Refubish/Repair (and modify) General Radio Variac V5H - Page 1
Forum section: electronics / repair

author: xabean
author_rank: Newbie
timestamp: December 17, 2017, 12:47:11 am
content: So, I inherited a bunch of WW2 era US Signal Corps and amateur radiovacuum tubegear, and the related stuff that goes along with it. I think I've got 9 vacuum tube testers, and thousands of vacuum tubes. Anyway. I apparently now own one of these:https://www.recycledgoods.com/general-radio-v5h-variac-auto-transformer-pri-230vac-sec-0-260vac/-- it looks to be in pretty good shape. I disassembled the case, took the power cord section home and replaced the cord. Easy peasy, lemon squeezy.Only until I looked at the aforementioned link, did I notice its specs:INPUT230VOUTPUT 0-270V2 AMPS...which means that it's got a tiny bit of travel beyond the input taps on the transformer to boost voltage. Problem is: the input voltage is 230VSo, the variac hasseventaps on it:Of course I left the body at home and can't ohm out the different taps on the transformer's windings; but anybody have any bright ideas if it's possible, or a good idea to try to rewire this thing for:INPUT110VOUTPUT 0-2??VIt came wired to terminals 2, 3, and 5; 2 + 5 were output common, 2 + 3 were input common, which I suppose makes sense for a variable transformer.It looks like the taps are:1) 10 o'clock tap2) winding start3) brush tap4) winding end5) 2 o'clock tap6) 7 o'clock tap7) 5 o'clock tap

author: Bratster
author_rank: Frequent Contributor
timestamp: December 17, 2017, 01:28:53 am
content: With the existing wiring configuration that it had, if you feed it 120 volts it will work just fine. Output will be 0 to 140ish.With all those taps you may be able to get output voltage higher, but the current capacity will take a huge hit.Sent from my Nexus 6 using Tapatalk

author: Electro Detective
author_rank: Super Contributor
timestamp: December 17, 2017, 03:14:23 am
content: AFAIK, you can't 'variac' the input to these, only the output, unless it has 120v and 240v taps for the input (unlikely)So if it's a 230/240 volt unit, you will need to use a 120 input to 240 output step up transformer (preferably an isolation type for a twofer function) to power up the variac,and then in theory and practice you will have an awesome variac that goes from 0 to 270 voltsYou will have to do some 'current' math and fuse the input and output on the variac correctly. Better to take out a fuse rather than kill a winding or carbon brush

author: xabean
author_rank: Newbie
timestamp: December 17, 2017, 04:18:14 am
content: I'm gonna ohm it out tomorrow, and try running low voltage AC through to get the winding ratios ... I'm considering wiring it up so that 2+4 the output, and seeing what voltages come out at that point.But yes, two amps is more than overkill for what I intend to use this for, I'll gladly put some 1A or 0.5A fuse in-line with it.

author: ArthurDent
author_rank: Super Contributor
timestamp: December 17, 2017, 04:23:06 am
content: I believe you remembered the connections wrong. I believe 2 is common to both in and out and 5 is 120V in. Terminal 3 is the wiper that goes from 0V to 140V. If I recall the plate on the knob is reversible depending on the taps you are using. Just divide the voltage it shows on the dial plate by 2 to get the correct voltage out with 120V in. The max current will be the 2A on the rating plate.If you were to connect the input between 2 and 3 you would burn the winding out as you turned the knob toward 0V or you would blow a fuse. I would strongly advise using a 3-wire cord with the neutral connected to terminal 2.Oh, and I'm editing to add one last thought. Because this is a transformer (variable autotransformer), when the wiper is toward the low end, the current flow in the lower part of the winding required to blow a 2A line fuse will be determined by the ratio of the in/out voltages. If you set the output to 12V with 120V in then it will take approximately 20A (10:1 ratio) on the output to blow the 2A line fuse and the winding on the lower part of the variac will be seeing 20A which could burn the winding open before the line fuse blows. Best solution is to use two 2A fuses, one for the line and one on the output in series with terminal 3.

author: SeanB
author_rank: Super Contributor
timestamp: December 17, 2017, 12:32:42 pm
content: Input side pin 2 is neutral, pin 6 is line. then output is pin 2 common and pin 3 variable, which will allow you to get 0V to 270VAC at the output, with the caveat that below 120VAC the max current is limited to 2A, and above 120VAC the input current is limited to 2A. Output is not isolated. If you want a lower max voltage connect input line to pin7 instead of pin 6, and this will limit max output voltage to around 210VAC, with the same caveats about current and voltage limits. Max current 2A on output or max power 240VA, whichever is lower.

author: xabean
author_rank: Newbie
timestamp: January 07, 2018, 10:52:14 pm
content: Found documentation!http://www.jamminpower.com/PDF/GR%20V5%20Variac.pdf

author: Electro Detective
author_rank: Super Contributor
timestamp: January 08, 2018, 05:32:38 am
content: Quote from: xabean on January 07, 2018, 10:52:14 pmNice PDF manual score mate, thanksHope your variac is coming along well, and please triple check everything before applying powerUse in-lne fuses and globes, anything to save the unitMiss Label and Miss Wire can be real b!tches sometimes

author: helius
author_rank: Super Contributor
timestamp: January 08, 2018, 06:31:43 am
content: The plate says "voltsshown for270V". As you should know, transformers are just voltage ratio devices: N primary turns & M secondary turns makes for an input:output voltage ratio of N:M. You can apply any primary AC voltage you like as long as the windings are insulated to at least that much.This one is an autotransformer: there is no primary or secondary, just a single winding with several taps. The schematic isshown for270VAC from the lowest to the highest tap, with 40, 75, 40, 75, 40 between each adjacent pair of taps to add up to 270. This is fully ratiometric, so it could be redrawn with 20, 37.5, 20, 37.5, 20 between each pair to add up to 135. The input and output cable wiringdoes not change.

author: xabean
author_rank: Newbie
timestamp: January 12, 2018, 04:34:20 am
content: Good news - finally got the unit at home, wiped down the brush surface with some isopropyl alcohol and a soft cloth to remove the 50+ years worth of dust. Ohmed out the taps in order of position along the transformer and saw a progressive higher ohms along the way as I expected.Scrapped the old power cord, scrapped the internal wiring, and put in 18ga black/hot white/neutral. Discovered the switch is dual pole single throw, so off is *totally* off, not just switching hot. Wired it up for 115v in, 0-270v out & hooked up a cheap outlet probe (the cheap kind that has a chime and like eight LEDs that light up from 12v to 400v) and everything seems to be in order. Next is to find some fuse holders I can ... somehow mount in the case. Thanks everyone!

author: helius
author_rank: Super Contributor
timestamp: January 12, 2018, 05:30:43 am
content: I would not advise using a variac as a step-up transformer, because it could burn out the winding. It's best to keep them wired as they are delivered, as output=input*(0 to 110%). Also beware of wiring the wiper to the input, instead of correctly to the output: this can quickly burn it as power is delivered to a short length of winding.

author: Ian.M
author_rank: Super Contributor
timestamp: January 12, 2018, 08:08:06 am
content: There is no problem using a Variac with fixed taps for up to 1:2 ratio step up, as long as you never exceed the rated wiper current (other than as specified for short term overload).Also, to avoid saturation, the input may not be applied to a winding tap with a lower rated voltage than the supply voltage.At ratios above 1:2 you have to derate the output current so the current in the lower part of the winding (between the L in tap and N) does not exceed the rated wiper current:From KCL, Ilower=Iin-Iout.Iin*Vin=Iout*VoutIin=Iout*Vout/VinIlower=Iout*(Vout/Vin-1)=> for Vout/Vin>2, the derating factor is1/(Vout/Vin -1)For a Variac like the O.P's one, that outputs up to 270V and 2A at 230V input, with 115V input applied to a suitable tap, so that the maximum output is still 270V, the derating factor is 74%, so the maximum output current will be fractionally under 1.5A at maximum voltage. At 250V it will be able to supply 1.7A. When set to below 230V, the full rating will be available, but if you want it to be foolproof, use a 1.5A breaker on its output.N.B. In a 1:2 or greater stepup configuration, the wire to the input tap will have to carry up to double the rated wiper current. If you've got a large Variac, make sure it has enough Ampacity to do so

author: Electro Detective
author_rank: Super Contributor
timestamp: January 14, 2018, 09:22:37 am
content: 110v input... 0 to 270v out...Never seen it done, watch out for weird humming and magic smoke once a load is connectedHope I'm mistaken

author: Ian.M
author_rank: Super Contributor
timestamp: January 14, 2018, 10:16:36 am
content: As long as you use an appropriate input tap, and derate correctly so that no part of the winding ever carries more than the wiper current there really shouldn't be any issues. Its not a great deal different to using a fixed autotransformer in a step-up configuration. However it would be prudent to initially power it up with a suitable incandescent bulb in series and check the output voltage range is as expected just in case one has made a mistake with the input tap connections.