Shopping for a variac

[engrguy42]

What specifically make it a junk?

It's too light.  That means the core (that wires are wound on) is too small.  It will saturate and overheat.  Wires are also too thin for the rating.  When I lived in Japan, I had a 250VA version.  It was larger and heavier than this thing.

My prediction is, it will get way too warm, possible smoke, possible broken wire, but I don't expect anything spectacular.  I think I have a 500 watt light bulb somewhere, so I'll use that as a load.  Thermo-couples on bottom, face plate, sides.  Clamp on meter for current at input, Voltage at the output.  Run it for one hour at half way point, and another one hour at whatever the bulb is rated.  (120V?)  Then rotate the slider from 0 to 120V back and forth few times.  This is when I expect something to go wrong. 

Breaking it isn't the point, and stressing until it melts isn't a point either.  My goal is to determine, if _I_ feel safe using this at the rating.  Obviously, at half way point, it won't be expending 500VA.  I am a licensed electrician but this is only a partial and personal test in home setting.  I don't endorse, approve, or guarantee anything and if anyone decides to buy it, they do so at their free will.

It might take me few days but I've been wanting to do this for a while, so I will do it.  Just don't stay up all night waiting for it. 

But for the iron core to saturate means the applied voltage is too high. So you're saying they can't actually handle 120 volts?? And are you sure the wire isn't rated for the current? What gauge do they use?

[Ian.M]

It will be the usual things for built down to a cost Chinese electrical gear:

* Not enough iron, so too close to saturation and prone to drawing excessive current and overheating if the incoming line voltage is higher than nominal.

* Not enough or low grade copper, so the winding resistance will be excessive and it will be unable to provide its rated current without overheating.

* Poor quality insulation, so a high probability of developing turn to turn or turn to core shorts with sustained use, especially at power levels that heat it significantly.

* Poor quality plastic parts, often of a plastic unsuitable for the application.   e.g. fuseholders that break in half due to the stress of the spring pressing on the fuse, control knobs that split, brush assemblies and terminal boards that disintegrate etc.

* Flimsy sheet metal work which wont hold fastenings reliably and distorts during normal handling.

* poor quality soft screws and nuts - good luck getting a good connection if tightening a terminal sheers it and there are few things more annoying  than a Variac control knob with too soft a grub screw that slips on the shaft if you try to turn it too quickly, or if you hit the end stop with too much force!

If you've got access to a thermal imaging camera, setting up to run it with the housing off or partially removed, and examining it for hot-spots running under load may be educational (and horrifying).

You can probably expect to get at most 50% of the max rated current without overloading a cheapo Variac, and may well have to do away with any boost capability completely by rewiring the input Line wire from the 'hot' end winding tap to the 'hot' end winding end to keep it out of saturation.  (The Neutral wire should always go direct to the 0V end of the winding.)

Edit:  three posts were made while I was typing this and there's quite a bit of duplication with my points.

[engrguy42]

Ian.M, again I'm trying to get down to actual facts. Is what you're describing speculation, or is it based on stuff you've witnessed with these variacs?

[tkamiya]

Magnetic core in transformer can saturate with excessive current.  (or said more precisely, as it approaches saturation)  It's not only the voltage that matters.  Please read what I said simply as "insufficient design."  I didn't run material analysis, flux density calculations, or measure wire gauges.

I should measure the wire size.  That's a good idea.

[tkamiya]

If anything, it will be fun. 

It will also be self-assuring ME to be using this.

If this was a local radio club or something, we'd all gather in back yard, do a BBQ, and run this test and have a good ol' fun.  Kind of hard to do that on Internet and during Covid virus pandemic. 

Like someone said, speculation and theorizing is one thing but real world test is quite another.  I'm a bit sad my spectrum analyzers, signal generators, frequency counters, and precise lab standards won't be part of this....

[engrguy42]

Magnetic core in transformer can saturate with excessive current.  (or said more precisely, as it approaches saturation)  It's not only the voltage that matters.  Please read what I said simply as "insufficient design."  I didn't run material analysis, flux density calculations, or measure wire gauges.

I should measure the wire size.  That's a good idea.

The magnetic core is a nonlinear I/V characteristic, analagous to a nonlinear resistance of, say, a metal oxide varistor. The reason excessive current would flow is due to the voltage getting up to the knee of the I/V curve into saturation territory. Above that the current increases quickly with small increases in voltage. And that current is comprised of odd(?) harmonics, which can cause heating and other bad stuff. So I'm not sure what you're referring to when you say it can saturate with excessive current. Excessive current with lots of harmonics is the RESULT of high voltage.

[Ian.M]

Ian.M, again I'm trying to get down to actual facts. Is what you're describing speculation, or is it based on stuff you've witnessed with these variacs?

Some I've witnessed (e.g. the slipping knob), some I've personally heard from other techs,  but much of the above is generalisations from watching the quality of electrical goods imported from China dropping year on year over the course of my career.

[engrguy42]

Ian.M, again I'm trying to get down to actual facts. Is what you're describing speculation, or is it based on stuff you've witnessed with these variacs?

Some I've witnessed (e.g. the slipping knob), some I've personally heard from other techs,  but much of the above is generalisations from watching the quality of electrical goods imported from China dropping year on year over the course of my career.

And I'm sure we could all point to some of our favorite, high quality equipment that is made in China.

[tkamiya]

Test jig.....

That light bulb is a 500 watts on its own. 

[drussell]

Magnetic core in transformer can saturate with excessive current.  (or said more precisely, as it approaches saturation)  It's not only the voltage that matters.  Please read what I said simply as "insufficient design."  I didn't run material analysis, flux density calculations, or measure wire gauges.

I should measure the wire size.  That's a good idea.

Indeed.  A critical analysis would be instructive. 

I expect they work fine though most of the range on an easy, simple, resistive load, especially intermittently but as soon as you try running motors or battery chargers or other horrible loads through them, like trying to troubleshoot an SMPS or something, they're useless...  I've been told by a reliable personal friend who is an EE that you can even see the obvious core saturation on current peaks on at least one particular example, although I have never actually attempted to verify this myself on my meager equipment stash and wouldn't spend my own money on one of those things just to test it, even though there are presumably several variations of that same design coming out of one factory with unknown, varying quality levels, as is customary for Chinese factory goods.

Personally I've seen a blown up Chinese model one in person, but I didn't blow it up. 

I've also used many, many old boat-anchor ones, including some complete rust buckets and don't recall ever having an actual problem or witnessing any fault in any one in actual operation including some chassis-mount ones in fairly harsh environments. 

I helped fix a couple of pieces of ones for a friend of mine years ago or else I never would have even seen the internal mechanics of the old ones, they normally just work.    They were "for repair/parts" type units with obvious smashing, breakage and general smooshy-smashage from a whole discarded box of them, but at the difficult-to-argue-with price of free, then  .-- .... -.--  .-- --- ..- .-.. -..  -.-- --- ..-  -. --- - ..--..

[engrguy42]

Test jig.....

That light bulb is a 500 watts on its own.

Nice.  I'm staying up all night waiting for your results. 

Are you done yet? 

[tkamiya]

Thing about Chinese, Japanese, Indian, whatever country's goods is that they make to the spec of whomever is ordering. 

I have two fabulous quality microscopes made in India, and piece of crap camera made in Japan.  This piece of crap auto-transformer is here because we, the buyers, demanded cheap products.  Some manufacturers will say "sorry, we can't make it for that", and for every one of those there are ten who will put something together and ship them.

I just have one question.  Who will hold my beer while I do this test?

[drussell]

And I'm sure we could all point to some of our favorite, high quality equipment that is made in China.

Don't get me wrong, there are actually some impressively high quality products that come out of China.

Certain products can end up being ridiculously good bargains overall, but those standard red-model Variac clones do not tend to be one of them, at least from what I've seen. 

Let the ritual of the smoke release commence! 

[engrguy42]

Thing about Chinese, Japanese, Indian, whatever country's goods is that they make to the spec of whomever is ordering. 

I have two fabulous quality microscopes made in India, and piece of crap camera made in Japan.  This piece of crap auto-transformer is here because we, the buyers, demanded cheap products.  Some manufacturers will say "sorry, we can't make it for that", and for every one of those there are ten who will put something together and ship them.

I just have one question.  Who will hold my beer while I do this test?

Don't forget the video 

[tkamiya]

I have no ability to shoot video or edit one.   I'll see what I can do but you might have to settle for a photo or two.

Dave?  Can you come to Orlando, Florida by tomorrow?

[engrguy42]

I have no ability to shoot video or edit one.   I'll see what I can do but you might have to settle for a photo or two.

Dave?  Can you come to Orlando, Florida by tomorrow?

Reminds me of a computer tech youtuber I used to watch (before he decided to pander to the mindless entertainment crowd). In the computer world people turn purple and pee in their pants over stuff like CPU cooling, and stuff they actually don't understand. "OHMYGOD, you need to have tons of cooling or the whole computer will melt!!!".

So he'd do stuff like take an old computer, yank all CPU cooling, and turn it on and see what happens. And of course it throttled the CPU and kept below damaging 100C temps, just like the engineers designed. Or he'd place a motherboard in the sink, under water, then plug it in and see what happens. Stuff like that where you actually get beyond the hype and paranoia and get to the ACTUAL facts. I wish more folks would do that. (wonder if Dave is listening).

Anyway...tkamiya you're doing a great public service    Thanks much.

Are you done yet ?? 

[drussell]

I have no ability to shoot video or edit one.   I'll see what I can do but you might have to settle for a photo or two.

Dave?  Can you come to Orlando, Florida by tomorrow?

Editing in a pinch is best just with virtualdub.  Easy peasy.  It's basically usually just one .exe file. 

Any phone, camera, anything with video capability?  Even a 640x480 potato like mine? 

(...although I don't actually expect much excitement with a light bulb...)

[tkamiya]

I think, all of us who are at or above middle age will understand this.  It's amazing we lived this far and through our own stupidity.  All those chemistry sets and managzines for science of all kinds, they included instructions and we actually did them.  We didn't watch it on You-tube, and there weren't adults telling us not to. 

Then we got creative.  Myself, I've tried to charge 1.5V battery by directly plugging into mains and see if I can "super charge" it.  Tried to catch a falling soldering iron and ended up grabbing the hot end.  Forgot to discharge capacitor and ZAP!  Find a tantalum cap and stick it into mains outlet (that one hurt quite a bit)

Somehow, the world was safer with us "scientists" running lose.

[tkamiya]

I found a D800.  I never used it for a video so I'll have to figure this thing out.

[Electro Fan]

I found a D800.  I never used it for a video so I'll have to figure this thing out.

Good find

[Electro Detective]

I think, all of us who are at or above middle age will understand this.  It's amazing we lived this far and through our own stupidity.  All those chemistry sets and managzines for science of all kinds, they included instructions and we actually did them.  We didn't watch it on You-tube, and there weren't adults telling us not to. 

Then we got creative.  Myself, I've tried to charge 1.5V battery by directly plugging into mains and see if I can "super charge" it.  Tried to catch a falling soldering iron and ended up grabbing the hot end.  Forgot to discharge capacitor and ZAP!  Find a tantalum cap and stick it into mains outlet (that one hurt quite a bit)

Somehow, the world was safer with us "scientists" running loose.

What evidence is there it's not still going on ?    

 

[drussell]

I found a D800.  I never used it for a video so I'll have to figure this thing out.

Good find

Indeed!   

I wish I could randomly find things like that around here!

[Electro Fan]

I found a D800.  I never used it for a video so I'll have to figure this thing out.

Good find

Indeed!   

I wish I could randomly find things like that around here!

This might be a brother to the "What did you buy today?" thread.... "What did you randomly find today?"

I'm guessing if the posts were ranked by good finds a D800 would be on the upper half of the list

[bdunham7]

I'm in the USA, so 110-130V is my mains.

Looking for a good reliable variac. Cost should be under $100 USD.
10-20 amps is okay (max. required is enough power for 200w CRT based tv)

New or used is fine-- even classic/vintage in good shape. Made in China okay if the quality/reliability is decent.
I'd rather it be shipped from within USA due to Coronavirus shipping delays from Asia.
eBay or Amazon are okay. AliExpress -- no!
Suggest some options.
Thx!

If you're working on old TVs and the like, try to find a BK Precision 1655 or 1655A (the older one is actually better, IMO) which will have 3A/4A peak and isolation, leak testing and an input circuit breaker.  Output protection is by fuse.  I got one with some minor, fixable cosmetic damage on eBay for $85, although I don't see any on there now anywhere near that price.  EDIT:  The BK 1653 seems to go for prices near your budget.  It is only 2A/3A peak, but if that's enough, it's a good unit.

If you get the Chinesium model, let us know how it goes. I've seen them at Harbor Freight and I wasn't tempted to let one follow me home.

[tkamiya]

I took covers off mine and made some photo and measurements.

Bare wire thickness is 0.505mm
With enamel coating it is 0.573mm
(both measured by Mitutoyo Micrometer)

That means AWG 24 equivalent? 

Also, the sliding contact does NOT have a heat sink (it's only a 5A version) and it's a cone shaped carbon piece, so it makes a point contact; maybe crossing two windings?  Most solder joints were cold soldered and it did not look right, but I have no ways to further analyze this.  Lead wire seems to be vinyl insulated wires so they are not high temperature rated.  I'm saying it's vinyl by feel of consistency and the fact at every (cold) solder joint, insulation has charred black.

I'll withhold my opinion until the testing is done.