Help Relay shows 1-6v across when it should show 28v

Thread Starter

gillman331

Joined Sep 16, 2019
3
I have a relay I'm using to power on various loads like a DC step down converter and and/or an inverter. The relay shows 1-6v across it when its closed with a load attached. It should be 28v,like it is when there is no load connected. I've tried switching the relay with a few power supplies and with my 28 battery. Both gave the same results. I'm at a loss for why I'm only getting the 6ish volts across the relay. I am using a SSR-40DA relay. Any help would be greatly appreciated!
20190916_213257.jpg
 

sagor

Joined Mar 10, 2019
115
You are measuring the voltage drop across the load. Your meter on bottom of relay is the same as measuring across the load. Voltage would be load resistance x current thru load.....
 

Thread Starter

gillman331

Joined Sep 16, 2019
3
You are measuring the voltage drop across the load. Your meter on bottom of relay is the same as measuring across the load. Voltage would be load resistance x current thru load.....
Then wouldn't the voltage drop appear on the other side of the relay? Plus I'd have to be drawing a ton of current to have my voltage drop by over 22v. I'm see 6 volts at my load, so low they won't even power on.
 

dendad

Joined Feb 20, 2016
3,069
I think the SSR-40DA is an AC SSR, not to be used on DC.
use a "SSR-40DD"
Or better, just a FET. Maybe like this?
FET_board.jpg
Readily available for 3D printer bed heaters. Look on Ebay.
 

Audioguru

Joined Dec 20, 2007
11,251
Does the battery measure 28V when the relay is closed?
Maybe your relay contacts are corroded and have resistance.

EDIT: Yes, it is a solid state relay that MUST use AC on its output.
 
Last edited:

dendad

Joined Feb 20, 2016
3,069
Wow I feel dumb now. You are correct!! This relay only says AC on it like 100 times. Thank you!!!
It just happens my 3D printer bed SSR failed last week, and I went to replace it with one I had for another job only to discover that one was an AC version. That is why I know it.
The fix was to build my own FET switch.
WombotBedFET1 S.jpg


WombotBedFET2S.jpg
 
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