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 Post subject: Re: leads from Variac ?
PostPosted: Dec Sun 29, 2019 9:04 am 
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Joined: Dec Sat 28, 2019 4:18 pm
Posts: 125
Newcomer.

I picked up a Chinesium "Variac". I added two sets of pin jacks, one set across the output so I could accurately monitor the voltage and one set with a .1 ohm 20 watt resistor between them in series with the output wiper to monitor the current.
100 mv AC equals 1 amp. Because the thing goes past 135 volts, I figure that I can afford to lose 1 volt.

John


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 Post subject: Re: leads from Variac ?
PostPosted: Dec Sun 29, 2019 10:47 am 
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Joined: Oct Sun 15, 2006 12:57 pm
Posts: 7088
Location: Liberty, Missouri
Quote:
...BTW, I have tried a Kill-A-Watt and had to get my variac up to 90 volts before it would show anything...

The Kill-A-Watt itself needs to have power, and is apparently designed to start operating around the 80 to 90 Volt region, which is its lowest intended input Voltage. If you do a search, you will find hacks to make it operate at much lower applied voltages, but I don't think they are actually worth the effort.

Note the position of the Kill-A-Watt meter in my schematic. The Kill-A-Watt meter is before the Variac so it is operational regardless of the output Voltage of the Variac, and, as I said earlier, regardless of its position, Watts drawn by the load, which includes the Watt or two consumed by the Variac, are accurately indicated.

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 Post subject: Re: leads from Variac ?
PostPosted: Dec Sun 29, 2019 11:40 pm 
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Joined: Nov Fri 10, 2006 12:24 am
Posts: 2685
Location: Thornhill, Ontario, Canada
Y2KEDDIE wrote:
... If you find a variable transformer without a case you might consider building a combination variable source with an isolation transformer (a must for safety to you and your equipment)...
A long time ago I acquired a naked variac (no case.) I built it into a large metal coffee can with an EIC 120 VAC input (actually 125 VAC here) and a dual receptacle output, both on the curved sides of the can... no space for meters. It's been on my bench for years feeding a separate 120/20 VAC 5A transformer used for series bucking to the main bench supply (with separate 1:1 isolation, of course.) This allow the full range on the variac knob to control zero to 20 volts of bucking to the bench (separate AV voltmeter on that.)
My regular service variac has volt and amp meters.
Cheers,
Roger

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Roger Jones,
Thornhill, Ontario
Ontario Vintage Radio Assoc. http://www.ovra.ca


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