I am trying to control a self latching gas relay by an aftermarket dual temperature controller. This is to replace a failed OEM temp controller in a dual zone 12Vdc compressor camping fridge. There are no parts available as replacements for these fridges.

The dual temperature controller I have sourced provides a relay contact closure for each of the 2 temp zones in the fridge (fridge/freezer). The original controller provided a single output which provided a pulsed 12Vdc output which would reverse the polarity depending on which zone requires the refrigerant. I cannot find an equivalent aftermarket controller so this is the best I can do at present.

The problem is how do I emulate this to the gas latching relay from the 2 separate relay output closures from the dual temp controller. It does not pulse the output but provides a contact closure while the temperature is above its set temperature and potentially both these contact closures could happen simultaneously at times which would be a problem without converting to a pulsed output and isolating the 2 opposing voltage sources.

So I am looking for an interface that would work and provide that conversion. Hope this makes some sense with the basic block daigram - Any thoughts

 

potentially both these contact closures could happen simultaneously at times

So what should happen if both relays are closed?
Should the last closure generate a pulse?

 

I'm sure there was another similar thread to this recently... yes, here it is. Seems that both @crutschow and I came up with solutions though the requirement was slightly different, but both may still be applicable. Its unlikely you'll find an off the shelf solution.

Ignoring, for now, the possibility that both relays may be activated together, here are some other solutions.:

The most obvious and simplest solution is two power supplies: (or 1 power supply and an inverting converter which can be very simple if the power requirement is low)


If that's not possible then this circuit will do it (one of many variations on the classic H-Bridge)

 

The most obvious and simplest solution is two power supplies: (or 1 power supply and an inverting converter which can be very simple if the power requirement is low)

He needs a pulse, so you could put a large capacitor (along with bleed resistor to ground) in series with each output.
That will also solve the problem of both switches being on at the same time.

 

Below is the LTspice simulation of another solution using two SPDT relays in a bridge configuration:
The series capacitors gives about a 100ms pulse to the gas relay (assuming the SPDT relay coils are about 300 ohms).

 

So what should happen if both relays are closed?
Should the last closure generate a pulse?

They would apply opposing voltages to the terminals of the latching relay at the same time so they must be mutually exclusive at the time they apply a pulse.

 

So what should happen if both relays are closed?
Should the last closure generate a pulse?

The fridge is operated from a single power source i.e. a 12 volt vehicle or caravan or just a battery/solar so the application of both pulses at the same time would probably damage the dual temp controller output.

Yes the the last closure should generate a pulse.

 

Below is the LTspice simulation of a another solution using two SPDT relays in a bridge configuration:
The series capacitors gives about a 100ms pulse to the gas relay (assuming the SPDT relay coils are about 300 ohms).

View attachment 268406

I like that looks like it should work. I will give it a try. Thanks Crutschow

 

Here's the same circuit implemented with a pair of TC4429 gate drivers. Can use much smaller C values, so whole assembly smaller. TC4429 is 8-pin DIP, can switch 6A pulsed or 400mA continuous and is $2.5/£2.4 1 off, in stock at Mouser and elsewhere.

 

Thank you Irving, that looks nice and straightforward and minimalist. I like it and will give it a try. Really appreciate your input and working this up for me.

 

They would apply opposing voltages to the terminals of the latching relay at the same time so they must be mutually exclusive at the time they apply a pulse.

OK I have had a look at this application over the weekend and discovered that the gas relay is not a latching relay in this fridge - it requires 12V to be applied to direct the refrigerant to one zone and then removed to direct it to the other - it doesnt latch, sorry my misunderstanding as its different to other fridges.

However isnt there a problem at switch-on regardless as both the fridge and freezer is out of tolerance and therefore gas relay would not be activated as initially 12V would be applied to both sides of the relay and therefore cause an issue? Trying to work through the various situations with this and the capacitors would not be required if the relay is held on. The temp controller applies a contact closure while out of tolerance independantly for the fridge and freezer (S1 & S2). Hope that makes sense.

Cheers Tim

 

OK I have had a look at this application over the weekend and discovered that the gas relay is not a latching relay in this fridge - it requires 12V to be applied to direct the refrigerant to one zone and then removed to direct it to the other - it doesnt latch, sorry my misunderstanding as its different to other fridges.

However isnt there a problem at switch-on regardless as both the fridge and freezer is out of tolerance and therefore gas relay would not be activated as initially 12V would be applied to both sides of the relay and therefore cause an issue? Trying to work through the various situations with this and the capacitors would not be required if the relay is held on. The temp controller applies a contact closure while out of tolerance independantly for the fridge and freezer (S1 & S2). Hope that makes sense.

Cheers Tim

Good grief what was I thinking! thats ridiculous as it has to start somewhere at switch on so it doesnt matter what state the relay is in at that stage and then it sorts itself out as it comes up to temp

 

Okay, it's just a relay that either directs the gas to the fridge or the freezer, but then how does the gas get turned off when the desired temperature is reached in both?

 

Okay, it's just a relay that either directs the gas to the fridge or the freezer, but then how does the gas get turned off when the desired temperature is reached in both?

I have had to incorporate another relay to activate the compressor on demand from either the fridge or freezer so working on that.

 

Then it would seem the compressor relay would be activated if either of the temperature contacts is closed, and the gas relay is activated whenever one of the zones is too warm.

Which chamber is being cooled when the gas relay is activated?

 

Then it would seem the compressor relay would be activated if either of the temperature contacts is closed, and the gas relay is activated whenever one of the zones is too warm.

Which chamber is being cooled when the gas relay is activated?

In this fridge either zone can be a fridge or freezer depending on how the temp is set so there is no specific physical reason for either zone to be specified as a freezer except one side is smaller than the other.

This is a dual zone camping fridge operating from 12V. Here is a link to the Youtube product video. Of course parts are not available!!!!

.

I recently received one of these to try:
https://www.aliexpress.com/item/10000195280317.html?spm=a2g0o.order_list.0.0.1f8a1802KpB6hh
I have changed the relays to SPDT and added an extra SPST relay to it to activate the compressor. This initially worked however the load from the additional relay killed the relay switching transistors as the manufacturer was running the 5V internal relays at approx 11V and drawing to much current. I have ordered replacement transistors and will add resistors to halve the operating current which should allow the relay switching transistors to comfortably handle both relays.

 

In this fridge either zone can be a fridge or freezer depending on how the temp is set so there is no specific physical reason for either zone to be specified as a freezer except one side is smaller than the other.

This is a dual zone camping fridge operating from 12V. Here is a link to the Youtube product video. Of course parts are not available!!!!

.

I recently received one of these to try:
https://www.aliexpress.com/item/10000195280317.html?spm=a2g0o.order_list.0.0.1f8a1802KpB6hh
I have changed the relays to SPDT and added an extra SPST relay to it to activate the compressor. This initially worked however the load from the additional relay killed the relay switching transistors as the manufacturer was running the 5V internal relays at approx 11V and drawing to much current. I have ordered replacement transistors and will add resistors to halve the operating current which should allow the relay switching transistors to comfortably handle both relays.

This is the current configuration I was in the process of testing prior to the relay driver transistors failing using the above module - I think it works!!!: