Replacing a 40A 12V relay

[WGJ]

I see a few familiar characters lurking here and I hope to get an encouraging reply or two.....

This is the question: I want to temporarily replace a relay rated at 40A/12V with a SSR to eliminate any effects that the mechanical relay may be having on the circuit that drives it.

This is an engine electronic control application where we are investigating how poor battery condition has to get before we can't get a start.
The relay in question is supplied via the engine control unit and I think I want to drive the SSR with as low a voltage as possible and have as little volt drop on the 'output' as possible.
Have mercy on me.


Bill

 

[itsmoked]

I don't think you want to do this....

Reasons. I believe you will be causing the very issues you are trying to avoid.

A mech relay will have virtually no voltage drop across its contacts. An SSR will definitely have a voltage drop across the semiconductor. This means that your driven circuit won't have as high a voltage provided to it during the entire test. This also means your controlled (whatever) will hit its malfunction due to low voltage point earlier than otherwise with a mech relay.

A mech relay will likely draw more 'control' current than an SSR but the bigger hit will be the output side of the SSR.

Also you will need to provide a heat sink for the SSR and seriously de-rate it for for any local elevated temperature that would be present.



Alway grab for a mechanical relay first unless you will expect more than about 100,000 (larger relay) or 1,000,000(smaller relay) actuations in the product life.

OR

Your control circuitry has zero guts and has the absolute lowest drive capability so an SSR starts to be viable even when the initial cost and the cost of heat sinking and guaranteed airflow don't overwhelm the balance.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[WGJ]

Hello itsmoked.

You have confirmed the suspicions that I had re the disadvantages of the SSR for this problem.

The investigation that I've been involved with centres on the way that this particular 12V relay behaves over a range of coil temperatures; how the pull-in and drop-out characteristics change and what this does in teraction with some of the associated systems that it is supposed to work with.

I had a quick look at the stuff in the Radio Spares and Farnell catalogues and at some data on the Crydon site and assumed that we would be wasting our time in pursuing this particular way of eliminating the mechanical relay for the purpose of our tests/data gathering.


Bill

 

[ScottyUK]

You could replace the relay with a contactor rated for heavier current and / or parallel the poles of the contactor. That takes the specific relay type out of the loop. The types used for electric vehicles and FLTs should be a good starting point.


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If we learn from our mistakes I'm getting a great education!

 

[itsmoked]

Hi WGJ.

I'm surprised your relay changes that much over T. I wouldn't normally even think about it more than maybe a check at an application's top temperature.

Is this actual personal experience you've had? Or a data sheet expression?

What brand/model is it?

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[WGJ]

Keith, it's an automotive relay and I don't know the make, but safe to say it's the cheapest one that the Purchase Dept could get that still met the specification.

The problem is that a field concern of 'will crank but not start' has been put down to battery voltage cycling (during cranking) below the drop-out voltage of this particular relay and coil temperature has been cited as a possible issue: - higher pull-in coil temp - higher resistance - less current - lower hold-in force - potential drop-out as cranking cycles battery voltage.
Once this particular relay drops out and even if it pulls in again as battery volts go up, the engine has to go through synchronisation again and more current is sucked out of the battery, etc., etc.

If we can reliably substitute a relay with a device that will take a mere whiff of voltage to drive it, with the resistance equivalent of a pair of mechanical relay contacts on the output, we'd isolate a potential cause for the purpose of the investigation.

Bill

 

[itsmoked]

Hmmm.

Okay.

Run the very same mech relay with a DC SSR. Just a wee one. Only power the second,(your original relay's coil), with a completely separate supply. Say a lantern battery, or a power supply. This would have your control signal needing to be >3V and supplying 3-5mA. The 50-100mA mech relay coil current will be yanked entirely from the system as it will be provided from outside the system. Of course the switched-power the mech relay controls still comes from your vehicle.(connections don't change)

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com