impedance of detector 88 can be adjusted by selectively shunting resistors r15 r16 and the opposite to prevent damage to an optoisolator 216 shunting is accomplished by jumpers j22 j23 which connect to corresponding jumpers at a relay device 218 jumpers j24 j25 convey a zero crossing detector output to controller 94 the output on j24 j25 transitions between shunted and open circuit at every zero voltage crossing of the ac line input on lines 76a 76b relay 90 also connects across lines 76a 76b as shown in fig5 the circuit elements shown are selected such that an ac line input greater than a threshold voltage level causes relay device 218 to shunt jumpers j26 j27 and isolate jumpers j22 j23 for an ac line input less than the threshold level relay device 218 relaxes to its default condition where jumpers j26 j27 are isolated and jumpers j22 j23 are shunted preferably the threshold level is between the widely used line voltages of 120 vac and 240 vac jumpers j26 j27 connect to corresponding jumpers in heater circuit 92 heater circuit 92 connects in series to cell heater 66 across the ac line input on jumpers j16 j17 heater circuit 92 conducts an ac current from the ac line input through cell heater 66 and through a heater control.