Electrical / electronic question.

[Norman_E]

In my thread on wiring a fridge http://www.ybw.com/forums/showthread.php?400773-Wiring-up-a-Fridge Rob Wheatley has kindly identified a relay that looks very suitable. The only possible snag is that it is rated to work on 9.6 to 13.2 volts, but when the batteries are charging on shore power or from the alternator the voltmeter usually shows 14.4 volts. I don't want to burn out the coil, which has a quoted resistance of 120 ohms plus or minus 5%. Am I right in thinking that a resistance of 12 ohms in series with the coil would drop the voltage at the coil to below 13.2?

 

[Rigger Mortice]

I make it 24ohms, but you had better wait and see what others come up with!

 

[nigelmercier]

I wouldn't pay too much attention to the specification from Maplin (spit). That is the operation voltage anyway, not the maximum. If you just want a 10A relay with a 12V coil, this one is good:
http://cpc.farnell.com/imo-precision-controls/srm-1c-sl-12vdc/minature-power-spco-12vdc/dp/SW02553

 

[William_H]

That relay should be fine on 14.4 volts. If it does seem to get hot after some minutes of operation at 14.4v then yes a ressitor will help.
12volts at 120 ohm gives .1 amp current. To drop 2.4 volts at .1 amp you need R=E/I 2.4/.1 is 24 ohms.
If however you want the relay to run on 13.2 volts then you need only lose half the voltage so 12 ohms would be correct.
In practice anything between those 2 would be fine. NB the 24ohm ressitor will dissipate .24watts so a 1 watt resistor would be recomended. But the 12 ohm would only dissipate something like .12 watts so not so hot. good luck olewill

 

[Norman_E]

Thank you William. I think that I may go for a small margin of safety and use a 15 ohm 1 watt resistor as I have found some in my box of "useful" bits.

As an aside the inverter which is said to be beyond repair has one badly burned out component, which I have now identified as a power relay like this http://www.digispot.co.uk/new-brand---power-relay-pcb-12v-spco---smi-s-112l-58637-p.asp
Only when I get the inverter back will I be able to see if it is exactly the same, but if it is I might still be stymied because every supplier I can find lists them as unavailable or out of stock, which is probably why the repairers say it cannot be fixed.

 

[nigelmercier]

That relay should be fine on 14.4 volts. If it does seem to get hot after some minutes of operation at 14.4v then yes a ressitor will ...

... Get hot instead.

 

[William_H]

Thank you William. I think that I may go for a small margin of safety and use a 15 ohm 1 watt resistor as I have found some in my box of "useful" bits.

As an aside the inverter which is said to be beyond repair has one badly burned out component, which I have now identified as a power relay like this http://www.digispot.co.uk/new-brand---power-relay-pcb-12v-spco---smi-s-112l-58637-p.asp
Only when I get the inverter back will I be able to see if it is exactly the same, but if it is I might still be stymied because every supplier I can find lists them as unavailable or out of stock, which is probably why the repairers say it cannot be fixed.

That relay is typical of a small relays made to fit onto a PC board in the quest for miniaturisation. I reckon at 10 amps DC it might have a very finite life being so physically small. I reckon if you can fit a real sized relay elsewhere in the box or even outside the box substituting for the failed relay you might get that inverter going again.
good luck olewill

 

[Norman_E]

That relay is typical of a small relays made to fit onto a PC board in the quest for miniaturisation. I reckon at 10 amps DC it might have a very finite life being so physically small. I reckon if you can fit a real sized relay elsewhere in the box or even outside the box substituting for the failed relay you might get that inverter going again.
good luck olewill

I have been thinking much the same, though I am unsure if the relay is the only failed part because when something like that burns out there may well be "collateral damage" to other components including the various integrated circuit chips. I will not be able to do anything until I get back to Turkey in September, and will probably just bring it back when I return in late October. It was still with the repair man when I left last week, so I had no chance the bring it home.

If it is not fixable I am thinking of a simple setup to run the fridge off the mains, and a rotary three port changeover switch to allow the fridge to be run from either shore power or a big new inverter. I suspect that the old one did not have enough margin of power because my research shows that not only do you need an inverter with a decent surge rating to cope with compressor start up, but some sources say you need a continuous rated output of at least three times the running consumption of the fridge. The old inverter was rated 800Va with 1100Va surge and I am thinking of a much higher rated inverter from Sterling or Victron, say 1500Va with 3000Va surge capability. The old inverter was also fitted in a very hot locker alongside the compressor, which probably cooked it.