Can a variac replace the power transformer

gnomus · 17th December 2002, 04:22 PM

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.

tiroth · 17th December 2002, 04:27 PM

no isolation. you still need an isolation xformer. this would be a very expensive solution to inrush current!

gnomus · 17th December 2002, 05:08 PM

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?

tiroth · 17th December 2002, 05:22 PM

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.

blank527 · 17th December 2002, 09:08 PM

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

mrfeedback · 18th December 2002, 01:30 PM

"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.

gnomus · 18th December 2002, 01:34 PM

Yes, I understand no isolation. And its best to use the variac to test designs.

Thanks

mrfeedback · 18th December 2002, 02:01 PM

"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.

janneman · 18th December 2002, 02:27 PM

It might help to think of a variac as a huge potmeter with a lot of inductance, which is what it really is.

Jan Didden

gnomus · 18th December 2002, 02:30 PM

Great idea. And much cheaper than a voltage meter.

17th December 2002, 04:22 PM	#1
gnomus diyAudio Member Join Date: Jan 2002 Location: New York City	Can a variac replace the power transformer 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. __________________ Paul

17th December 2002, 05:08 PM	#3
gnomus diyAudio Member Join Date: Jan 2002 Location: New York City	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? __________________ Paul

18th December 2002, 01:30 PM	#6
mrfeedback diyAudio Member Join Date: Feb 2002 Location: Perth, Australia.	Now Say After Me, "Auto-transformer".... "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. __________________ I believe not to believe in any fixed belief system.

18th December 2002, 01:34 PM	#7
gnomus diyAudio Member Join Date: Jan 2002 Location: New York City	Yes, I understand no isolation. And its best to use the variac to test designs. Thanks __________________ Paul

18th December 2002, 02:01 PM	#8
mrfeedback diyAudio Member Join Date: Feb 2002 Location: Perth, Australia.	"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. __________________ I believe not to believe in any fixed belief system.


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17th December 2002, 04:27 PM	#2
tiroth diyAudio Member Join Date: Dec 2001 Location: Pittsburgh, PA, USA	no isolation. you still need an isolation xformer. this would be a very expensive solution to inrush current!

17th December 2002, 05:22 PM	#4
tiroth diyAudio Member Join Date: Dec 2001 Location: Pittsburgh, PA, USA	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.

17th December 2002, 09:08 PM	#5
blank527 diyAudio Member Join Date: Nov 2001 Location: Haarlem	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

18th December 2002, 02:27 PM	#9
janneman diyAudio Member Join Date: May 2002 Location: Where Germany, The Netherlands and Belgium meet Blog Entries: 6	It might help to think of a variac as a huge potmeter with a lot of inductance, which is what it really is. Jan Didden

18th December 2002, 02:30 PM	#10
gnomus diyAudio Member Join Date: Jan 2002 Location: New York City	Great idea. And much cheaper than a voltage meter. __________________ Paul

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