It’s been a while since I been around here. I now have a question that I am unsure about and don’t want to end of messing up. Right now I am making a relay board that is 12V and using AVR to drive the relays. Each relay is rated 10A at 28V, but will be using 12V instead. Is there anyway that these relays can handle up to 15A at 12V? Anyway that not my main question. There are 11 relays on this board that I am making. There is only one input power supply at 12V to output via the relays. What I worry about is the traces on the board won’t be big to allow the current to flow to all relays and would end up melting the board over time or fried some parts. How did you know what size trace do you need for this kind of amps. Hope you understood my question.
Thanks
Shane
It would be safest to assume that a 28V/10A relay cannot carry more than 10A at lower voltage, unless you can track down the manufacturer’s data sheet and find that it says otherwise. After all, when the relay is closed, the full power supply voltage isn’t being seen by the relay - only the (hopefully few) millivolts of drop due to the contact resistance.
See http://circuitcalculator.com/wordpress/ … alculator/ to determine how wide your traces need to be to carry the current - note that there are a LOT of factors involved.
If you don’t have room to make the power trace as wide as needed, try this: remove any solder mask from the trace, run a heavy piece of bare copper wire along it, and solder it in place along its entire length. If you’re only slightly below the needed width, you might get by with simply running a bead of solder down the trace to thicken it.
madcitysw:
Right now I am making a relay board that is 12V and using AVR to drive the relays. Each relay is rated 10A at 28V, but will be using 12V instead. Is there anyway that these relays can handle up to 15A at 12V?
Greetings Shane,
Why are you using relays? Do you need the isolation from the 12V load circuit to the AVR control circuit?
Power MOSFETS would make the project better in many ways, and if isolation is really needed Opto-couplers on the driver circuits would work.
Relays are mechanical and prone to wear. The contacts, depending upon the load, may arc and cause pitting. High current relays can and sometimes do weld closed or ‘on’ due to current overloading of the contacts. Not to mention the racket eleven relays will make if more than one operates at the same time.
Let’s see. 15A * 12V = 180Watts per channel load. Times eleven channels = 1980Watts. Assuming all loads are the same and operated at the same time. I would think that connectors to safely deliver that power on and off the PCB will be a challenge. If something goes wrong, how are you protecting the loads (and relays, etc.) from damage? Fuses? (11 x 15 = 165Amps at the supply and ground return points - again assuming all output channels are operated together).
Why not mount the relays on a metal panel and hard wire the high current circuits with suitably sized wire and lugs instead of a PCB?
Comments Welcome!
Ya, that looks like the road I going anyway as I have order 11 of the t9ap5d52-12 relays from P&B, which were very cheap. I guess I won’t be using those PCB relays now.
bigglez:
Let’s see. 15A * 12V = 180Watts per channel load. Times eleven channels = 1980Watts. Assuming all loads are the same and operated at the same time. I would think that connectors to safely deliver that power on and off the PCB will be a challenge. If something goes wrong, how are you protecting the loads (and relays, etc.) from damage? Fuses? (11 x 15 = 165Amps at the supply and ground return points - again assuming all output channels are operated together).
Hold the phone. While it’s important that the relays can make/break 15 amps more than twice without melting, the board circuit doesn’t have to dissipate 180 watts. The voltage drop across the circuit (connector pin – trace – relay contact – trace – connector pin) would be very small, on the order of 0.01 ohms, or roughly a quarter of a watt dissipated. While you do want a certain minimum trace width, the batchpcb.com boards are 1oz copper, which helps. In very high-current applications, 2oz copper is sometimes used; however, I’m sure that’s not required here.
In general, I’m more concerned with how much the relay coil dissipates. Evan at 500ma, a dozen relays can easily draw (and dissipate) over fifty watts. I tend to try and select low-power relays for this reason.
There’s my two cents 
insertBillHere:
bigglez:
…I would think that connectors to safely deliver that power on and off the PCB will be a challenge. If something goes wrong, how are you protecting the loads (and relays, etc.) from damage? …
Hold the phone. While it’s important that the relays can make/break 15 amps more than twice without melting, the board circuit doesn’t have to dissipate 180 watts. The voltage drop across the circuit (connector pin – trace – relay contact – trace – connector pin) would be very small, on the order of 0.01 ohms, or roughly a quarter of a watt dissipated. While you do want a certain minimum trace width, the batchpcb.com boards are 1oz copper, which helps. In very high-current applications, 2oz copper is sometimes used; however, I’m sure that’s not required here.
Greetings Bill,
I agree with your points, but you may have glossed over my point (above). Placing relays on a PCB for this power level creates a problem in how to get the power on and off the PCB (lugs, connectors, solder pads?).
Also, what happens when something goes wrong and potenitally a couple hundred amps are dumped into the PCB?
For this load power level a PCB is not very attractive, and I’d suggest putting the relays on a metal panel and wiring them with suitably sized wire and lugs (or connectors).
I think the OP has seen the light and is not going to mount these on a PCB. I don’t think we found out from the OP why they prefer relays, solid-state switches (PowerMOS) would be better all round unless I’ve missed something.
Comments Welcome!
jasonharper:
See http://circuitcalculator.com/wordpress/ … alculator/ to determine how wide your traces need to be to carry the current - note that there are a LOT of factors involved.
That's a nifty tool right there. But I'm not sure what to do with the "temperature rise" field. What is a safe limit for pcbs like the ones from batchpcb? What will happen when the trace gets too hot? Delamination? I'm pretty sure it takes a lot more than 40 C to do that.