b) Using a Shorting Switch
When using a shorting switch instead of the 4300B flyback diode, the current stored in the inductor discharges
through the cables and switch. This current discharges with a time constant calculated from the formula
below which is dependent on the test current that was used to charge the inductor:
Test
Current Discharge
Time
Range
Used: (in
Seconds):
10 Amps
7 x L
÷
R
1 Amp
5 x L
÷
R
.1 Amp
2 x L
÷
R
< .1 Amp
Not required
Where:
L = Inductance in Henries
R =Total resistance of cables, switch
and inductor (in ohms)
In a typical application R might be approximately 1 milliohm (.001
Ω
) and L might be 1000 Henries. If the 10
amp range of the 4300B was used to measure the inductor, 7,000,000 seconds are required (11½ weeks) to
discharge it. The 4300B may be disconnected at any time provided the short circuit is maintained across the
winding.
