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Old 07-22-2010, 09:14 PM
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Variacs and possible alternatives

I was just thinking about this last night, variacs seem to be really expensive.
Just thought I'd compile my thoughts, maybe helpful for someone.

So, where would be a good place to find a cheap one.

I found one on ebay that I might just get, but for the time being is there anything I might have around the house that would work?

Well, there's the dim bulb tester which kinda does the same thing. You would just need varying wattages of light bulbs, possibly some big resistors mounted in light bulb sockets.
I almost want to make something like this and include a dimmer switch for greater control.

Well, why not just use a straight dimmer switch?
I guess the simple answer to this is it just isn't the same. It uses a variable resistor instead of an autotransformer. It also wouldn't be able to handle a lot of amperage.
But, I really don't know on that one.

The other idea I thought up, is why not find where the plate voltage from a tube tester goes, and tap into that? On my tube tester at least, it goes from 0 to 100 volts with some fairly accurate indicators along the way.
Well, tube testers seem to be more expensive than variacs and I really don't want to fry it. I'd imagine they're not meant to take a whole lot of amperage on the plate voltage circuits. With the way my tube tester specifically is made, I don't think its designed to take much amperage in any situation.
The control for the plate voltage also seems to be a dimmer switch and not an autotransformer.

So, I'm not sure. I'm gonna watch that one and see what comes of it.

What would you guys recommend and what do you use?
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  #2  
Old 07-23-2010, 01:26 AM
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I have a Tenma 72-1097, which is a variac & isolation transformer rated at 3.5 amps (yet it's fused for 6 amps.) I had a lower amp B&K but one side of the transformer went open - I think the variac part is still good.
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Old 07-23-2010, 09:50 AM
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The way you can think of a light dimmer is that it cuts up juice into longer or shorter pieces with blank spaces in between, longer pieces for brighter light, etc. The result is OK for an incandescant bulb but would be a buzzy mess for electronic work, and there's the power handling problem.

If you look around you could find a variac at a flea market. I got a good one in a metal case with pilot light, switch, and voltmeter for five bucks.

Your best bet until then would be the dim bulb tester. It protects as well as cuts voltage (bright light tells you to shut it down, now!) Some have used a three-way bulb, like 30/70/100 with three-way socket, of course. In that case you need to mark the switch so you know which position you're in. A bit more elegant would be made up of three or so sockets and switches with maybe 50, 75, and 100 watt bulbs; larger for larger TV's. You don't really use a variac much, usually just on first startup, so a simple one socket deal and a handful of bulbs (just change 'em to go up in voltage) is really sufficient.

Edit: I just looked at that auction you mention: that would be great. Mount in a metal box, switch, pilot light, receptacle, fuse, nice little project. Go for it!
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Last edited by Reece; 07-23-2010 at 09:53 AM. Reason: Added auction mention.
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Old 07-23-2010, 09:25 PM
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I second everything Reece said

I use a vintage RCA ISO-TAP WP-25A isolation transformer and a modern variac from Circuit Specialists.
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Old 07-23-2010, 09:26 PM
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Quote:
Originally Posted by danrulz98 View Post
I was just thinking about this last night, variacs seem to be really expensive.
Just thought I'd compile my thoughts, maybe helpful for someone.

So, where would be a good place to find a cheap one.

I found one on ebay that I might just get,

Well, why not just use a straight dimmer switch?
I guess the simple answer to this is it just isn't the same. It uses a variable resistor instead of an autotransformer. It also wouldn't be able to handle a lot of amperage.
But, I really don't know on that one.

The one you see on eBay is fairly lightweight, good enough for radios and most TVs, but I'd recommend a higher amperage (actually a higher VA rating) for everyday use.

PM me - I have a hefty powerstat (~10 Amps, IIRC) that's yours for shipping (USPS parcel or Priority Large F/rate) - it's still in the metal box someone mounted it in at NASA Langley years ago.

I use a 22 Ampere Powerstat I got from NASA, fused at 7.5A for power on testing, and I can change the fuses to fit my needed safety margin. Massive monster for sure!

Cheers,
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Old 07-23-2010, 09:37 PM
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The one I'm offering is a 116BU, same as this eBay item: 150258319426

PM me and it's yours for shipping.

Cheers,
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Old 07-23-2010, 11:20 PM
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Quote:
Originally Posted by bandersen View Post
I second everything Reece said

I use a vintage RCA ISO-TAP WP-25A isolation transformer and a modern variac from Circuit Specialists.
That seems like a nice cheap one.

What does the isolation transformer do? I know I've seen them and I have an idea as to what they do, but I've never been too sure.

Quote:
Originally Posted by Findm-Keepm View Post
The one I'm offering is a 116BU, same as this eBay item: 150258319426

PM me and it's yours for shipping.

Cheers,
PM sent.
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Old 07-24-2010, 12:04 AM
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Quote:
Originally Posted by danrulz98 View Post
That seems like a nice cheap one.

What does the isolation transformer do? I know I've seen them and I have an idea as to what they do, but I've never been too sure.
Some TVs and radios (AA5) ran right off the AC input without a transformer. Sometimes they would solder one side right to the chassis! Depending on which way you plugged in the non-polarized plug, the whole chassis could become hot An isolation transformer will make the set act as if it had a power transformer.

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Old 07-24-2010, 12:13 AM
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Ah, so its basically a transformer that takes 120 volts in and puts 120 volts out.
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Old 07-26-2010, 06:32 PM
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The "poor man's" isolation transformer can be made by stacking two high-current transformers back-to back. Years ago, I made one with two 50V transformers I got from old 48V telephone system supplies. I just wired the secondaries together, and used the primaries as in and out. Mine was probably good for only a few hundred VA.

Here's a cool youtube video describing the same:

http://www.youtube.com/watch?v=QDSmv9E5VgQ

Cheers,
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Old 07-26-2010, 11:49 PM
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I'm thinking two MOTs wired like that would work, those handle a few amps normally.
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Old 07-27-2010, 06:09 AM
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Quote:
Originally Posted by danrulz98 View Post
I'm thinking two MOTs wired like that would work, those handle a few amps normally.
Unmodified, I wouldn't recommend. But rewiring one for use as an isolation transformer has been done before. Just use high-temp 90 degree wire and remove the magnetic shunts to prevent overheating. There are numerous websites that show how it's done.

I've rewound only one MOT - I used it for a high current supply before I found a battery charger that had a more capable (heftier) transformer.

RCA Isotaps and B&K isolation transformers use to go cheap on fleabay at auction, but now most are fixed price - BIN sales, so the bargains went away. I got a Sencore PR57 off eBay for cheap and promptly resold it. I wish now I'd kept it!

Cheers,
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Old 07-29-2010, 11:50 PM
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Cool, just got Findm-Keepm's variac today.
It's a bit of an odd piece. The scale on it goes from 0 to 120, but the wiring diagram shows that it's set up to go to 140. So, there's pencil marks around it at voltages that must have been important at one time. So, I think I'm just gonna put a volt meter in the case along with a cord connector from a computer power supply (I have plenty of those to scavenge) and a power outlet.

It was lovely how the power cord crumbled away in my hand...

Last edited by danrulz98; 07-30-2010 at 03:14 AM.
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Old 07-30-2010, 08:32 PM
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Quote:
Originally Posted by danrulz98 View Post
Cool, just got Findm-Keepm's variac today.
It's a bit of an odd piece. The scale on it goes from 0 to 120, but the wiring diagram shows that it's set up to go to 140. So, there's pencil marks around it at voltages that must have been important at one time. So, I think I'm just gonna put a volt meter in the case along with a cord connector from a computer power supply (I have plenty of those to scavenge) and a power outlet.

It was lovely how the power cord crumbled away in my hand...
The variac was NASA Langley excess - It was part of a process oven controller asssembly - the whole thing was the size of a refrigerator. I got it in late 2006, and all of the wiring was 50-60's era stuff - big thick rubber coated stuff. After removing most of the wiring, I was rewarded with the sight of what seemed like thousands of 225 watt resistors. I removed everyone of them - the hardware alone filled up a 5 gallon bucket. Sadly, most of the resistors are extremely odd values - stuff like 405 ohms and 1140 ohms. They musta spent a fortune on building it all. The front of the unit had a Leeds-Northrup temperature controller I sold on fleabay for more than I paid for the whole unit, so the rest of the stuff was free excess to me. I still have the 300 or so power resistors - I may give them to the father-in-law for target practice.

I tend to search locally for odd stuff - I use fleaybay, Public Surplus, and rarely, CL. About a third of my test equipment came this way - cheap too. My big ol' variac that would have set me back over 360 bucks cost me nothing after I sold off other stuff I got in the lot of equipment. Frugal, yes, but fun for sure!

Cheers,
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