Variacs and Their Uses
What is a Variac? as the surname suggests, it varies AC voltage. The descriptive term would certainly be"variable AC transformer," yet the profession name Variac was provided by the basic Radio companyfor numerous years, and also eventually that came to be the generic name.
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Here are four variacs on mine workbench:
We"ll look at at every of them later in this article.
Why usage a Variac?
The heat voltage in American homes is a in the name 120 volts AC, however a variac lets you change that voltage up or under (usually, down) because that variouspurposes.Use 1: Taking precise Voltage Measurements
If you regain vintage radios or TVs, you"ll an alert that schematic diagrams normally contain voltagereadings. Often the an essential voltages are displayed in the schematic itself, and also some business manuals encompass achart showing the correct voltage for every pin of every tube. The readings point out the voltage you"d expectto measure at miscellaneous spots in the radio or TV once it is working normally.
For example, this schematic for a 1957 RCA television shows that you should measure around 200 volts DC at pen 5of V7, the 6DT6 audio detector tube:
When diagnosing problems, it"s advantageous to compare those ideal voltages to what you in reality measurein her radio or TV chassis. If the schematic states 200 volts DC and also your radio only produces 35 voltsat the spot, other is wrong.
To optimize accuracy, it"s preferable to collection your line voltage come the same level as the men who made every ofthose old measurements. Otherwise, you"ll it is in comparing apples to oranges.Often, the schematic speak you to keep the heat voltage in ~ 117 volts:
117 was standard when these old sets were made, but nowadays 120 voltsis the norm, and also your actual house voltage might differ, depending upon variables suchas how far you live indigenous the strength sub-station.
To test voltages accurately, you set the variac"s calculation voltage come 117 VAC and then plugyour radio or TV into the variac. This replicates the environment in i m sorry thoseold readings to be taken.Use 2: gradual Power-Up
You can also use a variac once trying out an unrestored radio or TV for the an initial time, once itsoperating condition is unknown. The idea is to set the variac in ~ a short level—say,50 volts—and then gradually increase the voltage, very closely watching your set for anysigns the trouble. This is much safer than merely plugging your set into the wall surface and turning it on,which subjects whatever to a sudden strength surge.
If you perform this test without a existing meter, then her eyes and ears room the just measurementtools. Smoke, frying sounds, or solid smells are risk signs, the course.
Having a present meter in her variac permits a an ext scientific test.My variac in the clean Lucite instance has an ammeter for checking a set"scurrent draw. Here, I have plugged mine RCA 14-S-7070G TV into the variac, which is set for117 volts. The ammeter in the facility shows the the TV draws around 1.4 amps.
What go a analysis of 1.4 amps mean? Well, a brand on the bottom the this TV states that that draws about165 watts. To transform to watts, us multiply the amps times the heat voltage. Multiply 1.4 * 117 provides us 163.8watts, really close to the stated value that 165. That"s fine within the best ballpark.
Similarly, let"s to speak you desire to strength up a radio choose my GE F-63. That set has 6 tubesand the draws around 70 watt according come its label. Once I it is provided it top top the variac in ~ 117 volts, theammeter read around .6 amps. This time, arithmetic tells united state that .6 amps times 117 volts equates to 70.2 watts, so that radiois additionally drawing a typical amount. (In hindsight, it would have actually been handier to construct this variac with awattmeter fairly than an ammeter; then I might read the wattage directly without law arithmetic.)
When you power up an unrestored set with a metered variac, the meter lets you watch because that excessive present draw.If mine metered variac confirmed that mine GE radio to be drawing, say, over 100 watts when I had increased the ACline voltage to just 95 volts, that would show a problem, most most likely in the power supply.
Conversely, if the radio draws tiny or no current, also at complete line voltage, that also indicates aproblem. However a dead radio is other you can diagnose v your eyes and ears.
The factor to use a metered variac during power-up is come watch because that excessive present draw.In that case, you must power under immediately and also end the experiment prior to you damages the setby running it at full power through defective components.
If your collection uses a pipe rectifier and a strength transformer, you may not notice much life until youincrease the heat voltage to roughly 80 or 90 volts. Below that point, the rectifier tube won"t operateand the rest of the set (which relies on the rectifier) will certainly draw little current. This is normal.
In a functioning TV or radio, you"ll also an alert that the current usage spikes upto a higher-than-specified level throughout the first 30 seconds or so, and also then settles ago down. Thisis additionally normal. Tubes that are cold use much more current, and also their usage easesdown after ~ they warmth up.Use 3: to run at diminished Line Voltage
If you own a valuable revitalized set, such together my RCACT-100 color television, friend may constantly want to power it up making use of a variac. Graduallyincreasing the line voltage offers the collection a softer begin than suddenly turning it top top a fullpower.
Some old TVs also had integrated "soft start" mechanisms. MyDuMont RA-103 has a circuit v atube and also a relay, to hold-up the rise of B+ present for a while. MyRCA CTC-7 has a easier B+delay, and also others choose the CTC-11 achievea similar result by putting a thermistor top top the strength line. I can"t think that anyradios with soft-start circuits, however.
After the collection has come on, you deserve to leave it come play at a diminished voltage such together 117 VAC. Somepeople believe this expand the life of tubes and also other components. If girlfriend experiment a little, you"llfind that countless radios and also TVs work simply fine in ~ a lower voltage, even as low as 110 volts.
In some vintage TVs, particularly early color sets, it"s an important to limit the existing drawn through thehorizontal calculation tube, to prevent damages to the "unobtanium" flyback transformer.Operating in ~ slightly lessened line voltage gives a much more comfortable setting for this TVs.
For a permanent installation, there space other means to alleviate line voltage, such as structure ahumbucking transformer, but that"s a object for another day.Use 4: Reforming Electrolytic Capacitors
Another use for a metered variac is come "reform" an oldelectrolytic capacitor to reclaim it to working condition. The idea is to boost the voltagegradually—even over a period of hours—with an eye ~ above the meter, until the capacitoroperates normally and it becomes safe to usage again.
I open minded can"t recommend this procedure through capacitors that room several decades old. Ns haveexperimented v it numerous times. In part cases, the capacitor never ever reformed—it wassimply as well deteriorated inside. In others, the capacitor appeared to revolutionary at first, however it inevitablyfailed prior to long.
New capacitors room cheap and easy to replace. Mine recapping articleexplains various methods to change electrolytics. Including methods that preserve the authenticappearance of her TV or radio. Don"t danger damaging your set by relying ~ above a reformed capacitor.
Types that Variac
Let"s take it a closer look at these variacs. Earlier, we experienced one i m sorry a friend of mine builtinto a Lucite case, in addition to a voltmeter and also ammeter. The big black knob top top the appropriate is therotary voltage control.
Behind the voltage knob is a basic Radio Corp. (GRC) version W-2 variac, rated for 2.4 amps.Its appearance is very similar to this larger, unwired unit: a model W-5 rated for 6 amps:
The amperage rating the the variac is important. You require one rated higher than the maximumcurrent consumption of the gadgets you plan to test. The 2.4-amp-rated variac in my Lucitemodel is enough for nearly all radios and many black and white TVs. But it"s also wimpy formy early color TVs, which have the right to burn an ext than 300 watts. Because that blowtorches like that, girlfriend needa beefier variac.
I to buy the bare variac in ~ a radio swap accomplish for $20, a cheap price. I arrangement to mountain it ina case together with meters, prefer the very first one.
Here"s a nice small Calrad design VC-6 in a metal case. It"s a 5-amp variac with twooutlets, each rated because that 2.5 amps:
The Calrad has actually a voltage calculation meter, but no consumption meter.
I saved the manliest variac for last. This Powerstat design 116 has no meter, only a rotating voltage control,on/off switch, and also an output plug. The is rated for a chuck 7.5 amps:
Using a Kill-A-Watt to Meter her Variac
If you very own an unmetered variac choose the Powerstat unit simply shown, friend canplug in a handy little maker called a Kill-a-Watt to serve as her voltageindicator, wattmeter, or ammeter. Here"s what it looks like:
The Kill-a-Watt is design to aid monitor electrical usage of household devices.Here, it"s set to measure the line voltage (which wake up to it is in 120.2), yet it can additionally measure amps and watts.You push a switch on the prior to adjust what it measures.
To usage it, plug the Kill-a-Watt into your variac, and also then plug your radio and TV right into theKill-a-Watt. Leaving it in ~ the voltage setting while you set your variac"s voltage, thenswitch come the watts setting to measure the wattage provided by her radio or TV.
Voila! friend now have a metered variac. The Kill-a-Watt is commonly available.I purchase mine for around $20 at home Depot.
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Wiring a Variac
If you have actually a suitable enclosure and also a ceiling variac, wiring it up is simple.These photos show the schematic used to develop my Lucite-cased unit, and itsvariac backplate through terminals numbered as in the schematic:
As quickly as I uncover a an ideal enclosure, I"ll build my unwired variac prefer the Lucite one.This time, I"ll incorporate a wattmeter quite than one ammeter. That will save medoing the arithmetic to convert an amperage analysis to wattage.