[AMRadio] 813 Final Problem

james.liles at comcast.net james.liles at comcast.net
Thu Mar 19 21:09:36 EDT 2015

Hi Jim,

It's when you open the circuit or stop current flow the huge voltages occur. 
A voltage applied with a 1us rise time into a 1hy winding will simply 
develop a back voltage equal to the applied.  Stop it by opening a switch 
and it will develop whatever voltage necessary to arc the gap.

Kindest regards Jim K9AXN

-----Original Message----- 
Sent: Thursday, March 19, 2015 4:14 PM
To: AMradio ; paul at paulbaldock.com
Subject: [AMRadio] 813 Final Problem


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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