Two Variacs for Coarse and Fine Adjustment....... - diyAudio
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Old 8th July 2010, 09:10 AM   #1
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Default Two Variacs for Coarse and Fine Adjustment.......

Hello All,

I would like to build a new tool for the shop in which I use the first variac for coarse line voltage adjustment from 0 to 130v and another variac for fine adjustment of the specified line voltage determined by the coarse adjustment. I would prefer the second variac to represent plus or minus 10 percent of the coarse adjustment over its full range so that the second variac acts as a sort of very fine vernier adjustment to the coarse adjustment.

Does this make any sense? Is it possible? Any suggestions?

Cheers,

David
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Old 8th July 2010, 09:34 AM   #2
AndrewT is offline AndrewT  Scotland
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I would be tempted to use a multi-tapped secondary for the coarse adjustment.
This could even be an autotransformer since that is no worse, safety wise, than a Variac.
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Old 8th July 2010, 10:23 AM   #3
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Quote:
Originally Posted by Gavinator68 View Post
Hello All,

I would like to build a new tool for the shop in which I use the first variac for coarse line voltage adjustment from 0 to 130v and another variac for fine adjustment of the specified line voltage determined by the coarse adjustment. I would prefer the second variac to represent plus or minus 10 percent of the coarse adjustment over its full range so that the second variac acts as a sort of very fine vernier adjustment to the coarse adjustment.

Does this make any sense? Is it possible? Any suggestions?

Cheers,

David
Yes it is possible. You'll need a high (mains) voltage variac and a low voltage variac and you use these two in series. (Sometimes you find a small variac in series with a mains transformer in a tube tester to regulate exactly the voltage used for testing).

I found a 15V variac on the local auction website and it is earmarked for just that purpose.

AM
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Old 8th July 2010, 10:50 AM   #4
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I have seen something like that set up. There was a high voltage variac and a low voltage one with a small transformer in between. I thought it worked the way I am thinking for my idea but i wasn't sure.

Click the image to open in full size.

Click the image to open in full size.

I have seen reference to limiting the range of any given variac using a supplementary transformer. This info is listed in any of the variac info sheets from GR but again, I was not sure if I would end up with the + or - 10% of line voltage using this arrangement. I would like the whole turn on the smaller variac to represent the very fine adjustment of the first. Is this what would result if I used them in series as you describe?

Cheers
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Old 9th July 2010, 09:09 AM   #5
glennb is offline glennb  Australia
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A typical variac can be set to about +/- 1 volt quite easily. In theory, a "fine adjustment" variac will allow it to be set to sub-volt levels, but is it really worth it? I find the local line voltage varies up and down a couple of volts over a one minute period, so what's the use of fine adjustment?
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Old 16th July 2010, 03:31 PM   #6
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If you have a centre tapped variac and a transformer with low voltage secondary you can connect the transformer primary between the variac centre tap and slider. If you then connect the secondary in series with your load then varying the variac will buck or boost the load voltage by up to the the nominal secondary votage (say +/_ 15 volts) O/P current is limited by the seconday current rating so only a small variac is needed. of the transformer.

Barry
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Old 16th July 2010, 03:50 PM   #7
Did it Himself
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Why? Your equipment is designed to cope with the upper and lower limits of the mains voltage.
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