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Components and Circuits For discussions about component types, alternatives and availability, circuit configurations and modifications etc. Discussions here should be of a general nature and not about specific sets.

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Old 28th May 2011, 3:44 pm   #1
Pamphonica
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Default Isolator then Variac or VV.

I am finally tidying up my radio test box (isolator plus lamp) and adding a variac.
Now, is it better to isolate first then have the variac, or vice-versa, and why?
The isolatinf transformer is rated at 100W and the variac at 0.5A. eg designed for lowish power use.
Well, which is better?
Jeremy
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Old 28th May 2011, 3:52 pm   #2
Leon Crampin
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Default Re: Isolator then Variac or VV.

Isolate first, then the variac.

If you connect them the other way round, at less than full voltage the isolating transformer will have excessive copper loss and poor regulation.

If the isolating transformer has an electrostatic screen, putting it at the supply end isolates the noise at source.

Leon.
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Old 28th May 2011, 4:13 pm   #3
Pamphonica
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Default Re: Isolator then Variac or VV.

Thanks, Leon. I'm wiring away now as per your very sensible and useful response.
Jeremy
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Old 28th May 2011, 4:47 pm   #4
LXSTEIN45
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Default Re: Isolator then Variac or VV.

Quote:
Originally Posted by Leon Crampin View Post
Isolate first, then the variac.

If you connect them the other way round, at less than full voltage the isolating transformer will have excessive copper loss and poor regulation.

If the isolating transformer has an electrostatic screen, putting it at the supply end isolates the noise at source.

Leon.
Can you please elaberate?
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Old 28th May 2011, 5:49 pm   #5
woodchips
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Default Re: Isolator then Variac or VV.

With the isolator first then both transformer and variac are running at full design magnetic flux density. Putting the variac first means that the transformer will only have have the flux at half the voltage, and the available VA out will reduce proportionally. Bob
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