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Variable AC Isolated Transformer (showing wrong voltage)

AcousticBruce

Apr 1, 2013
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I got this Variac in a package deal. I hooked my DMM up to the device and it reads quite a different voltage. For instance: In order get get 120V AC on the Variac I have to dial it to 140V on the LED display. Notice the two displays one being 140V on the Variac and 120V on the Fluke DMM

https://www.dropbox.com/s/zu90uvfly9yvyvl/20130401_234442.jpg


Why would something like this happen?
Is there any way to fix it?

Edit: I just added a straight link of the picture. Some found the previous one not to work.

Edit: This is a Global Specialties 1504 ->> PRODUCT PAGE

edit: There are better pictures later in the thread
 
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Harald Kapp

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Your picture is not showing.

2 ideas:
1) display on the variac not calbrated correctly
2) display on the variac only correct under certain load


Unlikey: fluke not calibrated correctly?
 

AcousticBruce

Apr 1, 2013
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1) This could be it and I would bet on this one. If you have any info on calibrating that would be awesome. I will do a search and try to find a manual for this.

2) This is interesting, I made a computer power supply into a bench-top power supply which acted funny under certain loads. When a piece of equipment acts different under certain loads, is that a result of cheap equipment?

The Fluke is calibrated and working great. So this is extremely unlikely.
 

Harald Kapp

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1) No chance without manual or at least a schematic.

2) Some power supplies, specially the switched mode type, require a minimum load for correct operation. That's not necessarily cheap (although a good quality ps can handle this with additinal internal circuitry). For a computer ps, for example, one can assume a certain inimum load in good faith and therefore forego the circuitry to make it stable under no load conditions.
 

AcousticBruce

Apr 1, 2013
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Very valid to think mains was low. I tested it with fluke as well as my bk precision bench top. Both read 120v ac from mains.

Thanks for the idea.
 

shrtrnd

Jan 15, 2010
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I was thinking like Harald Kapp, the variac may be wired to read in a loaded condition.
My second choice would be that the LED readout needs to be adjusted.
Good to get the manual, and see what it says.
 

AcousticBruce

Apr 1, 2013
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What is that called? Unregulated?

If that is called unregulated then is there a good reason to why you would use something that is unregulated vs regulated?
 

shrtrnd

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I can't access the picture you sent of your variac.
MOST variac's are 'unregulated', although some of the more expensive ones are.
Yeah, basically a regulated variac has it's own circuitry internally to maintain a set
voltage whatever the load applied.
Most people USE a variac to get in the ballpark of a desired voltage, using a voltmeter
across the load to get exactly what they want. All the money in construction of the variac is
in the transformer windings, and it can get pretty expensive to start adding circuitry for
regulation of that voltage, so cheaper is usually better for most applications (I think).
What I think YOUR issue here is however, is that the LED readout might need to be
tweeked to give you a more accurate read. There's probably a trimpot on the display,
or feeding the display, that will do that. You just have to be sure you're tweeking the
LED readout to the correct value.
 

AcousticBruce

Apr 1, 2013
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I can't access the picture you sent of your variac....

https://www.dropbox.com/s/zu90uvfly9yvyvl/20130401_234442.jpg

This is a Global Specialties 1504 ->> Product Page here with specs

Perhaps it will be a simple one. I would be curious to know if this is regulated or not. Are there other words for regulated that I should learn? Such as: "Triple isolated continuous variable AC Output" What does this mean?


Btw the reason I have this is because I went to buy this Tek 2235 and walked out with the Tek 2235 the BK Precision 1635 Benchtop PS and this Global Specialties 1504 all for $240 bucks. I think I did pretty good. Even though the scope is needing work, I probably did ok. Otherwise I wouldn't have bought a $500 dollar item without knowing what it is. It just seemed like a good deal.
 

shrtrnd

Jan 15, 2010
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That is an expensive variac. You can email Allied or Global Specialties and ask them
but I'd be shocked if it isn't a regulated voltage.
Make sure you've got an accurate DVM, and I'm sure you can tweek the read-out to
be accurate is heck.
The vaiacs I use are around $100, and only have circuit protection.
You have a little work to do on your instruments, but yeah, you got a great buy.
 

AcousticBruce

Apr 1, 2013
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Variable Isolated AC Power Source - Teardown and Troubleshoot

I had another post about this but it was closed... so here is a better one with more info.

Here is the manual with the schematics HERE

It is a 1504 Isolated Transformer Variac that is showing the wrong Voltage. So I opened it up and on the display circuit was 1 variable resistor so I tweaked so the display said 120V while the Fluke also said 120V.

So the update is now the unit shows 120V perfectly but as I go above or below that voltage it changes exponentially the farther away you get from that voltage.

Here are some measurements for the Fluke and the Variac at various volts.

(I will update these when I get home tonight - this is from memory)
Variac - Fluke
120V 120V
185V 150V
12V 2V

Does anyone have any idea what this could be???


Here are the pics of the inside.. MAN this transformer is awesome!!!

20130410_134631.jpg


If you look at the top left of the display circuit you can see the variable resister I adjusted.

20130410_135311.jpg


There are about 4 more variable resisters as you can see from this view.

20130410_135353.jpg
 

AcousticBruce

Apr 1, 2013
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Oh I see... closed was the wrong terminology. I just have no ability to edit the first post for some reason. I figured it was closed or something. I would rather it be added to this anyway.

How can I edit my first post? Is it locked?
 

(*steve*)

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It's better that you DONT edit your posts to add more information.

I, for one, certainly don't expect it. People will probably not read it again to see what you've added.

Also it makes the thread disjointed if you edit earlier posts to give information asked for in later posts.

Ian disabled editing of older posts for this and similar reasons.
 

AcousticBruce

Apr 1, 2013
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Well the pictures do not work on the original post. I would like to add these to the original.
 
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(*steve*)

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OK, start with the minimum voltage the device will output (knob all the way anticlockwise).

What does your meter read, and what does the display indicate?

Turn it up bit by bit and report at interesting places (e.g. if the reading remains at zero for a while, tell us when it first becomes positive, or if the output voltage stays zero for a while, do a reading when that changes.)

get 5 to 10 readings from 0 to the max voltage and let us consider them...
 

AcousticBruce

Apr 1, 2013
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Ok fair enough.


Variac - Fluke
10V - 54mV (lowest possible)
11V - 1.5V (one bees dick notch up)
12V - 3.8V (Again just a bit of a notch)
15V - 7V
20V - 12V
35V - 28V
50V - 44V
75V - 71.5V
100V - 98.6V
120V - 120V
130V - 130.5V
140V - 142V
150V - 152V
160V - 163.5V
163V - 167V (max rotation)

Here is a picture of the variable windings.

20130411_092554-1.jpg
 

(*steve*)

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OK, I can see 5 pots.

I suspect you adjusted the one on the rear of the display.

I would recommend that you attach the probes to the voltage input for this display module. I'm not sure which ones they are, but they'll be the ones which change as you change the output voltage :)

See if the voltage you measure here is the same as the display (it won't be because I think you adjusted the pot).

But it would be useful to know if the voltages you see here are the same. i.e. at the minimum value for the variac, do you get 54mV, 10V, or something different again.

Probably best to repeat the measurements for the same values on the display. You can set then probe, set then probe... and so on.

The unfortunate thing is, once you've changes one setting, you're in unknown territory. We really need to know what the trimpots adjust before you change them.
 
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