# Need Ultra Low Resistance DC/DC SSR

#### SurviverMan

Joined Dec 3, 2013
7
I am building a Remote Control for my Fireplace and Fan. I have it completed and working on a Breadboard. I switch the 120VAC Fan with a FORTEK SSR-40 DA and it works fine. I am using a CUON HHG1D-0/032F-20 SR to switch a Low voltage DC on-off circuit that controls the Gas Valve. Forget the wireless RF xmitter and receiver, power supply,etc. it all works.

My problem is... The Gas Valve uses a 520 mV DC current generated by a thermocouple in the pilot light to shift the valve from off to on and turn on the fireplace. It currently uses a wall switch (plain old light switch) to complete the circuit. The DC/DC relay is opto isolated but has 11.4 kohm on-state resistance (At least that's what I think it measures on my DMM) that will not pass enough current to shift the valve. Even putting my DMM in the wall circuit to measure the current output of the thermocouple adds enough resistance to prevent the valve from shifting. This is a very low power application and I need your recommendations on what device I should use for a switch. Input to the DC/DC SSR is 5V DC from a wall wart. It's a low resistance output switch function that's required.

Please note: I'm a component assembler, not a circuit designer. Retired Mechanical Engineers tend to just know enough EE to get into trouble.

#### Dodgydave

Joined Jun 22, 2012
9,876
Not with you here mate, do you want to replace the light switch for a relay, or just another voltage free switch?

#### crutschow

Joined Mar 14, 2008
26,069
That DC SSR has a Darlington stage switch at the output which likely has too high a drop when ON (the data sheet states 1.2V max).
You need a DC SSR with a MOSFET output.

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#### mcgyvr

Joined Oct 15, 2009
5,394
Yep mosfet based SSR
or mechanical relay (with gold contacts)

#### SurviverMan

Joined Dec 3, 2013
7
Thanks for the replies. Yep. I thought I was on the right track looking for an SSR. I need opto-isolation and didn't want a mechanical relay because it would have to remain in the on state for hours at a time. Mechanical relay coils tend to hum and add other complications to what I see as a straight forward application.
I purchase components on-line and am looking to contain costs. I could buy a control directly from the fireplace maker, complete and ready to plug in but it's over US$150 and doesn't include a control for the 120VAC fan. Mine will end up under US$50 and will control both the fan and the gas valve as soon as I find an SSR with low on-state resistance.
Thanks again. A recommendation on an actual part would be a big help especially if you have a favorite supplier for this sort of thing. You know... Good service, reasonable prices. That sort of thing.

#### crutschow

Joined Mar 14, 2008
26,069
One of these should work for you.