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Old 4th November 2005, 10:17 PM   #1
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Default What size Variac for DIY work.

Do I need a 10, 15, 20 or 30 amp Variac for most of the DIY work I may do in the future? I am currently building my first amp P101@180wpc. and will be moving on to a variety of other projects in the future.

Thanks for the help ..........

Cheers,

David
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Old 4th November 2005, 11:04 PM   #2
BWRX is offline BWRX  United States
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Old 4th November 2005, 11:08 PM   #3
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Hi Gavinator68,

Funny you should post this as I am currently wiring up my autotransformer. I bought 240V 8 amp one which I hope is big enough. My logic dictates that 8 amps out of the variac means a little greater than 8 amps in, and seeing my house wiring and switching is rated at 10 amps, its OK. I know some house circuits are 15 amp for 3 phase machinery such as air conditioners.

Most of my stereo amps have 240v 2 to 3 amp fuses so I know these amps draw less than that. In rush currents will be higher.

In the US, with 110v I guess that current rating must be more (double?).

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Old 4th November 2005, 11:12 PM   #4
anatech is offline anatech  Canada
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Hi David,
I use a 2A model for testing. I rarely need the 10 amp model I also have. Unless you are building some real class A stuff, or really large tube amps, a 2 amp unit will do.

The advantage to the smaller variac is that you can mount it in a smaller panel or box with voltage and current meters. That and the smaller fuse will blow if something goes wrong with your back turned. And it will sometime.

-Chris
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Old 4th November 2005, 11:42 PM   #5
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Hi Chris,

Are you recommending additional fuses on the autotransformer?

My PSUs always have fuses (2 or 3 A) and obviously the power circuit has a 10 A circuit breaker.

Thanks
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Old 5th November 2005, 12:04 AM   #6
anatech is offline anatech  Canada
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Hi Greg,
Not necessarily. Many amplifiers I work on may have 6 to 15 amp fuses fitted. Since for initial power up and some testing you aren't running the unit at full power, a small fuse can save parts.

When powering up my own things that can be true as well. Some amps can break into oscillation only when reaching a higher heatsink temp. Then, wing! off it goes when my back is turned.

Really it's your own preference. What works for me may not be what you want.

-Chris
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Old 5th November 2005, 02:46 AM   #7
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Thanks Chris,

I can see how having a intermediate fuse would be handy for you. You don't need to worry about the customer's amp fuse rating because you have a small one in the test gear. For a hobbiest with relatively few amps I could just use a smaller fuse in the amp for testing.

I was going to have all the bells and whistles on my autotransformer but it ended up a major project that I would put off forever so I decide to use a small power board with an overload and sealed plug etc.

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Old 5th November 2005, 03:58 AM   #8
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Consider how you will use the variac. 8 amps at 240v is almost 2000 watts. 2000 watts is a lot, and I don't care what your mains voltage is.

My 120v unit is fused at 8 amps. It is enough for anything I do. I will run a 4000 watt amp through it if I want to. I use the variac to test for excess draw without pulling the mains full up. I never use it to power the amp to full output. If a transistor is shorted or a rectifier or whatever, it will show up at idle, and the amp will draw heavily. I simply have no reason to dial the mains down while an amp is trying to produce 3000 watts.

Once the excess draw problems are sorted out with the help of the variac, I then power the unit from the mains directly. So a 2 or 3 amp unit should be fine for most applications in audio service.
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Old 5th November 2005, 03:08 PM   #9
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I bought a 20A unit just because I got a good deal on it. The thing is, I can put a two amp fuse in it if I want to.

I always use a light bulb in series with it when I'm first powering up an amp. The light bulb will ususally tell me if there is something amiss. My variac also goes up to 140VAC output.

Blessings, Terry
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Old 5th November 2005, 03:37 PM   #10
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Hi Greg & Enzo,
That's about it. I still have my 10A unit, no voltmeter. It would be extremely difficult to mount one, as it is, the current meter is in a separate box. Greg, I can see why you put that on hold.

The smaller variac is permanently mounted in a rack mount plate on my bench. It them becomes very efficient to use. The larger one is a pain.

Terry,
The light bulb thing works well most of the time, but some supplies don't like that too much. Switchers may complain. I power them up with two supplies. One for the housekeeping, the other for the main supply. They will draw high currents at lower voltages. Sometime they will do strange things through a light bulb and you end up troubleshooting a non-fault. As long as you are aware of this, no problem.

-Chris
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