Time to get a "Variac" - DIY?

[JayBee66]

I have seen plenty of secondhand ones on the bay and that chap in Las Islas Canarias selling them for a reasonable price but how hard is it to make one?

After all, isn't it just a rheostat and a few meters.

I have no interest in repairing or watching TVs and have never repaired anything over 60W.

Surely, a 100W single-turn, ceramic, rheostat off the bay is fine. Am I good to go or good to die? I know what the wits will say.

Are there Earthing issues?

I don't have an isolation transformer either, relying on sitting on my spare hand. (I do know that "Variacs" do not isolate.) The question, "Is my household Earth connected to neutral or not" has yet to be answered for fear of finding out the hard way.

What sort of characteristics should I look for in a rheostat and pitfalls when wiring it up to meters and sockets?

I am sure that I have overlooked something.

[cmjones01]

A variac isn't a rheostat. It's a variable transformer. It has an iron core. The brush has to be carefully designed not to short-circuit adjacent turns together. That makes it more complex and heavier than a rheostat, which is why it costs more.

If you just connected your 100W rheostat across the mains, it would get very hot, dissipating 100W. It would also have very poor load regulation: as your radio (or whatever) drew more current from it, its output voltage would drop, so it would make fault-finding unpredictable. Variacs don't have these problems.

Chris

[Nickthedentist]

Originally Posted by JayBee66

I am sure that I have overlooked something.

It's a variable autotransformer, not merely a rheostat (variable resistor)

EDIT crossed with CMJ, sorry

[ms660]

A Variac is not a rheostat.

EDIT: Post crossed.

The chances are your household earth (at the mains outlet socket earth) is connected to the supplies neutral at the DNO's incomer in a sealed box, if that's the case then that should be the only point that the earth and the neutral conductors are connected together.

Lawrence.

[paulsherwin]

Although I have a variac, I rarely use it in old radio work. I find the simple lamp limiter approach works just as well for preventing disasters, and it gives more immediate feedback in case of any problems.

[JayBee66]

Thanks for the replies. I knew I had to ask here before jumping in with both feet.

For the time being, I shall stick with my dim bulbs and wait for my local auction house to throw up a proper Variac.

[Andrew B]

I agree with Paul. I have a selection of olde-worlde filament lamps of different wattages from a 12W night light to an eye busting 300W. I find them really useful for reforming caps up to SMPS testing.

[Goldie99]

Originally Posted by JayBee66

.... and wait for my local auction house to throw up a proper Variac.

That's indeed where mine came from - it's often worth spending some time 'expanding' your search criteria on the bay, there are plenty of people selling items with no idea of what they are... so it's not unusual to find thing like variacs, isolation transformers, etc., misdescribed, and consequently cheaper, but being misdescribed they're harder to find....

Alan

[threeseven]

Same here. I found mine at large country house auction where virtually all the buyers were there for fine art and furniture, so I had almost no opposition for the treasure I wanted! The deceased owner had been an enthusiastic dabbler, there was a wood shop, a metal shop and an electronics shop. I picked up a very high quality, Zenith Duratrak, fully enclosed, 300 watt unit for £5, the only bid! The auctioneer had no idea what it was, or anyone else evidently. Same applied to loads of other stuff I picked up, including a Garrard 301, with Leak TL50, Varislope and Tuner, in a cabinet for £25!! I just wish I'd had the space and logistics to handle all the stuff in the metal shop! I often think of the owner, he must have been an interesting chap. At least some of his stuff found an appreciative home. That was an exceptional auction but I've had similar luck on a smaller scale at other house auctions.

[Lucien Nunes]

The question, "Is my household Earth connected to neutral or not" has yet to be answered

If you have a TN-S or TN-C-S supply, where the earthing conductor connects to the service cable in one way or another, it will be solidly connected as the DNO (distribution network operator) is required to do this. The connection may be at the intake in your property (TN-C-S) or all the way back at the substation (TN-S) however you can assume they are solidly connected.

If you have a TT installation where your earthing conductor connects to a an earth electrode or rod that is part of your own installation, then the resistance from N-E may be higher due to the resistance of the rod. 200Ω is considered satisfactory for a typical domestic rod although resistance values vary widely. For protection against direct contact shock, for practical purposes E & N are still connected.

[MotorBikeLes]

There is something else missing here, Jaybee66; Your "100W rheostat" is a bit meaningless. In order to meet the 100W spec, off 240v mains (250v for convenience), let's do a simple calc. W=V*I, or I=V/W=250/100=2.5A
R=V/I =250/2.5=100ohms
So, does that 100W rheostat in fact have a specified resistance of 100ohms. You did not say.
If you set that (100ohm rheostat) at 1/10th of its rotation, you should get 25v between the wiper and neutral (assuming it is sensibly connected), but that does not mean you can draw say 1 amp into a necessarily 10 ohm load, because you would now have a new calculation to do. You can only get the 25v with NO LOAD in that position.
OK, I think the idea has been abandoned already, but don't abandon the related facts.
Les.

[Biggles]

I would agree with the series lamp limiter being just as useful for general repair work. I made a variac, well, put a factory-made variac into a box etc a few years ago and I have used it about half a dozen times. I use a lamp limiter if I think there is going to be trouble. Horses for courses though, it depends how you work I suppose. My capacitor reformer is probably used most. If the capacitors pass the test there is less likelihood of fireworks.
Alan.

[FrankB]

The Variac (tm) is an auto transformer, meaning the windings are connected across the mains. It is NOT ISOLATED.
You need to use an isolation transformer before the Variac to insure you don't fry yourself, or blow up the equipment you are using.

Back in the days of stone knives and bearskins, when I was in tech school, the teacher knew nothing about isolating equipment (Gotta love those old retired USAF electronics theory instructors with no practical experience). I hooked up a Telequipment scope to the hot chassis TV and immediately caused the entire building to go black. Blew the main breakers.
He had no idea why it happened.
I am lucky I took safety precautions with 1 hand in my pocket before hooking things up. After I got in the "real world" of servicing, I learned what and why it happened.

[Herald1360]

Originally Posted by MotorBikeLes

There is something else missing here, Jaybee66; Your "100W rheostat" is a bit meaningless. In order to meet the 100W spec, off 240v mains (250v for convenience), let's do a simple calc. W=V*I, or I=V/W=250/100=2.5A
R=V/I =250/2.5=100ohms.

Oops, Les, that's upside down I =W/V so 100/250 = 0.4A and R= 625ohms.

[Stevie342000]

Not quite what you are looking for but an interesting perspective of auto transformers and lighting at the turn of the 20th century.

https://ia801201.us.archive.org/31/i...0-%20Ebook.pdf

Auto-Transformer Design by Alfred H. Avery

[TrevorG3VLF]

Many years ago I saw an article on the construction of a Variac.
Take a long strip of thin aligned silicon-iron.
Insulate the strip and wrap it into a toroid.
Make sure one side is dead flat.
Wrap with insulating tape.
Wind with round wire with turns tight and spaced precisely.
Machine the winding to give a flat top.
Plate the machined part with alloyed gold.
Make a carbon brush with anisotropic carbon.
Make a brush holder to scan the brush from one end of the winding to the other.
Mount into a frame.

A half hour job!

[MotorBikeLes]

Herald, well spotted. Stupid error, but so simple it should not lead anybody astray.
The lesson still stands, in spite of my stupid mistake.
Les.

I have never used a variac when fixing radios but I do have two in use, both for variable speed fans. The interference from 'chopper' type fan controllers ruined my radio reception.

[Jonster]

I've got two here, lovely Japanese made units, I hardly use them to be honest, one is a 1kVA the other is a 2kVA, both fully metered. I'll probably pass the 2kVA one on at some point soon as its a big old thing!