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Old 17th August 2015, 01:31 PM   #1
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Default variac question

Obviously not the Diyaudio moderator

Got my shelf a nice nos variac of excellent quality which actually is a III phase variac and has 3 "pots"with one mechanical axis

Point is that the variac is kinda small probably 150 W per coil ...150W is relatively enough to start any amplifier just for a safe start up and to observe offset and bias ( probably in class AB only )

My idea was to bridge all 3 of them to have like 450W available ( single phase of course ) My gut says that is a bad practice and you cannot parallel circuits that are not equal but since current demands are low anyway any body thinks that i might get away with this ?

Kind regards
Sakis
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Old 17th August 2015, 01:36 PM   #2
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Sorry, a slight error in the voltage output between the variacs and they will get very hot and may destroy themselves.
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Old 17th August 2015, 05:07 PM   #3
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Possibly, but it is very easy to test:

Wire them up in single phase (ie in parallel) EXCEPT for the wipers - keep these separated.

Now measure the voltage between wipers as the control is rotated. Ideally it will always be 0v if everything is perfect and symmetrical. More than a volt or so and I would start to get worried and give up!
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Old 17th August 2015, 05:26 PM   #4
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Hi Sakis,
Hey, you can soft start an amplifier, preamplifier and a tuner all at once! Cool!

The one I use on my bench is closer to 300 watt, but it is normally all I need. I keep a beast (10 ampere) for those larger tube amplifiers, or class A amplifiers. Normally all you will need is about 200 watts, so if you could combine the three windings the derated power would be just about perfect.

To combine the windings, do tests with maybe two analog pointer type meters between the windings and run it up slowly. The analog meters will be easier to keep an eye on and you can see trends. Then try the same test with a load on it, but use series resistors to help make the transformers share - and watch the meters again as you run it up. You could use a flood light as a load as your variac may go above 100%. That's good for testing units when the mains go a little high. You may want to keep some resistance in series with each coil just to make current sharing better.

-Chris
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Old 17th August 2015, 05:47 PM   #5
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I was thinking to wire up ballast resistors in the output ...Can anyone suggest a value like 1R could be ok ?

The variac is really precision made first look up gave some difference between 3 circuits like no more 1-2% in various points on the scale

Will see i will put it at work and see how it goes

Kind regards
Sakis
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Old 17th August 2015, 06:25 PM   #6
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Hi Sakis,
Well, 150 watt per circuit, so about 0.65 ampere at 230 VAC - right? Using 10 ohms (just a number from the air), you would have 6.5 V drop dissipating approx. 4.25 watts at full power (likely). So 10 ohms might be high, but you get the idea. 4.7 ohms would dissipate about 2 watts each with 1/2 the voltage drop. That sounds better. You might want to make these 10 watt units so they don't act as fuses, then fuse the input side to cover all circuits. A breaker might be better come to think of it. I'll be putting a circuit breaker in my unit, I've gone through a lot of fuses over the years. I might make that change right now as a matter of fact (parts are just sitting there ready to go).

-Chris
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Old 17th August 2015, 06:44 PM   #7
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Οκ ..That sounds fine will give it a shoot to see how far i will go from there

thanks for the input
Nice to get help from you Chris Thanks !!!
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Old 17th August 2015, 07:03 PM   #8
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Hi Sakis,
It is rare that you need assistance, and a real pleasure to be able to give you a hand!

Let me know what you decide for this project. The 450 watt assembly is just perfect for the test bench. I often wish mine was a little bit larger.

-Chris
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Old 17th August 2015, 07:27 PM   #9
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I have on my bench a 2000W variac one of the very first tools i got extremely useful Point is that the III phase variac is far more elegant space wise considering that my bench is packed

I could actually save some space which is needed.... badly!!!
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Old 17th August 2015, 07:44 PM   #10
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Hi Sakis,
I understand completely!

Mine is built into the shelving to hold instruments. The big guy is stored off-bench for exactly the same reasons you have put forward.

You are going to love having one built in. You can plug things in without having to hold the darn variac.

-Chris
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