[Amps] Soft start, how to determine... if needed.

[Dr. David Kirkby]

Jim Thomson wrote:

> ##  Gerald, what he is implying is that when T=0.. on turn on, the 
> ENTIRE  240 vac  is dropped across the 1 ohm, leaving little or 
> no voltage across the pri.   This is how any step start works. 
> Place one dvm across the main 240 input, and 2nd dvm across the
> step start R... and a 3rd dvm across the PRI.   On turn on, the entire
> 240 vac is across the step start R.   Zero on the PRI. A  few msecs
> later, the  vdrop across the pri goes up, while the V drop across the
> step start R  goes down.   The 2 always total 240 vac. 

That is not correct. The *vector* sum of them will be 240 V, but due to the 
inductance of the primary, the voltage across the primary and the voltage across 
the resistor, as measured on a DVM, do not have to sum to the supply voltage.

Of course, you need a pretty special DVM to measure the voltages with a 
soft-start circuit.

Dave