[AMRadio] 813 Final Problem

JAMES HANLON knjhanlon at msn.com
Thu Mar 19 17:14:19 EDT 2015

You are indeed experiencing an "inductive voltage spike" problem across the modulation transformer winding when you attempt to make an abrupt change in the amount of current flowing through the coil.  The cure, at least for operating CW, is to put a short across the secondary winding of the modulation transformer.
The physics behind what's happening goes like this.  The relationship between the voltage, V, across an inductor, L, and the current flowing through the inductor, I, is expressed by the following formula.
V = L (dI/dt) . 
"dI" is the instantaneous change in current in the amount of time, "dt."    If you attempt to change from one current level to another, for example if you attempt to go from zero current to 100 ma, and you attempt to do it in one microsecond and through an inductance of  1 Henry, then a voltage spike would build up across that 1 Henry inductor of about V = 1 (0.1/.000001) = 100,000 volts.  The polarity of that voltage spike would be such as to oppose the build up of current in the inductor, or in other words to drive the current in the opposite direction to the current change you were attempting to make.  Obviously such a large spike would be enough to break down something in the circuit, in your case the insulator in your connector.
The cure for CW operation is to switch a short across the modulation transformer secondary while operating CW.  
For phone you will of course have to open that short.  The only time you may experience a problem during phone operation is when you are turning the carrier on at the beginning of a transmission and turning it off at the end.  Too rapid a transition at either turn-on or turn-off would generate a voltage spike across the modulation transformer secondary that would break something down and cause an arc.  Old-time transmitters, like the Johnson Viking I and Viking II for example, turned the high voltage power supply on and off with a switch in the transformer primary winding to switch the transmitter on and off for phone.  
Hope that helps,
Jim, W8KGI

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