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Solid State Relay Question

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Njguy

Member
Relays - Solid State - SSRDC100V40A

The link above has the cheapest SSR that I could find so far that has decent amp output, and is DC-DC. Just a simple question though, I see the input connectors, and the output. But where exactly does the line go that trips the relay? I can't tell on any SSR. Thanks guys.
 

Boncuk

New Member
There are two trip lines for the relay, marked "INPUT", "4 - 32VDC" "-4" and "3+".

Connect 4 - 32VDC to the approriate screw terminals to trip the relay.

Connect "2+" to the positive power supply terminal, and the "-1" terminal to the load. From the load connect the other terminal to ground.

Boncuk
 

Njguy

Member
Ok, I get it, and how do you secure the wire to it? Do you just wrap the wire around the screw and tighten?

Also when the led is lit, that means the relay is active?
 
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MikeMl

Well-Known Member
Most Helpful Member
Right off the data sheet you posted the link to:
 

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KMoffett

Well-Known Member
Also, note "1." under precautions.

Ken
 

bountyhunter

Well-Known Member
We used a lot of those to run the heater units in our ovens way back when. I built up a controller to turn on a vaporizer with one. They were very unrelaible and always blew up after a while. We had to replace them in every system we sold.
 

Njguy

Member
We used a lot of those to run the heater units in our ovens way back when. I built up a controller to turn on a vaporizer with one. They were very unrelaible and always blew up after a while. We had to replace them in every system we sold.

Did you use heat sinks on them? They are supposed to be more reliable than a mechanical relay, no moving parts and all that.
 

bountyhunter

Well-Known Member
Did you use heat sinks on them? They are supposed to be more reliable than a mechanical relay, no moving parts and all that.

Yes, we had massive heatsinks. Back then, the SS relays were optically coupled TRIACs which are solid state devices and they were not reliable at all. Maybe they are better now, I don't know.
 

bountyhunter

Well-Known Member
Guess I'll find out.
Maybe they make them better now. Problem is they fail with no warning and just go stone dead. They may be more reliable than mechanical relays but that's like being the tallest person on the island of midgets.... mechanical relays are pretty terrible for reliability.

Regarding SS relay reliability, I found this reference:

when used with inductive loads such as electric fans, care must be taken to assure that the TRIAC will turn off correctly at the end of each half-cycle of the AC power.

A snubber circuit (usually of the RC type) is often used between A1 and A2 to assist this turn-off. Snubber circuits are also used to prevent premature triggering, caused for example by voltage spikes in the mains supply. Also, a gate resistor or capacitor (or both in parallel) may be connected between gate and A1 to further prevent false triggering. That, however, increases the required trigger current and / or adds latency (capacitor charging).

It's possible they didn't get the snubber right, but something definitely blew them out at random times. Looks like this problem should not occur in a DC application.
 
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bountyhunter

Well-Known Member
I know this sounds like wishful thinking, but are there solid state relays to turn on 5V dc motors yet?
Maybe, but always remember: a SSR is a solid state device, the "switch side" looks like an SCR that is turned on. When it is turned on, it has a couple of volts of drop across it, so it isn't real practical driving low voltage loads. In other words: If your load needed 5V, you would have to provide maybe 6.5 - 7V to run it to allow for the voltage lost across the SSR.
 

bountyhunter

Well-Known Member
Interesting, the SSR's I saw looked like an SCR so they had some voltage drop even at light currents but did not need to be continuously driven to stay on. This one is a light-activated FET so it has a low on resistance but needs constant drive to stay ON. You could build one of these with a 4N25 opto coupler and a FET.
 
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KMoffett

Well-Known Member
Once an SCR or Triac are triggered ON, the only way to shut them off is to cut off the current to the load. These are used only in AC-output SSRs. With AC, the current is cut off every half cycle (~8-10 mSec). If you use an SCR-SSR or Triac-SSR with DC circuit, once triggered ON it stays ON.

Ken
 
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