I think my variac is trying to kill me - Page 1
Forum section: electronics / beginners

author: slim
author_rank: Contributor
timestamp: October 13, 2014, 05:46:01 pm
content: Hello all.I picked up a General Radio variac model W10MT3W.It has the supplemental transformer 60-120V which I would like to take out of the circuit as I want a 0-150 range.I opened it up and it looked really good inside the 10 and 3 amp fuses looked ok but the 10A cap felt a little funny when I opened it.I plugged it in...I read 50V when the variac was set to 0 and 80V when fully on.I decided to check out the continuity of the windings and other stuff the main autotransformer checked fine. the supplemental showed no continuity.I then checked the continuity at various places. at the output there is continuity between all three slots.... ground/neutral/hot...I then checked continuity at the 4 terminal on off switch. there is continuity between all lugs when in the on position.I took the switch out of circuit and it checks fine, when on only continuity between same sides but not across as it did when in circuit.I then took a look at the 10A fuse that had felt funny and it was broken inside and no continuity between posts with a good fuse.I changed the fuse holder with a known good one and reinstalled the switch. the thing shows the same behavior of continuity betweenneutral and hot at the output and the switch.I am not sure how to proceed from here. any help would be greatly appreciated.I will try and post the pdf manual. a crappy wiring diagram I drew and some pics..Thanks in advance.Sorry if this is in the wrong forum topic. if so I will move it.PDF manual -http://www.w9slm.com/variac/manual.pdfCrappy wiring diagram-Pics -

author: saturation
author_rank: Super Contributor
timestamp: October 13, 2014, 09:24:12 pm
content: Hi,Some thoughts. Safety first, as always, given you are working with mains voltage.A problem seems to be the variac particularly since it did not zero when dialed to zero. Check brushes, winding, are they clean [ the photos look quite clean] and insure the mechanical arm travels across the full perimeter of the variac.Are you reporting output voltage from the built in analog meter? Less likely a culprit, but check its calibration anyway, parallel it with a good ac voltmeter and confirm.

author: johansen
author_rank: Super Contributor
timestamp: October 13, 2014, 09:54:51 pm
content: from your wiring diagram and the manual, i suspect someone connected it wrong, and you have an open circuit.its also not impossible that the 60v transformer is burned out, but it doesn't look like it.you can also use the 60v transformer to boost the output of the variac by 60 volts, this will give you 60vac to 200vac (typically variable transformers are connected for 120v in and 0-140vac out, but you can also connect them as 120 in and 0-120 out)

author: Richard Crowley
author_rank: Super Contributor
timestamp: October 13, 2014, 09:58:57 pm
content: If you look at Figure 5 in the PDF manual, it appears that your unit is wired per variation "e".And you seem to be asking for variation "b" or maybe "d".Circuit "b" gives you output from zero to the mains voltage, while circuit "d" would give you a bit higher than the mains voltage.Remember that this Variac is NOT AN ISOLATION DEVICE. So it offers ZERO PROTECTION against risk of shock from mains-connected circuits.

author: slim
author_rank: Contributor
timestamp: October 14, 2014, 01:05:35 am
content: Quote from: saturation on October 13, 2014, 09:24:12 pmThanks.I measured 50V at the output when the v meter was showing 0V and 80V when the meter said 150VQuote from: Richard Crowley on October 13, 2014, 09:58:57 pmYes, D is what I am looking for.Quote from: johansen on October 13, 2014, 09:54:51 pmThanks for the reply. The 60V trans looks great but I am getting no continuity across it at any of the three terminals. it is however still in circuit and I suppose it could grounding out somewhere. I guess the next step would be to put the switch back in and take the supplemental trans out of the circuit. find out if its good or not then make a decision on what to do with it.I dont see why the rest of it wouldnt work if I disconnected the red wire and tie the blue and purple at the 60v tranny together. does that sound about right?Also could someone explain how the watt meter works in this design???Thanks for the help

author: slim
author_rank: Contributor
timestamp: October 14, 2014, 01:30:31 am
content: Hey again all.so with this watt meter it has voltage and current lugs.the voltage source should be in parallel and the current in series with respect of the transformer wiper and the hot side output?Also. If I find that I don't want or cant use the 60V transformer, Can I use the 10A fuse to break the hot side mains before the switch and keep the load side 3A fuse?Also if so, what size fuse would be recommended for that application..About the safety aspect of things. My shop/shack is powered by old two wire mains and I have separate ground buss bars to separate ground rods connected to separate outlets for the radios and oscilloscope and one outlet for DUTS. I know.. I know but after reading and turning my mind to mush with the whole grounding thing. I just figured what the hell.Thanks again

author: Richard Crowley
author_rank: Super Contributor
timestamp: October 14, 2014, 02:43:44 am
content: Quote from: slim on October 14, 2014, 01:05:35 amBecause of that goofy transformer. TAKE IT OUT!QuoteYou don't need that goofy transformer for that configuration. TAKE IT OUT!QuoteThe volt meter is connected across the output connector.The current section is connected in series with the output line going to the output connector.QuoteWithout a more conventional circuit schematic diagram it is hard to tell what those fuses are doing, and where.It seems strange that they are so different in value.You should protect the variac with a fuse in the output that is appropriate for the current rating of the variac.

author: slim
author_rank: Contributor
timestamp: October 15, 2014, 03:03:09 am
content: Thanks for the reply.I took out the "goofy" transformer and I still have continuity between the hot and neutral at the output and across the on off switch.I will keep hunting

author: Monkeh
author_rank: Super Contributor
timestamp: October 15, 2014, 03:08:50 am
content: Define 'continuity'. There is obviously no low-impedance AC path or the thing would pop a fuse immediately.

author: slim
author_rank: Contributor
timestamp: October 15, 2014, 03:21:55 am
content: .5 ohms between the hot and neutral at the switch and also at the output

author: johansen
author_rank: Super Contributor
timestamp: October 15, 2014, 03:33:52 am
content: Quote from: slim on October 15, 2014, 03:21:55 amthat's what its supposed to read, but i would expect to be able to read maybe half an ohm of winding resistance. connect the meter leads together, is it less than .5 ohms?

author: slim
author_rank: Contributor
timestamp: October 15, 2014, 03:39:35 am
content: B W1| 2|03| 4|the leads crossed is at about .2 ohms I was using the buzzer on the volt meter before and not looking at the ohmsthe main on off switch at the mains. when the switch is off there is no continuity between any of the terminals (1234)I pulled the switch out of the circuit and when on, I had continuity only between 1 and 3 and 2 and 4 when I returned the switchto the circuit I have continuity (.5 ohms) between 1 and 3, 1 and 4, 1 and 2.I have now removed the volt meter, watt meter, range switch both fuses and the output plug and I still have this behavior.I have to assume the autotransformer is bad I guess. there is continuity between all 5 lugs of the transformer but I suspect this is normal.

author: slim
author_rank: Contributor
timestamp: October 15, 2014, 03:44:23 am
content: I think I am just over thinking this. I just wasn't sure how the the variac worked with all the bells and whistles. its just a big ole potentiometer and I did plug it in at the start and didn't blow any fuses or a breaker in the box..Edit: thanks for the help. it is working as intended.

author: johansen
author_rank: Super Contributor
timestamp: October 15, 2014, 04:06:40 am
content: its not a potentiometer. read the line current when its plugged in, its probably less than 1 amp.

author: slim
author_rank: Contributor
timestamp: October 15, 2014, 04:36:12 am
content: yep, just a figure of speech thanks