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Old 27th June 2009, 07:02 PM   #1
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Default 220v Variable Isolation Transformer

Hello everyone,

As I am designing my first tube amplifier, I am getting more and more scared of the lethal voltages involved. I have decided to make the investment of buying a variable isolated transformer, so I could easily 'slow start' my prototypes.

Unfortunately, such an item, available for online ordering, proves rather hard for me to find. Does anyone have a suggestion where I should look at, or any personal experience with such a product?

Many thanks
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Old 27th June 2009, 07:05 PM   #2
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You are (probably) talking about a variac, but it doesn't provide isolation.
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Old 27th June 2009, 07:06 PM   #3
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I've seen variac products that do provide isolation, but for 110v only
Unless a proper solution is found, should I get both a 220:220 isolation transformer and a variac?
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Old 27th June 2009, 07:21 PM   #4
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most units sold here, contain both, a variac and isolation transformer, in one case.
But these units are usually very expensive.
http://www.conrad.de/goto.php?artikel=512303
DIY the whole thing really saves money
A lower wattage combination:
http://www.conrad.de/goto.php?artikel=516546
regards
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Old 27th June 2009, 07:38 PM   #5
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If you amp doesn't have an isolation transformer, then you should get both a variac and a 220:220 transformer. Assuming you need isolation.
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Old 27th June 2009, 07:50 PM   #6
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those variac&isolationtr.-combinations are used if you are exposed to mains voltages. From an insurance point of view this happens, as soon you open an amplifier case and connect the mains plug.
There is always touchable mains voltage in it, either per hand or with a tool.
regards
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Old 27th June 2009, 08:03 PM   #7
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I'm actually thinking of building a bench power supply for testing line powered switchers. The features I'm thinking of are:
* 100-240v input with active PFC
* 0-400v at 0-10A (1500w maximum) for main power
* 0-30v at up to 3A for control power
* Both main and control outputs are isolated from the line and each other
* Minimal capacitance on outputs to reduce peak fault current
* Digital control with a microcontroller (maybe Arduino?) and optionally RS232 communication
* Ground fault protection on the main output (negative side connected to ground through about 10k resistance, power down if current exceeds 1mA)
* Emergency stop inputs, both open and closed loops
* Small and light enough to be portable (about the size of a shoebox or smaller and 20lbs or less)

It should also work for testing tube equipment.
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Old 28th June 2009, 10:16 AM   #8
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hmm, indeed very expensive.
I guess I could live (pun intended ) without isolation as the amplifier also has its own transformer, just need to make sure the live side of it is untouchable...
So I guess i'm down to recommendations for 220v variacs..
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Old 28th June 2009, 03:09 PM   #9
star882 is offline star882  United States
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It would probably turn out to be cheaper than a variac. And it would have isolation. And digital control.

It would probably be a long time before I actually build the bench power supply. For now, I just use an isolation transformer constructed from two old halogen lamp transformers to test line powered switchers.
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