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carmusic · 8th May 2008, 07:35 PM

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?

mneary · 8th May 2008, 09:17 PM

You're going to have trouble making pcb traces for 30A.

That said, the Potter&Brumfield T9AS series includes some rated for 30A at 240VAC. I don't know the DC rating.

carmusic · 9th May 2008, 12:50 AM

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

mneary · 9th May 2008, 02:00 AM

The Digikey webpage lists a number of T9A's that are good for 30A at 240VAC. Are these only rated for 20A at DC? That would be unusual.

Leftyretro · 9th May 2008, 03:33 AM

Quote:

Originally Posted by mneary

The Digikey webpage lists a number of T9A's that are good for 30A at 240VAC. Are these only rated for 20A at DC? That would be unusual.

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.

Lefty

mneary · 10th May 2008, 01:15 AM

Quote:

Relay contact ratings for DC current are almost always lower then their AC current ratings.

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.

Leftyretro · 10th May 2008, 01:59 AM

Quote:

Originally Posted by mneary

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.

Lefty

shokjok · 17th May 2008, 01:49 AM

What are you interfacing that requires 30A at 12VDC? Will two relays paralleled together fit your application?

Hero999 · 17th May 2008, 11:01 AM

Paralleling two relay might not help because one will open before the other so the slower relay will bear the brunt of the arc.

Does it really need to be PCB mounted?

You can get 28V 30A relays pretty cheaply.

What about using a power MOSFET or do you really need the isolation?

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8th May 2008, 07:35 PM	(permalink)
carmusic	pcb relay high power 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?

8th May 2008, 09:17 PM	(permalink)
mneary	You're going to have trouble making pcb traces for 30A. That said, the Potter&Brumfield T9AS series includes some rated for 30A at 240VAC. I don't know the DC rating.

9th May 2008, 12:50 AM	(permalink)
carmusic	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

9th May 2008, 02:00 AM	(permalink)
mneary	The Digikey webpage lists a number of T9A's that are good for 30A at 240VAC. Are these only rated for 20A at DC? That would be unusual.

17th May 2008, 01:49 AM	(permalink)
shokjok	What are you interfacing that requires 30A at 12VDC? Will two relays paralleled together fit your application?

17th May 2008, 11:01 AM	(permalink)
Hero999	Paralleling two relay might not help because one will open before the other so the slower relay will bear the brunt of the arc. Does it really need to be PCB mounted? You can get 28V 30A relays pretty cheaply. What about using a power MOSFET or do you really need the isolation? __________________ I also post at the following sites: http://www.stop-microsoft.org http://www.heated-debates.com Screen name: Aloone_Jonez