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Registered Member #187
Joined: Thu Feb 16 2006, 02:54PM
Location: Central Ohio
Posts: 140
Yes! And it is something that had not occurred to me, thanks. But to put it another way, I would never use my "current limiting" variac to control the current to something that has a power rating far exceeding the variacs rating. So in essence I knew it would be bad, but you put it into better words.
Registered Member #72
Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659
Using a variac as a variable inductor in series with a load controls something, but it sure ain't current, or even approximating to current.
Consider just the voltage across the inductor (variac) for the moment. When 100% of mains number of windings are dialed in, there is sufficient inductance for the current to be very low indeed with any voltage up to full mains. Now set the variac to 50%. With 50% voltage across it, the current will be low. With somewhere between 50% and 100%, the inductor will saturate and the current will be very large. As the number of turns is reduced, so the voltage that can be withstood will reduce.
So you see, what is controlled on the variac dial is the maximum voltage across the inductor before the current drawn rises dramtically. Given that the voltage from the wall is relatively constant and from a low impedance, the setting on the variac controls the *minimum* voltage across the load. The variac is therefore a soggy voltage control, which will quite happily send very large currents into a short circuit.
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Variac wired for current limiting
