Shopping for a variac

[bdunham7]

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? 

So the wire is good for 1 amp--if it is copper.  After you are done blowing it up, can you check and see if the wire is copper or CCA?

[Cubdriver]

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.

Wow - that's a 5 A unit?  That brush looks tiny compared to the one on a 3A, 0-140V Powerstat I just dug out.









-Pat

[tkamiya]

Hahaha.... 

I'd be laughing so hard if it's CCA.  But it isn't.  (read on...) 

Scarcely thing is, the contact is a POINT, not a surface.  Another thing is, see how your windings are plated with something?  Mine is just bare copper ground about third of a way down....  so the actual contact area and wire thickness is smaller than it appears.  The size of point will, at maximum, touch, but not fully cover 2 windigs. 

I'm actually thinking leaving the cover off and focus on inside.  I can point the infrared thermometer that way and if something interesting happens, it will be recorded.  It might get a bit exciting. 

I'm trying to be fair minded and not make judgement before the test but I do believe some pre-caution will be necessary.  Likely endings are fairly obvious.

[Cubdriver]

Shame you don't have an IR camera - that might make for an interesting bit of evaluation.  I seem to have misplaced mine on my last CA trip - am hoping I left it in the office out there, but that's a somewhat dim hope.  The downside of the tiny FLIR-1 it's easy to lose the damned thing!!

And FWIW, I got my dial caliper out and the wire on the 3A Powerstat appears to be #24 as well, FWIW. I suppose on a variac they can get away with less derating since it's a single layer winding and it can thus dissipate heat better than a conventional transformer with many layers on a bobbin would be able to.

-Pat

[drussell]

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.

The Sencore PR57 (or the PR570 descendant) is another good TV-service-bench specially designed, isolating type unit to look out for if you happen to find one for a reasonable price. 

The current asking prices on theBay are insane.

[tkamiya]

I am now looking for a new Variac.  I have a local surplus shop for that sort of things so I'll have to pay it a visit, after covid thing settles down.  It's one of those things I rarely use, but when I need it, there is no substitute. I've also been wanting isolation transformer.  Should I look for shiny new one from China on Amazon?   

I actually knew I had a D800 but I didn't associate it with using it for video.  It can be done but some features are quite lacking, like not being able to auto-focus during shooting.  Just few weeks ago, I was thinking of re-re-restarting film photography.

[Cubdriver]

I am now looking for a new Variac.  I have a local surplus shop for that sort of things so I'll have to pay it a visit, after covid thing settles down.  It's one of those things I rarely use, but when I need it, there is no substitute. I've also been wanting isolation transformer.  Should I look for shiny new one from China on Amazon?  

I actually knew I had a D800 but I didn't associate it with using it for video.  It can be done but some features are quite lacking, like not being able to auto-focus during shooting.  Just few weeks ago, I was thinking of re-re-restarting film photography.

 

Sure, go for it!   

-Pat

[Cubdriver]

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.

The Sencore PR57 (or the PR570 descendant) is another good TV-service-bench specially designed, isolating type unit to look out for if you happen to find one for a reasonable price. 

The current asking prices on theBay are insane.

Damn!!  They have gotten a bit insane, haven't they?  I got one of the 570s a few years ago after seeing it on Shahriar's channel, and love it for powering things up for the first time - go slow and watch things and shut down if they look wrong.  Thanks for prompting me to look, though - bought a new front panel overlay to replace the buggered up one on mine.

-Pat

[drussell]

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?

Wow - that's a 5 A unit?  That brush looks tiny compared to the one on a 3A, 0-140V Powerstat I just dug out.

Yeah, that tiny brush contact point is anemic.  I don't remember the one I saw the inside of being a little cone like that, but I only looked at it to laugh at the charred remnants of the windings for a moment, several years ago... 

I always think of Variac brushes looking more like these:



A reasonable selection of replacements for the typical old ones are available here, by the way:

http://www.carbonbrush.com/variacbrush.htm

[drussell]

Another thing is, see how your windings are plated with something?  Mine is just bare copper ground about third of a way down....  so the actual contact area and wire thickness is smaller than it appears.  The size of point will, at maximum, touch, but not fully cover 2 windigs.

A properly made Variac winding will usually have the contact tops of the windings pressed down flat to maintain most of the cross-sectional area, not just filed off halfway through.     That's where the one Chinesium-grade one that I saw had blown up.  Cooked the tops right off of a bunch of windings.

[BravoV]

Yeah, that tiny brush contact point is anemic.

Yeah, really small contact area.

Found another photo of my Matsunaga teardown attached below.

[BravoV]

Another thing is, see how your windings are plated with something?  Mine is just bare copper ground about third of a way down....  so the actual contact area and wire thickness is smaller than it appears.  The size of point will, at maximum, touch, but not fully cover 2 windigs.

A properly made Variac winding will usually have the contact tops of the windings pressed down flat to maintain most of the cross-sectional area, not just filed off halfway through.     That's where the one Chinesium-grade one that I saw had blown up.  Cooked the tops right off of a bunch of windings.

Long time ago, I heard straight from a variac repair technician from Matsunaga, apart from the importance of contact area, also for example at the Matsunaga design, the rectangle shaped tip is slanted just few degrees in order for it to make new contact at the next adjacent wire before leaving the previous contacted winding wire. Photo attached below.

[drussell]

Wow - that's a 5 A unit?

Well, since the only rating I know of for these units is "500VA", that would be about 4A at the nominal voltage, so perhaps typically OK to use up to about 2A, given the way things like this often seem to be rated in Chinese watts (power supplies, speakers, power amplifiers, etc.,) similar to battery capacity rated in Chinese mAh.

I generally understand things like metric <-> imperial conversion factors.  They're typically simple ratios, but things like Chinese watts or VA to standard SI units tends to be more of a sliding scale, generally starting at about 2:1, but it varies. 

[drussell]

Long time ago, I heard straight from a variac repair technician from Matsunaga, apart from the importance of contact area, also for example at the Matsunaga design, the rectangle shaped tip is slanted just few degrees in order for it to make new contact at the next adjacent wire before leaving the previous contacted winding wire.

Yes.  They are rotated noticeably off-straight.

Edit: This is also part of the "resistance-between-windings" thing of the carbon brush instead of it just completely shorting adjacent winding turns.

[Cubdriver]

I just popped the contact out of the Powerstat - it looks to be about 6mm long and 1.5mm wide.



-Pat

[Cubdriver]

Another thing is, see how your windings are plated with something?  Mine is just bare copper ground about third of a way down....  so the actual contact area and wire thickness is smaller than it appears.  The size of point will, at maximum, touch, but not fully cover 2 windigs.

A properly made Variac winding will usually have the contact tops of the windings pressed down flat to maintain most of the cross-sectional area, not just filed off halfway through.     That's where the one Chinesium-grade one that I saw had blown up.  Cooked the tops right off of a bunch of windings.

Long time ago, I heard straight from a variac repair technician from Matsunaga, apart from the importance of contact area, also for example at the Matsunaga design, the rectangle shaped tip is slanted just few degrees in order for it to make new contact at the next adjacent wire before leaving the previous contacted winding wire. Photo attached below.

Interesting.  The brush on the Powerstat is perfectly radial - no angle at all that I can see, while your Matsunaga has an obvious angle relative to the centerline.



-Pat

[drussell]

I actually knew I had a D800 but I didn't associate it with using it for video.  It can be done but some features are quite lacking, like not being able to auto-focus during shooting.  Just few weeks ago, I was thinking of re-re-restarting film photography.

They can be fantastic for video!  What lens do you have on it? 

With a compatible lens, you should be able to put it into continuous auto-focus mode (AF-F) where it runs the lens in constant servo-mode and derives focus from the sensor.

[drussell]

Interesting.  The brush on the Powerstat is perfectly radial - no angle at all that I can see, while your Matsunaga has an obvious angle relative to the centerline.

Yeah, that is definitely a slightly different engineering design setup, the brush contact is much "wider" than the narrower, pointier style that I've always pictured when I thought of a Variac, but if you look in that list from carbonbrush that I posted above, there are obviously a lot of different styles!  I'm no Variac expert! 

I'm sure that David Riddle guy from the earlier links above knows a thing or two about Variac brushes. 

[Ian.M]

https://sound-au.com/articles/variac.htm is worth a read, and his comments on the use of anisotropic carbon brushes to reduce the 'shorted turn' circulating current in the turns bridged by the brush are especially noteworthy.

[Electro Detective]

GE Variac got it right back in the day with dual (duo track?) carbon brushes,

good luck skipping and arcing that setup with the rated load applied, smooth as silk tracking from 0.5v to 270 at full stick 

fwiw use the TooHungLow red devils at 40% of their 'optimistic' label rating and you might be ok         (...... )

[engrguy42]

Cool, thanks.

You sure about the wire diameter? Sure looks bigger than #24.

Anyway I had a couple of variac videos pop up when I went to youtube (apparently their AI whatever noticed I was looking at variacs), and I saw a couple talking about the cheap red Chinese models.

One was from a few months ago, and he unboxed a new one. Turned it on (no load) ran the wiper to max and min and the fuse blew when he got to minimum voltage. Turns out the factory had miswired it, and connected the incoming 120v line to the wiper. So when he turned the dial to minimum it was actually stepping up voltage to some huge value on the primary winding. And I assume the fuse blew when the 120v shorted at the minimum dial setting. And I think the guy's lucky he wasn't touching the wrong thing when it stepped up the line voltage to whatever it went to. He was grabbing the metal case with the thing turned on, and had the front panel open. Geez.

Anyway, I never thought of that one...

Another one from a few years back showed a guy with a similar model, and as he rotated the dial a connected light bulb would flicker, indicating a problem with the wiper/brushes. He talked about presumed low quality of these Chinese products, and took it apart to repair. Though while taking it apart he threw up a text graphic on the screen saying "In all honesty, I've owned this unit for about 10 years, and I've overloaded it quite a bit...". So apparently it was working fine for 10 years and he beat the crap out of it and it the windings needed some cleaning, as well as tightening a loose wiper.

So yeah, be careful with stuff that can hurt you. But I think a blanket dismissal of stuff like this is tough to justify. 

[engrguy42]

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? 

So the wire is good for 1 amp--if it is copper.  After you are done blowing it up, can you check and see if the wire is copper or CCA?

Huh? Where do you get a 1 amp rating for #24?

Anyway, it looks like we're talking about a device with a 5A fuse (according to the front panel), meaning it's only rated to carry maybe 2-3 amps. That's peanuts. Why the concern over a point contact with the brushes? 

[Electro Detective]

...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 

[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?

[BravoV]

...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.