Which way is best?

[Norman_E]

I have posted before on my fridge problems, http://www.ybw.com/forums/showthread.php?400773-Wiring-up-a-Fridge and have now found out that the fridge can be run without the failed CLD inverter if I use a relay or two to switch it on. Essentially the thermostat is a 12 volt one, and if I provide it with a 12 volt supply it closes the circuit when cooling is demanded. If I use the thermostat circuit to close a 12 volt coil relay and switch the 230 volt supply for the compressor I can either use another pair of contacts on the same relay to switch 12 volts for the seawater circulation pump that keeps the compressor cool, or use a second relay to do that.

The question is which is better? Two relays keep the 230 volt and 12 volt circuits well apart, but if one fails I could have the pump running without the compressor, or worse, the other way round causing the compressor to overheat. A single relay ensures that both work together. I am thinking of one of these http://cpc.farnell.com/jsp/displayProduct.jsp?sku=SW02627# which would allow me to use one pair of contacts for the 230 volt compressor, leave the centre pair unused, and put the 12 volt pump circuit on the other pair.

 

[superheat6k]

Electrical separation between the two poles of the relay will be OK. Looking at the Farnell item selected the electrical dielectric strength quoted is 2,500 VDC, your electrical risk is 240 RMS x 1,73 (root 3) = app 375v - v - 12 v, so no where near 2,500 v

 

[Norman_E]

Thank you for that. I thought it was OK but did not know that calculation. I actually chose that make of relay because the coil can withstand enough over voltage to cope with the situation where the battery charger is on and the 12 volt system is actually up to 14.4 volts.

 

[William_H]

Electrical separation between the two poles of the relay will be OK. Looking at the Farnell item selected the electrical dielectric strength quoted is 2,500 VDC, your electrical risk is 240 RMS x 1,73 (root 3) = app 375v - v - 12 v, so no where near 2,500 v

Just be very careful that the wiring to the relay contacts is very robust and well clear 240V from 12v. I would use 2 relays for this reason alone. Good luck olewill

 

[john_morris_uk]

Electrical separation between the two poles of the relay will be OK. Looking at the Farnell item selected the electrical dielectric strength quoted is 2,500 VDC, your electrical risk is 240 RMS x 1,73 (root 3) = app 375v - v - 12 v, so no where near 2,500 v

I don't mean to be Mr Picky but to calculate peak from RMS you multiply by the square root of 2 not the square root of 3! ie 1.414. The peak voltage of 240 volts is approx 340 volts. Your conclusion is correct in that the relay will be within spec. I'm not sure that it's very good engineering practice though...

 

[DJE]

Am I missing something? This is not my area of expertise but why can't you put the 12V relay coil in parallel with the 12V circulation pump?

 

[Norman_E]

Am I missing something? This is not my area of expertise but why can't you put the 12V relay coil in parallel with the 12V circulation pump?

I am not sure what the contacts inside the thermostat are capable of handling, so I was planning to run the water pump through a relay rather than directly off the thermostat contacts. The thermostat looks like the Frigoboat A020300, but is an old one and I have no idea of its technical specification. The pump has a 5 amp fuse so it cannot be more than that.