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Sonoffo, The front mount Coil is prone to overheating and here are some numbers that suggest you need protection for 12V Coils. Watts = (V X V)/R Normal 6V Coil with the Correct Ballast Combo for 6V or 12V Operation
(2.6 X 2.6)/.8 = 8.45 Watts (Worst Case) Key left on and points closed.
Operating (3.0 X 3.0)/.8 = 11.25 watts with a 38 % duty Cycle = ~ 4.27 watts.
(This Coil should last electrically for a lifetime)
The 12V Coil ( 12.5 X 12.5) /2.5 = 62.5 Watt(Worst Case) Key left on and points closed.
Operating (14.5 X 14.5)/2.5 = 84.1 watts with a 38 % duty Cycle = ~ 31.95 watts
The 12V Coil with a Ballast of .5 to.75 Ohms.
(10.5 x 10.5) /2.5 = 44.1 watts with a 38 % duty Cycle = ~ 16.75 watts
I would say that you cannot leave the Key �ON� with a 12 V coil but how much heat (watts), before it fails, will be determined by users (MTBF). We do know that if you bypass the ballast on a normal 6V Coil it will fail, and these are those numbers.
(6.5 X 6.5) /.8 = 52.8 Watts (Worst Case) Key left on and points closed.
Operating (7.2 X 7.2) /.8 = 64.8 Watts with a 38 % duty Cycle = ~ 24.62 watts
Hope this Helps,
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