i have searched for a PCB relay good for 30A at 28vdc, but i found a lot of relay good for 20A@28vdc (the g8p series from omron for example) or relay good for 40-50A at 14Vdc (automotive relay). Does it exist or i must use an external relay?
well i dont have the choice i use full brick dc converter that are solderd to a pcb board and output 30Amp, i make the trace 200mil on each side of the pcb with 2 oz copper
the t9as are only rated 20A at 28vdc
No, actually quite the rule instead. Relay contact ratings for DC current are almost always lower then their AC current ratings. The zero voltage (and current) crossing of AC helps to break the contact arcing upon opening and closing, where as DC is always a full draw break or make and tax the contacts much harder.
I understand the concept of AC breaking the arc, that's why the DC and AC voltage ratings are so different. I don't normally expect the current ratings to be different.
I understand the concept of AC breaking the arc, that's why the DC and AC voltage ratings are so different. I don't normally expect the current ratings to be different.
It's really mostly all about the current. Max voltage ratings is dealt with by physical separation of adjacent contact poles and is not a difficult or expensive specification to meet. However when large currents are flowing you have lots of energy (heat) during the make and break that really taxes the contact material. The amount of contact heat generated is much higher for DC then AC at the same current level, independent of the working voltage.