Your question makes no sense. Tell us what you want to do and why and we'll try to help.

SPST refers to switches not relays

I used a thermal switch (75deg c, normally open N/O) attached to the condenser to achieve what you are trying to do. Available at JayCar etc.

OK, that's a little clearer.

My understanding is that you wish the relay to energise when the fan
switches on.

Also the fan draws about 300mA and the power supply is 500mA maximum.

Is that correct?

If so:
Wire the relay in parallel with the fan ie. the positive connection
of the fan is connected directly to one side of the relay coil and
the negative of the fan to the other side of the relay coil.

This relay will do the job:
https://www.jaycar.com.au/12v-dpdt-power-relay-10a-240vac-24vdc-contacts/p/SY4065

And you can buy a base in which to mount it and wire to - easy :)

Although this is a DPDT relay you simply need to use just one pole
and wire between the normally open and the common contacts.

T1 Terry wrote:

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No one has mentioned the need for a diode across the relay coil to protect the electronics that power the existing fan. Does that mean they don't see a problem or that the need for the diode has missed? I only use solid state relays where electronic low current circuits are involved, had a few small transistors fail trying to power up mechanical relay coils. Something like this www.ebay.com.au/itm/Heat-S-Input-3-32V-DC-Output-5A-200V-DC-SSR-Solid-State-Relay-N1N5-F3E5/282694982241~P-
there are other even smaller is size relays they we use quite bit but I can't find a listing any more, we buy them in bulk direct.

Please understand the way solid state relays mark their connections, the control side is + and - as marked, but the load side refers to the more positive and more negative in regards to current flow. For example, to power the fan the battery positive would go to the load "+ " and the fan positive would go to the load "- " because the current is flowing from the battery to the fan. the fan negative would go to the battery via the shunt if one is used in your system.

T1 Terry

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The 12V power source is already feeding the existing fan!

The back kick from the coil would be no different to that of the fan ,that the relay coil is connected in parallel with

. That fan itself would absorb the back EMF

But if the OP is concerned ,a diode, reverse connected across the relay coil is a very small additional  expense, but it needs to be selected  to handle the max back emf

A diode & series resister takes some of the load off the diode as the energy is dissipated in the resistor

Yes ,lots of experience with BIG DC solenoids  & contactor coils  , they can generate a very nasty back EMF & give the actual opening contacts hell., unless arc suppression is used

Before the days of diodes ,capacitors  were used for that purpose.

Mike Harding wrote:

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It is unlikely a back EMF diode is required in this case.

[a[Terry: you are correct in your assertion the relay coil will produce a high voltage as its magnetic field abruptly collapses at switch off but it will be of low energy and will easily be absorbed by the fan motor or, with a 0W9 relay coil, by the PSU I am sure.

Peter: the fan motor will not behave in the same manner as the relay because once the motive force is removed from the motor coils it will being to act as a generator as it slows to zero, there will be no sudden change to the current flow through the fan motor it will be a gradual one and the di/dt rate will be slow so no spike of significance will occur.

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Hi Mike

[a]Totally agree

  Yes, I stand corrected on that part ,on it's significance , I was referring to that generated voltage

T1 Terry wrote:

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Ah, you plan to use the fan motor as the drain, do you think the in rush current for the fan motor plus the current to pull the relay coil will total less than 0.5 amp instantaneous load? The cell monitors we use have a relay contact supposedly rated at 500mA and they have been reported to have a nasty habit of failing instantly when attempting drive a relay, but there could have been other forces involved such as rapid switching etc. I have always used the mini SSR to switch the loads to avoid destroying a rather expensive circuit board and I would wire all the fans to the relay output if I was doing the job, no point in some being relay powered and others being circuit board powered and makes fault finding just that bit harder.

T1 Terry

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 Perhaps I have misread the op, but it seems to me that he is saying he will run the additional fans of 12V , so we are only talking about the relay coil current from the existing fan source

 I also suggested he could  run all the fans by the relay leaving the existing current limited source to just supply the relay coil

Perhaps you were so eager  to have another go  at me ,that you missed   that 

That should make fault ?? finding even easier'.

T1 Terry wrote:

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Sorry if you fell I'm having a shot at you Peter, that was not the intent and I did not follow that you were suggesting to remove the fan from the electronics supply and only use that for switching the relay. I was only putting forward an alternative method that would reduce any risk of damage to the fridge electronics because I'd imagine they would not be cheap to replace.

T1 Terry

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 Accepted