Shopping for a variac

[engrguy42]

...Why the concern over a point contact with the brushes?

When they skip under full load due to poor contact,

you get a free nano arc welding and or plasma cutting demo 

So what? And why are you assuming poor contact?

Goggle for "make before break" term , its quite popular in electronics.

You're assuming the contact is insufficient to bridge the wires. And we're talking about a mere 2-3amps. There's no arc welding or anything close (it requires like 100 amps or something).

[Electro Detective]

Here's some Youtube videos that tell the story for any disbelievers 

youtube.com/watch?v=PS4UeV8epTU

youtube.com/watch?v=po5Xxl_Syxg

youtube.com/watch?v=qRTKHNoiRQI

youtube.com/watch?v=vsnEMttQC_I



FYI + YMMV: even after repairs and fixits, they are still a future trainwreck waiting to happen down the 'track...'   

[engrguy42]

Here's three Youtube videos that tell the story for any disbelievers 

youtube.com/watch?v=PS4UeV8epTU

youtube.com/watch?v=po5Xxl_Syxg

youtube.com/watch?v=qRTKHNoiRQI


FYI + YMMV: even after repairs and fixits, they are still a future trainwreck waiting to happen down the 'track...'   

The first two are the ones I already referred to. The second one is irrelevant, since it was a repair after 10 years of beating the crap out of it. The first was legit.

Vague generalizations like "trainwreck" without data to support it is fairly useless.

[Electro Detective]

Fair enough, try this one for size > youtube.com/watch?v=vsnEMttQC_I

who loves ya? 

[engrguy42]

Fair enough, try this one for size > youtube.com/watch?v=vsnEMttQC_I

who loves ya? 

The guy replaced the voltmeter with a digital one. So what?

[Electro Detective]

it had a faulty earth ground connection too,
you didn't watch all of it before rushing back to stick in the boots   

come on, there is no denying they are just more TooHungLow dumped junk to money grub frugal or clueless electronics enthusiasts 

and put newb lives at risk with shoddy sweatshoppe alleyway assembly line fails

These red  -Very-Aches-  are not even worthy as door stops 

[engrguy42]

it had a faulty earth ground connection too,
you didn't watch all of it before rushing back to stick in the boots   

come on, there is no denying they are just more TooHungLow dumped junk to money grub frugal or clueless electronics enthusiasts 

and put newb lives at risk with shoddy sweatshoppe alleyway assembly line fails

These Vary-Aches are not even worthy as door stops 

I try to get the facts first before deciding, rather than making up my mind first then looking for stuff to support what I want to believe.

[Electro Detective]

Buy one, open it up carefully, take a look inside, give it a test, and go from there

If the usual suspects don't give you grief to fix, call it a keeper for now

Otherwise find the receipt if bought local, return it re-assembled exactly as you found it,
and let the next guy cop it, to learn the hard way that going too cheap on AC control gear is a NO GO 

[BravoV]

@Electro Detective, just do not easily get provoked. 

[Electro Detective]

Who, cuddly Teddy me ? 

 

[engrguy42]

Buy one, open it up carefully, take a look inside, give it a test, and go from there

If the usual suspects don't give you grief to fix, call it a keeper for now

Otherwise find the receipt if bought local, return it,
and let the next guy cop it, and learn the hard way that going too cheap on such gear is a NO GO 

I 100% agree with the idea of "don't buy junk". However, these devices are so simple that, aside from serious design flaws, getting them working well seems to be pretty simple. You can always point to a "yeah, but...". But this is different from a complex scope, for example, that you can't repair with a quick soldering iron. And the benefit of saving $200+ might outweigh the cost of some minor repairs.

The big question (for me at least) is whether there are any serious design flaws that apply to many/most units. 

[engrguy42]

@Electro Detective, just do not easily get provoked. 

And that should be easy as long as we stick to facts. 

[engrguy42]

it had a faulty earth ground connection too,
you didn't watch all of it before rushing back to stick in the boots   

He meggered the case, and showed that it has good insulation. That's good. Exposed bare metal is bad. He also showed there was a bad connection on the case ground, and repaired it. I'm guessing it took less than 10 minutes to clean/repair the connection.

So at this point I figure the best approach when buying one of these is "Be careful. Check the wiring and case ground first, and if there's a problem you may have to make some simple repairs".

Unless we can find some serious design flaws in many of these units.

[ArthurDent]

On fusing variacs/Powerstats, here are edited snippets of a post I made a couple of years ago in the forums about properly fusing a 240V center tapped variac with 120 V input connected to a center tap of the winding to get 0-240V output as a step-up autotransformer.

“Fuse both the input and output for the current rating of the Powerstat…

 If you have a 10A Powerstat wired for 120 V in and try to get 240V out, the current ratio is 2:1 and the INPUT 10A fuse will blow if you try to get more than 5A out at 240V, there is no way to get more than 5 A out at a 2:1 ratio-period.  The input 10A fuse will protect the Powerstat when set to output more than the input voltage.

If you have this 10A Powerstat wired for 120V in and try to get 12V out, the current ratio is 1:10 and the OUTPUT 10A fuse will blow if you try to get more than 10A out but the input current will only be 1A with 12V at 10A out. If you don't fuse the output and short it, up to 100A will try to flow through the output to cause a 10A current in the input at a 1:10 ratio and the lower end of the variac winding will burn out before the input fuse will blow. The output fuse will protect the Powerstat when set to output voltage less than the input voltage.

The only way to completely protect a Powerstat is to fuse both the input and output for the current rating of the Powerstat.”

[drussell]

However, these devices are so simple that, aside from serious design flaws, getting them working well seems to be pretty simple.
...
The big question (for me at least) is whether there are any serious design flaws that apply to many/most units.

The operating principle of these devices is very simple.  Producing a quality, reliable implementation is not.

As others have suggested since the beginning of this thread, anyone who decides to take the plunge on one of these cheap Chinese "Variac" clones should have a working electrical knowledge and be prepared to thoroughly check their unit own for any possible issues, especially given the history of known defects out of the box and apparent complete lack of QC or testing of final products before shipping.

If the cheap Chinese mechanism is sufficient for your application, great.  Use one.

But to suggest that they are perfectly adequate design, just as good as the "big boys", is nonsense.

[engrguy42]

However, these devices are so simple that, aside from serious design flaws, getting them working well seems to be pretty simple.
...
The big question (for me at least) is whether there are any serious design flaws that apply to many/most units.

The operating principle of these devices is very simple.  Producing a quality, reliable implementation is not.

As others have suggested since the beginning of this thread, anyone who decides to take the plunge on one of these cheap Chinese "Variac" clones should have a working electrical knowledge and be prepared to thoroughly check their unit own for any possible issues, especially given the history of known defects out of the box and apparent complete lack of QC or testing of final products before shipping.

If the cheap Chinese mechanism is sufficient for your application, great.  Use one.

But to suggest that they are perfectly adequate design, just as good as the "big boys", is nonsense.

If they meet their specs, then isn't that an adequate design? Why do you presume these are inadequate designs? I'm just looking for facts, not vague comparisons to "big boys", whatever that is.   

[drussell]

It seems silly to me to buy one of the Chinese ones for $60 when you can get one of these for $60 instead:

https://www.ebay.com/itm/Choice-Powerstat-Autotransformer-Variac-7-1-2-amp/402261290041

I realize the shipping is probably free from Amazon and the ones in that listing are used and look "old" instead of shiny and new, but I'd take an old 7.5A PowerStat over a 2~4A (maybe?) Chinese knock-off any day.

Even if I had to pay for shipping. 

Heck, even if I had to send the brush away to be re-tipped for $25, I would still be happier in the end.  YMMV.

[drussell]

If they meet their specs, then isn't that an adequate design? Why do you presume these are inadequate designs? I'm just looking for facts, not vague comparisons to "big boys", whatever that is.

LOL...  That's just it!

They don't even HAVE specs!

A proper Variac like the 7.5A Powestat models above will be able to do it's full rated output current of 7.5A, even on the very bottom end of the winding.

The Chinese ones don't even tell you how much current they're supposed to be able to deliver.

[engrguy42]

If they meet their specs, then isn't that an adequate design? Why do you presume these are inadequate designs? I'm just looking for facts, not vague comparisons to "big boys", whatever that is.

LOL...  That's just it!

They don't even HAVE specs!

A proper Variac like the 7.5A Powestat models above will be able to do it's full rated output current of 7.5A, even on the very bottom end of the winding.

The Chinese ones don't even tell you how much current they're supposed to be able to deliver.

They tell you exactly. There's a big 5AMP fuse label on the front. Which means you're limited to 2-3 amps.

And do you know how old those Powerstats are? I'm guessing 1940's. That company has been around since the 1930's. It's up to you, but buying a device that's been kicked around for maybe 70 years doesn't seem like a good choice to me.

[engrguy42]

BTW, word to the wise...

If you look at the ebay photo of the two Powerstats, the one on the left has magic black smoke all over the plug. 

Don't waste your money on that one. And I'm guessing it's the older one based on that weird black case design. Although maybe good for a museum or something.

[BravoV]

They tell you exactly. There's a big 5AMP fuse label on the front. Which means you're limited to 2-3 amps.

Btw, what happened to your other thread that wanting to buy a variac ?

You don't even own a variac, and probably never used one too, let alone publicly admitted you have Dunning Kruger. 

Also, say they labelled it 5 Amp, why derate it just to 2-3 Amps ? Why not just 1 or even 0.5 ?  Just because you said so ? 

[Cubdriver]

If they meet their specs, then isn't that an adequate design? Why do you presume these are inadequate designs? I'm just looking for facts, not vague comparisons to "big boys", whatever that is.

LOL...  That's just it!

They don't even HAVE specs!

A proper Variac like the 7.5A Powestat models above will be able to do it's full rated output current of 7.5A, even on the very bottom end of the winding.

The Chinese ones don't even tell you how much current they're supposed to be able to deliver.

They tell you exactly. There's a big 5AMP fuse label on the front. Which means you're limited to 2-3 amps.

And do you know how old those Powerstats are? I'm guessing 1940's. That company has been around since the 1930's. It's up to you, but buying a device that's been kicked around for maybe 70 years doesn't seem like a good choice to me.

So, the 5A fuse means that they're good for 2-3 amps?  Exactly?     2-3A doesn't seem very 'exact' to me - seems a rather large margin, actually.  You buy your cheap but shiny and new far east variac, and I'll stick with old but dull used ones in decent condition with a proven track record and conservative specs that they will meet all day every day.

-Pat

[engrguy42]

They tell you exactly. There's a big 5AMP fuse label on the front. Which means you're limited to 2-3 amps.

Btw, what happened to your other thread that wanting to buy a variac ?

You don't even own a variac, and probably never used one too, let alone publicly admitted you have Dunning Kruger. 

Also, say they labelled it 5 Amp, why derate it just to 2-3 Amps ? Why not just 1 or even 0.5 ?  Just because you said so ? 

Ah, so now the personal attacks start.

Anyway, the reason why is because a fuse generally starts thinking about melting at around 50% of its rating. So 50% of 5 amps is 2.5 amps. And based on what I've seen the wire can handle that current, so there should be no problem running it up near 2.5 amps.

And we're back to whether there's some unknown design characteristic we're unaware of that's limiting it below 2.5 amps. Well, if they put in a 5 AMP fuse they're pretty much saying how much it's rated for. And since nobody seems to have any data to prove that it IS limited below 2.5 amps, it's all vague generalizations.

[drussell]

They tell you exactly. There's a big 5AMP fuse label on the front. Which means you're limited to 2-3 amps.

And do you know how old those Powerstats are? I'm guessing 1940's. That company has been around since the 1930's. It's up to you, but buying a device that's been kicked around for maybe 70 years doesn't seem like a good choice to me.

Having a fuse holder marked for 5A is not exactly a thorough characterization of device capabilities.

I suggest you take a look at this example 1958 POWERSTAT Catalog to see how the "big boys" have been specifying these things for decades.

You go buy the red Chinese one and post the specifications from the manual, then see how long it lasts.

My money is on that black one on the left outliving your shiny red one by decades, even with a 50+ year head start.      

I'd take that one any day.  Hands down.  No question.

YMMV.

[engrguy42]

If they meet their specs, then isn't that an adequate design? Why do you presume these are inadequate designs? I'm just looking for facts, not vague comparisons to "big boys", whatever that is.

LOL...  That's just it!

They don't even HAVE specs!

A proper Variac like the 7.5A Powestat models above will be able to do it's full rated output current of 7.5A, even on the very bottom end of the winding.

The Chinese ones don't even tell you how much current they're supposed to be able to deliver.

They tell you exactly. There's a big 5AMP fuse label on the front. Which means you're limited to 2-3 amps.

And do you know how old those Powerstats are? I'm guessing 1940's. That company has been around since the 1930's. It's up to you, but buying a device that's been kicked around for maybe 70 years doesn't seem like a good choice to me.

So, the 5A fuse means that they're good for 2-3 amps?  Exactly?     2-3A doesn't seem very 'exact' to me - seems a rather large margin, actually.  You buy your cheap but shiny and new far east variac, and I'll stick with old but dull used ones in decent condition with a proven track record and conservative specs that they will meet day all day every day.

-Pat

Come on, this isn't rocket science. You can go to the Littlefuse website, find a 5 amp fuse, and look at the time/current curve. It tells you exactly the time it takes to melt at any current. But anyone with the least amount of electrical knowledge should know that your limitation is around 2.5 amps, and if it's that important you can test it and buy another fuse.