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kalee20 · 29th Dec 2018, 6:10 pm

You say it's a 500VA isolation transformer and a 2kVA Variac.

For maximum usefulness put the isolation transformer FIRST. Here's why:

The isolation transformer, rated at 500VA, will be able to cope with a load of just over 2A, without the windings burning out - whatever the voltage - because that's what the wire gauge will have been chosen to handle.

The Variac brush (usually the limiting factor for a Variac) will be able to cope with just over 8A, as it's a 2kVA Variac.

So if you have Variac first, transformer second, you'll only ever be able to pull 2A from the combination, whatever the Variac setting. But if you have transformer first, Variac second, then if the Variac is set to give 250V you can pull 2A; at 200V 2.5A; at 100V 5A; at 62.5V 8A, (all these being limited by the 500VA rating of the transformer), and then anything below 62.5V you can draw up to 8A (the Variac limit) without overloading anything.

Which, I think you'll agree, is more useful and flexible!

29th Dec 2018, 6:10 pm	#9
kalee20 Dekatron Join Date: Feb 2007 Location: Lynton, N. Devon, UK. Posts: 7,082	Re: Variac first or Isolation first ? You say it's a 500VA isolation transformer and a 2kVA Variac. For maximum usefulness put the isolation transformer FIRST. Here's why: The isolation transformer, rated at 500VA, will be able to cope with a load of just over 2A, without the windings burning out - whatever the voltage - because that's what the wire gauge will have been chosen to handle. The Variac brush (usually the limiting factor for a Variac) will be able to cope with just over 8A, as it's a 2kVA Variac. So if you have Variac first, transformer second, you'll only ever be able to pull 2A from the combination, whatever the Variac setting. But if you have transformer first, Variac second, then if the Variac is set to give 250V you can pull 2A; at 200V 2.5A; at 100V 5A; at 62.5V 8A, (all these being limited by the 500VA rating of the transformer), and then anything below 62.5V you can draw up to 8A (the Variac limit) without overloading anything. Which, I think you'll agree, is more useful and flexible!