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WARNING re SOLID STATE RELAYS

PotvinGuy

Well-Known Member
Staff member
I've been using a solid state relay (SDP4040D) as my primary power switch. Bu today I was adding some new stuff and checked the voltages around the system. The solid state relay was dropping 1.5 volts! I checked the specs and sure enough it can drop up to 2.5 volts at full current. This is not good when we only have 12 volts to start with. So I'm replacing it with an ordinary 100 amp relay. I don't need 100 amps, but the unit has the right terminals to just swap in.
 
Won't you save a few bucks by using the standard relay.
 
That's good information especially if you are having electrical problems with something.. It gives you a place to start looking.
 
They were both about $10. I got the SSR on eBay and the 100a relay on Amazon. I never leave the house anymore.
I was thinking I looked at those when I was doing my wiring project and they were higher price. Guess they could have come down since then or my memory is failing. :) I'm with you there, unless I need it that day I usually order from Amazon, Summit or Jegs.

Did you research other SSR's for voltage drop, are they all similar in the respect?
 
No, made a bad assumption, but now know all SSRs are going to have some drop. Too bad, it's a neat gadget, but I don't want to give away 2v when we only have 12v. Here's a nice one for $13:
Here's the new one:
 
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The solid state relay was dropping 1.5 volts! I checked the specs and sure enough it can drop up to 2.5 volts at full current.
Are you talking about the voltage drop on the control side or the load side?
 
Load side. I've replaced the SSR with the new 100A mechanical relay and no drop. Headlights are brighter. And the lady in the GPS has a sexier voice...could be my imagination.
 
Load side. I've replaced the SSR with the new 100A mechanical relay and no drop. Headlights are brighter. And the lady in the GPS has a sexier voice...could be my imagination.
Recalculating!!! Lol. When I worked on X-ray equipment, solid state relays were used extensively and were a frequent cause for service due to failure. In comparison, the processors primarily used coil activated mechanical relays and rarely failed even after decades of constant use in medical facilities. The processors were also wet X-ray film processors and used acidic chemicals, heat, forced air, and water, which created a harsh environment for all things electronic. Hard to beat good old mechanical relays, imho. Mechanical relays are electronically noisy, so solid state are chosen for many sensitive machines. Other than that, I personally see no advantage to using them.
 

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