm pitch).2. NX5080-03 (3 pin, 5,08 mm pitch).3. FS1536 (Faston 6.3mm x 0.8 mm) or LS1560 (Faston 4,8mm x 0.8 mm).

author: redgear
author_rank: Frequent Contributor
timestamp: August 31, 2020, 04:48:11 pm
content: Quote from: Vovk_Z on August 31, 2020, 09:13:32 amBoth are loadsThe power supply is on the pcb itself powering the micro and the relay. By load I meant the external load(Lamps) which are controlled by the relay.Quote from: Vovk_Z on August 31, 2020, 10:02:47 amThanks a lot for the recommendations, I will check them out.

author: Vovk_Z
author_rank: Super Contributor
timestamp: August 31, 2020, 06:40:26 pm
content: 1. First one is female without pins, but I think you can find male part - NS39-W3P. Contact part of it can be NS39-T.2. Male PCB part for second is NX5080-03SMS. Cantact part if I'm not wrong is MX-08-50-0105

author: redgear
author_rank: Frequent Contributor
timestamp: September 03, 2020, 06:27:14 am
content: Quote from: Vovk_Z on August 30, 2020, 10:02:40 pmWhat should be the minimum clearance between neutral and phase wires? Can I go with a 18 or 20 AWG wire for my purpose? Can the connectors be rated for the total nominal current ~3A?

author: Vovk_Z
author_rank: Super Contributor
timestamp: September 03, 2020, 09:27:00 am
content: 3 A rated connectors for 2 A continuos possibly can be too loose (too weak). I wouldn't use them.For example, good (trusty for 2 A) ones - A3960WV-03P - are of 7 A rated.But it depends on exact type. There are more expencive 3 A rated ones with low contact resistanse (for example Molex MX-52 series, MX-5264-03) - those one possibly can do if there is a need. They are up to 0.35 mm2 (up to AWG22).

author: redgear
author_rank: Frequent Contributor
timestamp: September 03, 2020, 09:29:45 am
content: Quote from: Vovk_Z on September 03, 2020, 09:27:00 amThanks. What about the wires, are 18 or 20 AWG's fine? What should be the minimum clearance between AC and N headers?

author: Vovk_Z
author_rank: Super Contributor
timestamp: September 03, 2020, 09:32:50 am
content: There was several topics somewhere here about creepage and distances. It could be better if you search. Sorry, I can't add something new.

author: redgear
author_rank: Frequent Contributor
timestamp: September 03, 2020, 09:52:40 am
content: Quote from: Vovk_Z on September 03, 2020, 09:32:50 amCool, I will search for them. No problem, thanks.