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 Post subject: Variac voltages
PostPosted: Mar Tue 29, 2016 12:30 pm 
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Location: Pocatello ID
TDGC2-2 VARIAC VARIABLE AC POWER SUPPLY/2000VA

I just got one of these 20 amp variacs. It seems fine physically, but I notice it is 5 to 10 volts off at the higher voltages e.g. at 110 volts I'm getting 115. I checked my line voltage at the socket and got 118.3.

I'm intended to use this for bringing up antique radios for testing and/or plugging in some of my antique receivers/transmitters to run them at the lower line voltages of 110.

For these purposes does this variation matter? If it's a defect I can send it back, but I realize variacs are sort of dependent on the input voltage. I'm thinking about getting one of those $12 digital voltage/wattmeter meters, putting it on the power cord of the radio and calling it good.


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 Post subject: Re: Variac voltages
PostPosted: Mar Tue 29, 2016 1:10 pm 
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The dial on a variac is often calibrated in percentage of maximum, which might be something like 140 volts. Or maybe percentage of input...... I'm not sure I remember seeing one with the dial marked in voltage.
And, yes, the output depends on the input.

I would simply put some marks on the dial, or use it with a voltmeter

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 Post subject: Re: Variac voltages
PostPosted: Mar Tue 29, 2016 1:25 pm 
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I always use mine with a multimeter as a voltage monitor, especially during calibrations. I find mine will fluctuate during hours of use and I need to adjust during the repair. Sometimes I might bump the dial in the work area or the line voltage may change.


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 Post subject: Re: Variac voltages
PostPosted: Mar Tue 29, 2016 2:20 pm 
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Your Variac G-R brand of variable transformer may be wired for boost and the wrong scale is in use. Often there are two scales one on each side of the scale plate and the 0-100 or 0-120 may be exposed. There is no need to have it wired for boost either. Not unless you intend to cook something. The complete winding of most variable transformers, commonly have a tap some 15 to 20% from each end. There are two taps because the unit can be mounted to a panel which would reverse its function. If the compete winding is energized by the line at whatever the line is the highest voltage will be the line, therefore, the 0-100 scale would be used. if the line is wired to one end and the FAR tap on the winding the unit will boost the line some 15 to 20 % once the brush has passed that far tap. Again these connections to the tap would be reversed of the unit is panel mounted and the other tap would be use.

Under no conditions connect the each tap to the line or a tap and the nearest end to the line. This would over excite the core and the windings would burn. Same for the brush, it never gets connected to the line. Finally, no variable transformer of the brush type is designed to be set and forget. The brush will oxidize the copper winding in time and begin to overheat. For a permanent voltage reduction or increase use a buck/boost transformer.

Do check your radio/transmitter for the correct filament voltage on a tube that is furthest from the filament transformer when running reduced line voltage. Stay within the manufactures specs. for the tubes. Exceptions are 201a types that can be run at reduced voltage, that's how the gain is controlled.

Yes, I would use a voltmeter and an amp meter. Because of the large range of this unit, newer digital meter(s) would work well. There just is not enough resolution on a 20 amp analog meter to work with a small radio. You may want to opt for a switchable Standard Edison socket for a series load to protect the radio or other device under torture... You can put a 15 amp salvaged circuit breaker on the input. Keeps one from putting the lights out in the shop if a short is either made or develops... Do run your shop with a GFI. Then you will be able to post here tomorrow.

Trust me I never built a rig like that, I have always hay wired my bench tools for the particular need, to do the job, then take them apart and put under the bench. I often grab a variable transformer for who knows what, like controlling to some extent the 425 watt soldering iron. It will glow red hot after 20 minutes on the line... The "variac" helps keep the heat down. Sometimes I will boost a battery charger... Or control a motor, doesn't work well with motors, O.K. for very light loads like slowing down the jig saw..

GL, stay safe,

Chas

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 Post subject: Re: Variac voltages
PostPosted: Mar Tue 29, 2016 3:36 pm 
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Mine goes from 0 - 140 volts, and the dial is calibrated in percent, 0 - 100. It assumes a 120-volt input

100% = 140 volts
120 volts = 86%

Line voltage here is ~ 125, so there's a small correction needed

I have a pencil mark on the Variac dial for various output voltages. Someday, I'll get a voltmeter hooked up to it.

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"Measure voltage, but THINK current." --anon.


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 Post subject: Re: Variac voltages
PostPosted: Mar Tue 29, 2016 3:48 pm 
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Bridkarl wrote:
I'm thinking about getting one of those $12 digital voltage/wattmeter meters, putting it on the power cord of the radio and calling it good.

Some of us get a little more elaborate. The Variac on my bench incorporates an isolation transformer, fuses, a voltmeter and an ammeter:

Image

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 Post subject: Re: Variac voltages
PostPosted: Mar Tue 29, 2016 8:59 pm 
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It's always good to have meters after the Variac in the panel, saves a lot of monkey motion, and they are easily calibrated to be "close enough for Gubment work". :roll:

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 Post subject: Re: Variac voltages
PostPosted: Mar Tue 29, 2016 10:50 pm 
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A Variac is just a transformer (although adjustable) and shares the same characteristics as other transformers. With a fixed setting, the output voltage is proportional to the input voltage and all line voltage changes will be reflected in the output. Also, it has losses, so the output voltage will drop when it is loaded; the heavier the load, the lower the voltage. The dial plate is only a rough guide to the output voltage.

Jim Mueller

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 Post subject: Re: Variac voltages
PostPosted: Mar Wed 30, 2016 12:51 am 
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On mine, not even rough.....

With 120 in, 120 out is 85.7 on the dial.
With 125 in, set the dial to 82

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"Measure voltage, but THINK current." --anon.


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 Post subject: Re: Variac voltages
PostPosted: Mar Wed 30, 2016 11:05 am 
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Sounds like the over-voltage tap is connected on that variable autotransformer and/or the dial plate needs to be flipped. The dial plate is only approximate; actual output voltage depends on input and load, which is why a voltmeter across the output is useful. But it should not be that far off.

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 Post subject: Re: Variac voltages
PostPosted: Mar Wed 30, 2016 1:10 pm 
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But, on some--including mine--the dial is not intended to read voltage.......

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"Measure voltage, but THINK current." --anon.


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