View Full Version : trying to understand SSR's

08-06-2006, 09:32 AM
hi all,
anyone know if you can take a SSR's rating for ex .. 45 A at 240vac and just multiply and use that power rating to adjust and find out if it will carry 90 amps at 12 Vdc· ??
in other words can you use its watt rating as a gauge ?
got to run a dc motor at 12 volts, i think the circut breaker for it is 50 Amps, might be higher
and i want to be able to run it forwards and back and brake... sounds like a H bridge i think

Bruce Bates
08-06-2006, 11:17 AM
Dan -

I can't speak for SSR's per se, but generally relay contacts are rated in ampacity, NOT in wattage as you suggest. Besides, I'd NEVER mix AC and DC ratings, regardless!


Bruce Bates

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Mike Green
08-06-2006, 12:54 PM
Generally, SSRs are made for either switching "mains" AC in the 120VAC to 240VAC range with some maximum current or they're made for switching DC with some maximum voltage and current. The design of AC SSRs is different from DC SSRs and you can't mix them up. You can't use the "wattage rating" really for anything. Applying significantly more than the voltage rating will very quickly (maybe instantly) destroy the device. Applying significantly more than the current rating will overheat and eventually destroy the device.

Generally, designing high current switches (like maybe 50A). You want your devices to spend as little time as possible in their "linear" zone so that they switch very rapidly and thus don't produce much heat. For an H-bridge, you want to design it very carefully so that the two legs between + and - can never switch on at the same time. For the kind of current you're talking about, if you don't have some experience, I'd suggest sticking with a commercially produced device or a carefully designed, published circuit.

08-06-2006, 06:20 PM
ok, thanks guys, no experience here in this, but on the other hand 6 months ago, no experince in the microcontrollers either.. so it sounds like lots of reading and surfin' is in order.

PJ Allen
08-06-2006, 10:28 PM
Typically, the SSRs you will see for sale at the various usual suspects are for switching AC loads.· One doesn't find the DC sort around much.·

I was doing some shopping and saw this item:

60V, 3A· solid-state relay --· http://www.alltronics.com/cgi-bin/item/96B002/search/60VDC%2D%40%2D3A%2DSolid%2DState%2DRelay

08-07-2006, 12:45 AM
i'm looking to controll a motor that is 12v and has a 50A circut breaker... so i think i need at least that in amps

but thanks to the forum sending me off looking in a different direction than i had been surfing... i noticed a schematic that uses a DPDT mechanical relay to control direction of rotation, and a single power mosfet to control current flow...

you just turn off the mosfet b4 switching the relay to extend the life of the relays... so whereas DC SSR's are expensive, and i would need 4, by using automotive relays with high current capabiltys ( cheap on ebay ) to switch direction, and a single mosfet, to interupt the current, its much less for the pocketbook