| gnomus |
Can a power amp or pre amp use a variac in place of the power transformer? This would seem to solve the problem of power surge at turn on. And producing the right voltage for your power supply. Are there any draw backs?
Thanks in advance. |
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| tiroth |
| no isolation. you still need an isolation xformer. this would be a very expensive solution to inrush current! |
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| gnomus |
In rush current is a small benefit. The real benefit is you can produce the exact voltage you need.
Are isolation transformers a lot more expensive than a power transformer?
If so then the cost of both the variac and the isolation transformer will be far more than just the power transformer. Plus the isoliation transformer will take up more space too. I do not know how large isolation transformers are.
But, I do not see why the variac does not provide isolation by itself. Its just an adjustable power transformer? |
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| tiroth |
Variacs are more or less comparable in price to a standard transformer, so you are at least doubling the cost and lowering the efficiency.
Take a look at how a Variac works and you will see from the wiper connection that there cannot be isolation. |
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| blank527 |
No, you can't.
I've been thinking about it too, but it is also very unlikely when you think about it.
If you want for example have a 10V ac, and using it for lots of current, you only use a little part of the variac, from wich the power it less.
Instead if you turn it up at full, it can dissipate more warmth, then setting it on half or less.
You need a (very) big variac to do this.., I've got a 2KVA variac (220ac) but I'm not gonna try it out. Just buy a regular transformer.
For example, if you use a variac, you can go from a aleph 3 (out of head 25V rail?) to a aleph 2 or 1.2!!
Hope I explained it a little bit.
Any comments welcome!
Remco |
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| mrfeedback |
"But, I do not see why the variac does not provide isolation by itself. Its just an adjustable power transformer?"
No, no, no.
A variac is an autotransformer, which means a single winding with a wiper, and hence no DC isolation.
I hope you undersatand the danger in that.
Eric. |
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| gnomus |
Yes, I understand no isolation. And its best to use the variac to test designs.
Thanks |
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| mrfeedback |
"And its best to use the variac to test designs."
And repairs too.
I use a variac with a lamp (40, 60 or 100W) in series with the primary, and this is a perfect tool for running up repaired equipment, especially amplifiers.
If there is a fault, the lamp glows and dissipates most of the power, and low voltage only is available to the device under test.
Eric. |
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| janneman |
It might help to think of a variac as a huge potmeter with a lot of inductance, which is what it really is.
:cool:
Jan Didden |
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| gnomus |
| Great idea. And much cheaper than a voltage meter. |
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