# Electrical question

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I have a standard electrical outlet supplied by a 20 amp breaker, outputting

118 volts. For something being planned, that circuit is needed to output 125 volts at 20 amps capability. Is there a good method for this, and what is the most economical, feasable, and most practical method? It needs to be fairly close to the 125 volt figure, as much as practical. Obviously, a step-up transformer might be the solution, but what about the ability to find one to output the 125 volts? Is there a practical homebrew buck and boost method? I've done a decent amount of industrial, automotive, house wiring and troubleshooting, but I'm a relative newby at this type of theory, so be gentle.

Thanks, RJ

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A 20 amp 6.3 volt filment transformer used in series with the line as a boost trans. should do the trick. The trick will be to find such a tran. in this day and age. What is so critical that you can't use 118 volts. Signal Tran. Co. has a very good selection of power trans.

Chuck P.

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You only need a transformer to supply the difference (7 V) at 20 A.

6.3V should be near enough and some old gear had absolutely massive transformers for the valve heaters. I dismantled some old test equipment back in the 80's that had an 8A heater winding. Ask Gunner if he's got any suitable transformers.

When you've got one, connect the secondary in series with the live wire to the load and the primary accross the suppply. If the output voltage is reduced from the input, reverse the secondary connections, its phased wrong. Warning: you will draw approx 5% more current from the supply than the load takes.

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I haven't seen anything that can't take a 5 percent drop in voltage. IIRC, 117 is actually the mathmatical number for what we call 120 volts.

Wht is the application?

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You need a device called a Variac, which is a variable autotransformer.

a new one would be pricey, but they've been around for a long time and can be found at surplus outlets, auctions and even junkyards that have electical/electronic stuff.

Trying to do it with a parasite transformer is tricky as the line voltage is rather variable with time of day and local load.

Is the precise 125 volts really necessary? Is regulation critical?

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As a refinement you can use a small Variac (adjustable transformer) to feed the 120 volt feed to the 7 volt transformer. Variac's in the 3 Amp size are readily available.

Bill K7NOM

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I built a buck transformer system, using a commercial new autotransformer, to deal with a high voltage situation, 15 amps rating. Info on my web site below. If you know your voltage in and out and amps, call the supplier and they will recommend a specific autotransformer.

There is nothing wrong with using a 5 or 7 volt transformer to buck or boost, as long as it is rated for the required current.

Check the wiring twice and check the voltage in and out before you try to use your buck/boost transformer system.

I wonder also what is so critical that it cannot run on 118 volts. Most equipment will run at +5% - 10% voltage.

Richard

Backlash wrote:

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Get yourself a Sola 'Constant Voltage Transformer'. I got mine from as local company 'cleaning out old junk' and it's good for about 30 Amps! Weighs about 80 friggin' ponds( think sack of cement as big too!). probably find them on Ebay, local colleges,electrical test shops,etc. Any other method of stepup transformers,Variac etc will NOT be regulated which is what I think you want. Anyone that sells Variacs should know/have the Sola

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Why do you need exactly 125VAC??? Line voltage from the power company is somewhere between 108VAC and 135VAC and equipment designed to run on the line voltage will run correctly anywhere in this range. As such, hook up your equipment and enjoy. If you have a specific requirement for exactly 125VAC I'd love to hear about it. Please note that a piece of equipment that says it runs on 125VAC is more likely an old piece of equipment that doesn't have the more mondern

108-135VAC tag on it. Electric power is electric power for all practible purposes.

-- Why isn't there an Ozone Hole at the NORTH Pole?

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For a 20A demand from a 20A breaker, you're most likely gonna have other problems. Small variacs or small transformers won't be much good.

Any application that requires 20A should never be connected to a standard household wall receptacle. Installation practices often vary from insanely wrong to barely adequate, and device design specifications are questionable, even when the parts are new.

In case you don't already know, line voltages fluctuate. All 'ya gotta do is stick a sensitive chart recorder on your supply, and watch it for a week.

If you absolutely need to have 125VAC minimum, you might consider coming down from 240V (apparently 236 in your case) instead of going up from 118V.

..the most economical, feasable, and most practical method?

What about safe? Electrocution might not be so bad, but I suppose death by fire is nasty

WB .............

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For lower current requirements I have made buck-boost transformers from old filament power transformers. Say you have 112 volts and need

124, you tie one end of the 12 volt vilament winding to the primary, and if you got the right ends together going frm the other end of the primary to the free end of the secondary, you will have 124 volts. If you got the wrong end connected, you will have 100.

With a big old radio transformer with 5 volt, 6 volt, and 12 volt secondaries, you have a fair amount of flexability.

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Where are you? I've got 3 old Solas sitting under my workbench. The buggers run warm, and being old and not perfectly clean they smell a bit - but if you can get to Waterloo Ontario you can have one for free.

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