Replacing multiple pole relay with solid state device

[BruceS57]

I need to replace a relay in an existing automotive power circuit with a solid state device. Device can be pre-packaged, or one that I can build from hobbiest parts such as found at Radio Shack. Result needs to be as lightweight and compact as possible.

The relay has 2 sets of contacts with both normally open and normally closed sides. With no power on the coil, commons 1 & 2 are each connected to their respective outputs. With power on coil, common 1 is connected back to common 2 but common 2 is not connected to an output.

The relay has a 12VDC, 5mA coil, and the contacts are rated 20A at 50VDC.

Suggestions?

Thanks!

 

[Nigel Goodwin]

Stick with the relay!, it's cheap, it's simple, it's effective, it's reliable, it's small, it's effective - what have you got against a relay?.

BTW, it's doubtful the current for the coil is only 5mA?.

 

[BruceS57]

Points don't last

Nigel,

Thanks for replying!

Want to go solid state because relay has proved unreliable. Contacts are subject to arcing, resulting in bad connections. Also, want to control it with logic-type voltage and current.

Any ideas?

Bruce

 

[Nigel Goodwin]

Nigel,

Thanks for replying!

Want to go solid state because relay has proved unreliable. Contacts are subject to arcing, resulting in bad connections. Also, want to control it with logic-type voltage and current.

Any ideas?

Use better relays!, there's really no sensible alternative - if you want logic control just feed them via a transistor. Millions of cars are out there on the roads, with loads of relays in every one, yet a failure is a rare event - I've never had one fail in 30 years of driving!.

Trying to build a solid-state alternative is going to be expensive, probably quite large, and I would expect far more unreliable!.

 

[BruceS57]

Would still like circuit

Nigel,
Thanks for the advice, but I'd still like to see a circuit.
Bruce

 

[Russlk]

The relay circuit is my interpretation of your description. The solid state circuit is below.

 

[BruceS57]

Thanks!

Thanks, Nigel. Very helpful.
Bruce

 

[Hero999]

I don't see how that circuit provides the isolation a relay would.

Have you looked at the circuit driving the relay?

Is there a diode in reverse paralell with the coil?

If so then connect a resistor the same value as the coil (about 2k4)in series with the diode. This will make the magnetic field decay at a faster rate so the contacts will open quicker and hopfully reduce arcing.

 

[zevon8]

One issue that a descrete design may have problems with is a highly inductive load. Driving a motor or similar can result in alot of back EMF that can be fatal to a circuit not designed to handle it. Relays will do this easily, and with a simple snubber ( capacitor, capacitor-resistor in series) you can expect years of releiable service.

Automakers know that it is hard to beat a relay for simplicity and price when it comes to large inductive loads. Relay wins hands down.

 

[JedOs]

BruceS57, you could use a couple of opto-isolators with darlington pairs. You would need to incorporate them into a design such as the one Russlk posted.
I will have to agree with Nigel though, find a better relay, the only time I have had trouble with relays is when I needed them in a small space.

 

[BruceS57]

Thank you!

My thanks to all that replied to my question! I found a SSR on the market that solved my problem. It was 10x the cost of the relay circuit, but will never have to be replaced and doesn't generate a spark.
Bruce

 

[Nigel Goodwin]

but will never have to be replaced

I think you are probably dreaming?

I would be very suprised if it's as reliable as a properly specified conventional relay? - and when it fails it's VERY likely to go S/C.

and doesn't generate a spark.

You're quite right there - but was that a problem?.