Quick Sanity Check

[Wilksey]

Hi Guys,

Just a quick sanity check,

If I wanted to PCB Mount a relay to switch 230V AC @ 16A Max, I have calculated that I would need a 9mm trace, so I have provisioned 10mm, but the trace is huge and I was wondering if that little legged relay can switch such a high current (it would probably be more like 10A max for most AC appliances, 16A is the relays max rating), if I had a pad next to the relay leg and put a trace the size of the pad to link them could I use 16/02 wire to link the power supply to the relay poles?

Would this be a better option?

I've switched a 3.5A Laptop PSU (3.42A to be precise) using a small-ish trace and it hasn't overheated or been damaged, and the circuit is still working 2 years later with no probs, so I just wanted to get someone who has done this kind of thing before's opinion.

Also, what would be really handy if someone could knock up a quick PCB design in Eagle demonstrating track widths and pad sizes for a standard sized 12V switching relay.

As I am fairly new to Eagle it would be good to get a visual representation.

Many Thanks

Wilksey

 

[Ian Rogers]

You do realize .. 230 @16A = 3.6kW.... 10mm is about right... What is it switching?

 

[DerStrom8]

That's a lot of power you're switching. First things first, is your relay rated for that kind of current and voltage? Seems that the contacts would burn out pretty quickly, unless they're made specially for that kind of power. As Ian asked, what is it for? You might want to mount the relay separately, unless you plan to make a huge PCB to fit all those traces

 

[Wilksey]

Well,
It is a remote power switcher, for mains rated appliances, the relays I have say 230VAC/30VDC @ 16A, of course, most AC stuff unless it's amplifiers or meaty power stuff will never get that far, I would rate a max of 10A but in reality it would probably be a few amps on the AC side, just worried about inrush current, which I'll have to investigate, each relay will switch one load (if someone wishes to put a power distribution block on it that's up to them!).

So i'm not switching anything specific it is a design I have been asked to look at for a friend who wants to build relay boards that are controlled by Micros such as a PIC, so I was thinking that 10mm would sound about right but rather than laying thick traces and risk it being wrong I could use some wire to link the power.

The relays have only small legs and are thin, so I wonder how they manage to withstand (even a 15/16A rated should support at least 10A continuous wont it?) such high current.

Wilksey

 

[canadaelk]

I use the Aromat relay JR1aF (where do they come up with these part numbers?) in such an application (non-inductive with a snubber circuit across the contact)
The trace with depends on copper thickenes, plating thicknes and temperature (start and rise). I find the free PCPToolkit from www.saturnpcb.com most helpful. E

 

[Wilksey]

Excellent link canadaelk, many thanks for that!

Do you have a diagram of the snubber circuit you use please? I usually put a diode reverse across the coil.

Regards

Wilksey

 

[canadaelk]

Ther snubber I mentioned goes across the contacts (you still need the diode across the relay coil). It is an RC in series, R: 22-220 Ohm, C: 0.1 to 1uF/300VAC, values are selected to cause the least arking between contacts at opening. Maybe someone on this forum has a formula that gives values for the R/C depending on load V/A. E

 

[canadaelk]

Wilksey:
I am sure you know all that, but other members who follow this thread may not. Motorola (ON?) app note AN978 discusses the distances of pc traces between primary and secondary circuitry necessary to meet VDE. Reading that saved me a lot of grief! E

 

[edeca]

Don't forget that the area of a small circular pin can be fairly large. Taking ∏r^2 for 2mm is 12.5mm area. If there are multiple smaller pins the power can be distributed between them.

Whatever happens you should build in a fuse (either on the board or in the wiring) and a snubber as suggested here!

 

[Wilksey]

Thanks for all of your replies!

Will take all on board when designing.

edeca, so, if I for example had a 2mm pad joined to the relay pad, I could take a piece of wire and eliminate the 10mm traces?

I would rather run a piece of wire between two pads than go crazy on the board copper!

I guess that the relay is spaced accordingly with it's pins to allow for 250VAC to be switched without much noise?

Regards

Wilksey

 

[PatM]

Sounds like something to be mounted separately - not on a circuit board.
Maybe not even a relay but possibly a contactor ?

 

[Wilksey]

I will be alright mounting it on a board, its just a case of finding the best method, I would have thought it better to link the pads with wire rather than run 10mm traces across the board. If you start looking at internal traces then you start effectively doubling the width.