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Can I use a 240v input variac on 110vinput?

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Hi Daley,

I noticed that you referred to the receptacle as "non-standard." That is a standard, but infrequently used NEMA 6-15 receptacle designed for 15 A in our 220-240V system. Even Home Depot has them. Since most appliances that require the higher voltage in the US, e.g., electric ranges and heaters, also require higher current, it is not often used. Small tools, e.g., table saws, don't require such high current, and that is where you will often see it used. Since it is infrequently used and is not compatible with any of the common 115 V connectors, it is also used for low-voltage DC connections, as in campers and such. I don't think code applies to such use, but it prevents accidentally plugging such things into a typical 115V receptable. Here is the link to Cooper that shows one: **broken link removed**

Back OT, I think the safest step is to photograph and try to draw what you have as a record. I would then remove all of the black wires and rewire from scratch using the common US color code and the datasheet posted above. Just as a reminder, our code is: Color (often black or red) = L (line), White = common ("grounded" wire) and Green = ground wire. Note the distinction between "grounded" and "ground".

Since Variac's are very expensive new, you may want to make a test supply cord so you don't smoke it. Yes, the wiper is fused. If you have a small regular light bulb, say 25W to 60W bulb, wire it in series in the black wire of your supply cord. Then if you have a short, you will limit the current by the bulb (i.e.,25W at 120V is only 0.2A).

John
 
That light-bulb trick is handy in other stuff too like repairing amplifiers and such. But there's downside it reduces the power DUT takes, so all equipment won't work as intended (i'm no expert so this can be rubbish.....)Variac is one handy device, just remember output is not isolated. I have 500w variac in same ''circuit'' as my 2kw isolation transformer, and that isolation transformer(weights about 20kg) is fed throught self-made ICL, without it, it takes about 400A current worst case IIRC. That ICL is automatic; time relay that switches/shorts power resistor. In fact, time relay can't take 2kw power, so time relay controls bigger relay that shorts power resistor, resistor is in series with transformer (plus fuses)
If interested here's ICL as well as isolation transformer/variac combo (enclosure frame is microwave oven's frame)I prefer analogue meters, they tend to give better overall situation feedback
https://www.electro-tech-online.com...mprovement-for-isolation-transformer-icl.752/
https://www.electro-tech-online.com/blog-entries/isolation-transformer-variac-combo.726/
.....As there was talk about variac, so not entirely off-topic, and not bragging either (that isolation transformer-case is ugly, but it works) :)
and light-bulb current limiter, with bypass switch (e27 of course has safety issues.....)
https://www.electro-tech-online.com/blog-entries/helpful-piece-of-equipment.274/
 
That light-bulb trick is handy in other stuff too like repairing amplifiers and such. But there's downside it reduces the power DUT takes, so all equipment won't work as intended (i'm no expert so this can be rubbish.....)Variac is one handy device, just remember output is not isolated. I have 500w variac in same ''circuit'' as my 2kw isolation transformer, and that isolation transformer(weights about 20kg) is fed throught self-made ICL, without it, it takes about 400A current worst case IIRC. That ICL is automatic; time relay that switches/shorts power resistor. In fact, time relay can't take 2kw power, so time relay controls bigger relay that shorts power resistor, resistor is in series with transformer (plus fuses)
If interested here's ICL as well as isolation transformer/variac combo (enclosure frame is microwave oven's frame)I prefer analogue meters, they tend to give better overall situation feedback
https://www.electro-tech-online.com...mprovement-for-isolation-transformer-icl.752/
https://www.electro-tech-online.com/blog-entries/isolation-transformer-variac-combo.726/
.....As there was talk about variac, so not entirely off-topic, and not bragging either (that isolation transformer-case is ugly, but it works) :)
and light-bulb current limiter, with bypass switch (e27 of course has safety issues.....)
https://www.electro-tech-online.com/blog-entries/helpful-piece-of-equipment.274/


A light bulb is a good idea as it gives a degree of protection. One downside is that modern CCFLs don't make good loads and the old tungsten bulbs had the disadvantage that the resistance can be a tenth of the operating resistance (positive temp co). This means that you will get an initial surge of current right at the time when you don't need it.

Your set up sounds ideal: Variac with isolation transformer. I also have an old 240V 50Hz 10A Variac that produces 0V to 285V, but no isolation transformer. I have got an old hp scope that hasn't been turned on for years and then it was on 120V in the States, so, one day I plan on leaving it set to 120V and gradually increasing the input volts to 120V and then leave the scope running for a couple of days. Then do a similar thing on the 240V range. That approach give the electrolytic capacitors plenty of time to reform. I'm expecting to have to change a couple of caps though. If the scope is otherwise OK I will change all the big caps as a matter of course.
 
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Only thing with this setup is that RCD doesn't care what's happening on isolated side, so there is that safety issue as there is no ''ground'' on isolated side, but it's still safe(r) once getting used to and think before doing anything 'specially with mains-voltage stuff. There is ground, but only on enclosure and doesn't effect isolated side, it's only if isolation transformer gets faulty....in my understanding
 
Hmm interesting. As you say the trip won't know what is going on on the isolated side. Not to sure of a way around that either.
 
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well since there is no current going to ground on isolated side since there is no ground, i can't see how RCD would even trip if there is ground-leak in isolated side...But i haven't even tested it, but that i do know isolation works, but don't feel like testing my luck :D
 
well since there is no current going to ground on isolated side since there is no ground, i can't see how RCD would even trip if there is ground-leak in isolated side...But i haven't even tested it, but that i do know isolation works, but don't feel like testing my luck :D
Yeah to get a belt from the isolated side you would have to grab both ends of the secondary voltage. I was thinking of the situation where the UUT has some kind of earth return. A current imbalance trip would do the job there, but I was concerned about the burden on each line and also the possibility of nuisance tripping. I have know chaps who weren't bothered about 240V 50Hz. The most I can take is around 50V.
 
I was thinking of the situation where the UUT has some kind of earth return. A current imbalance trip would do the job there, but I was concerned about the burden on each line and also the possibility of nuisance tripping
care to tell more? if not here, then in PM.
I have know chaps who weren't bothered about 240V 50Hz. The most I can take is around 50V.
Look at my avatar, guess how much I took? I know, lame joke...seen pictures of fellow EE-guys :/ horrible...
 
Your input pins should be 1 & 4 with AC Mains Neutral to 1 and AC mains hot to 4. Your output should be 1 & 3 with 1 being neutral (same AC in neutral) and 3 being AC hot and 3 is the wiper. The wiper as mentioned should be fused. John already covered the socket you have with the straight blades that look peculiar. Yes, you can run the 220V Variac on 120 Volts. So 1 & 4 are AC mains in with AC neutral on 1 and 1 & 3 are Output with Output neutral on 1. Just remember that pin 1 is the common neutral.

This is what you have. Connections and ratings page take a look at CONNECTION 1 on the right side of the page. It looks like in your pictures they were using 1 & 6 for input which would be for 120 V input CW Clock Wise (again see connections and ratings section). So as it is it should have worked. Again, here is a link to the PDF, I could not get your link in post 1 to work.

Ron
 
Thank you everyone, I think I killed it though.

IMG_0900.JPG


I wired it up as Ron above suggested and I got sparks again from the point right next to the screwdriver tip, it is at the zero point which is where the negative is connected I believe. I don't know if this was here when I bought it or not? I most probably caused it somehow though.

Is there anyway of salvaging this? Can I move the zero point where the negative connects to a copper strand further up?

Thanks again for everything and sorry to bail last night but it was 2 am and I had to be up at 7.30am.
 
Ok so I just tried moving the negative to terminal 5 which is at approx 20% and it worked great with a little test light. Am I correct in thinking this will be safe to use like this? There were no sparks or noises.
 
Hi Daley- its 7:30 in Somerset UK and i just woke up. I will have a coffee and a think and get back to you.
 
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Thank you everyone, I think I killed it though.

View attachment 96664

I wired it up as Ron above suggested and I got sparks again from the point right next to the screwdriver tip, it is at the zero point which is where the negative is connected I believe. I don't know if this was here when I bought it or not? I most probably caused it somehow though.

Is there anyway of salvaging this? Can I move the zero point where the negative connects to a copper strand further up?

Thanks again for everything and sorry to bail last night but it was 2 am and I had to be up at 7.30am.

Hi Daley,

It would be best if you could repair the terminal 1 connection to the transformer and if it is like the Variacs that I have examined there should be a fairly simple connection to the transformer coil. The mains connection for 110V is terminal 1 (neutral) and terminal 6 (live). This will give a range of 0V to around 285V on the wiper. Thus you will have a usefull device that will operate 220V to 240V equipments if ever required.

When you do the repair to the terminal 1 connection make sure nothing is shorting out in that area. You can connect slightly further up the transformer if necessary, but try and keep the loss of turns to a minimum.

If you could post a pic of the connection as it is now that would be useful. It may just be that the connection clamp to the transformer coil has been loose from new.

If push comes to shove, and the terminal 1 connection cannot be repaired, you could make terminal 4 neutral and terminal 7 live (110V). The Variac would work exactly the same except the dial would work in the opposite direction. (max output on the dial would be minimum output and vice versa)
 
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In your photos it looks like they used terminals 1 & 6 as the inputs. That application does show up in the manual for a 120 Volt input. Looks in your recent pictures like the winding is charred (black) where you are getting sparks. Here is something you can try. Use terminals 4 and 7 as the inputs with terminal 4 being the common neutral terminal. Your outputs will be 4 and 3 terminals. What this will do is change the rotation. Normally rotating the knob clockwise (CW) will increase the output, Now zero out will be in the full CCW and rotating CCW will increase the output. Since you are seeing sparks something is cooked or wrong with the unit.
Variac Basic.png

You can also try using 4 and 5. The idea is to get around the burned part or problem part. The above is what you have. I have a 120 VAC version laying here and somewhere I have a large 240 VAC 20 Amp version laying around.

Ron
 
Hi Ron

Thanks for your input, very helpful.
I have it connected right now with negative at terminal 5 and positive at terminal 4 and it is working well with a test light. I was just a little concerned that something might still be happening at the points before terminal 5 connects and cause sparks or a fire. I think I understand how this works a lot better now but I am cautious to leave it unattended until I understand it and can be sure its safe.

If I don't turn it CCW to a point before where terminal 5 connects I should be safe is that right? I don't need to run it at 0v, the lowest I would need is probably 25v.
 
On a slightly separate issue, I found another variac that has two broken strands/winding of copper on the side, would it be possible to bridge those and get it working again? if so would it be a case of soldering a piece of copper wire to connect the broken piece or would the solder affect it?
 
Hi Ron

Thanks for your input, very helpful.
I have it connected right now with negative at terminal 5 and positive at terminal 4 and it is working well with a test light. I was just a little concerned that something might still be happening at the points before terminal 5 connects and cause sparks or a fire. I think I understand how this works a lot better now but I am cautious to leave it unattended until I understand it and can be sure its safe.

If I don't turn it CCW to a point before where terminal 5 connects I should be safe is that right? I don't need to run it at 0v, the lowest I would need is probably 25v.

That should work as long as you are past or beyond the toasted part. :)

Also, yes, you can repair a variac / autotransformer just make sure you bridge the break using an adequate wire gauge also in the interest of safety use enameled wire or once done place some corona (insulating) dope over the job.

Ron
 
Let us know how it works out for you.

North Carolina huh? Have given some thought to moving down there. I have a collection of friends in Winston Salem area. All transplanted New Yawkers like me. :)

Ron
 
Oh it's great here, I'm up in the mountains of Asheville and I like it. Coming from UK the weather here can't be beaten. My wife's from New Haven CT but I don't like it up there, too busy for me and too much traffic.
 
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