How small relay pins can handle large currents

Thread Starter

zazas321

Joined Nov 29, 2015
869
Hello. I have come across this relay:
https://www.elpro.org/gb/4179-omron-g6qe-series

It states that the maximum current load is 30A AT 277VAC, I had previously even found a simmilar relay with 36A maximum load.

Can someone explain me how such tiny contacts can withstand such high loads? Usually when we have that much current flowing, the wire must be very thick!
 
Last edited:

Luckey.vm

Joined Feb 22, 2021
1
There are several factors to consider, surface area is a major factor, as well as meterial the pins and contact are made of, an as our technology has advanced we are able to take crap material and coat it in other meterial ( plate it) and have a much better material to use in small applications
 

Marley

Joined Apr 4, 2016
411
It does seem fairly amazing. I think if I was designing some equipment I would want at least a 2x safety margin for reliable performance. Note that the data says 30A "resistive" at 40degC. for 100k cycles. The PCB pins will solder to wide PCB tracks to carry the current and dissipate heat.
 

Thread Starter

zazas321

Joined Nov 29, 2015
869
Thanks for the replies.
It does seem fairly amazing. I think if I was designing some equipment I would want at least a 2x safety margin for reliable performance. Note that the data says 30A "resistive" at 40degC. for 100k cycles. The PCB pins will solder to wide PCB tracks to carry the current and dissipate heat.
Could you explain a little bit more on what do you mean "resistive" load. What differences does that make?
 

Marley

Joined Apr 4, 2016
411
It means not inductive or capacitive. The load is a simple perfect resistor only.
An inductive load (something like a motor, solenoid, transformer or anything with a magnetic coil - even a very long wire) will generate a high-voltage "kick-back" or "back-emf" when the contacts open. This high voltage will spark across the contacts, damaging them and severely reducing the life.
A capacitive load will take a very high inrush current when the contacts close. Probably causing the contacts to weld together.
Other types of load can also cause high peak currents when the contacts close example:
  • Tungsten lamps - technically a resistive load but the cold resistance is much lower than the hot (working) resistance.
  • Motors - at the point of starting (when the contacts first close) the motor is "stalled" (not moving) so the current is only limited by the resistance of the windings which will be very low.
 

Marley

Joined Apr 4, 2016
411
Actually, switching tungsten lamps or small DC motors is probably what these relays are designed to do. But you will specify the relay based on the cold current or stall current. Therefore once the load is up and running the normal current will be much lower (10x or more) and the relay will run cool.
 

eetech00

Joined Jun 8, 2013
2,491
Hello. I have come across this relay:
https://www.elpro.org/gb/4179-omron-g6qe-series

It states that the maximum current load is 30A AT 277VAC, I had previously even found a simmilar relay with 36A maximum load.

Can someone explain me how such tiny contacts can withstand such high loads? Usually when we have that much current flowing, the wire must be very thick!
Additionally,
PCB trace width would be appropriately sized.
PCB copper may be thicker.
PCB copper plane may be used with thermal pads widened or not used at all.
 

sagor

Joined Mar 10, 2019
469
But it still begs the question as to how the large pins, which have a cross sectional area of 1.2 mm squared (0.8mm x 1.5mm), can carry such currents. 1.2mm square area is about equivalent to a 16 gauge wire, which cannot carry 30A. I guess large copper traces on the board will remove some of the heat from the pin, and I assume inside the relay, the pin gets a lot bigger. Still a strange rating for those small pins
 

crutschow

Joined Mar 14, 2008
27,415
16 gauge wire, which cannot carry 30A
16 gauge wire can indeed carry 30A, if its a short wire.
The main concern is the voltage drop and heat dissipation due to the wire resistance.
For a short wire, the drop and dissipation are low.

For example, 16 gauge copper wire has about 4mΩ/ft of resistance, so a half inch of it would have a voltage drop of 5mV and dissipate 0.15W at 30A.
 
They can carry that much current because the circuit board dissipates all the excess heat which would otherwise burn up a conductor that size.

Take two 14 gauge wires. Wrap one in insulation and immerse the other in water. Which has the higher ampacity? Same concept.
 
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