How to wire variac with voltmeter and ammeter?

[Phil Nelson]

Secret Santa generously sent me a NIB General RadioW2 variac (thanks, man!).
I would like to wire it up with a voltmeter to show output voltage and an
ammeter to show current draw.

The internal connections are shown here:

Loading Image...

I guess terminal 3 is the variable output, and I would put the ammeter in
series with that terminal and the line going to the radio. I suppose the
voltmeter would be wired across that terminal and one of the others.

My feeble brain doesn't quite grasp why there are four other terminals . . .
.

Any help?

Phil Nelson
Phil's Old Radios
http://antiqueradio.org/index.html
to contact me via email, kindly go to http://antiqueradio.org/contact.htm

[Jeffrey D Angus]

Post by Phil Nelson
http://antiqueradio.org/art/temp/Variac.jpg
I guess terminal 3 is the variable output, and I would put the ammeter in
series with that terminal and the line going to the radio. I suppose the
voltmeter would be wired across that terminal and one of the others.
My feeble brain doesn't quite grasp why there are four other terminals . . .

Connect Input NEUTRAL to terminal (4)
Connect Input HOT to terminal (2)
Connect Output (hot) to Terminal (3) and (neutral) to Terminal (4)

If you use an ampmeter, put it in series with Terminal (3) and the
output receptacle.

If you use a voltmeter, Put it across terminal (3) and (4)

If, for example, your line voltage is 110 VAC, connect the input
HOT to Terminal (1) instead of Terminal (2). This will allow you
to go from 0-130 VAC instead of 0-110 VAC.

Put a 3 Amp circuit breaker in series with the OUTPUT lead on
Terminal (3) The overall current rating of the windings is
limited to 2.4 Amps. A 3 Amp circuit breaker gives you a little
bit of margin, but will still protect the windings if something
silly happens. (Like shorting the suicide cord you're using.)

Put a 4 Amp fuse in series with the INPUT lead on Terminal (2)
(Or (1) if you're boosting). This will protect the variac and
internal wiring in case something dreadful happens.

For metering, might I suggest the MFJ limited range AC volt
meter. http://www.mfjenterprises.com/products.php?prodid=MFJ-850
It reads from 95-135 VAC. For the Amp meter, 3 Amps full scale.

Jeff

[Michael A. Terrell]

Post by Phil Nelson
Secret Santa generously sent me a NIB General RadioW2 variac (thanks, man!).
I would like to wire it up with a voltmeter to show output voltage and an
ammeter to show current draw.
http://antiqueradio.org/art/temp/Variac.jpg
I guess terminal 3 is the variable output, and I would put the ammeter in
series with that terminal and the line going to the radio. I suppose the
voltmeter would be wired across that terminal and one of the others.

Yes, the ammeter goes in series with the load, and the volt meter goes
from terminal 3 to the neutral connection on the unit.

Post by Phil Nelson
My feeble brain doesn't quite grasp why there are four other terminals . . .

You can use it for 0 to 120 volts, or 0 to 140 volts out. If you
need to, you can use it for 20 to 120 volts out, or 20 to 140 volts out.
The simplest way is to connect the line and neutral to terminals 2 & 4
for line and load for 0 to 120 operation. If it operates in the wrong
direction, switch the wires to these two terminals.

Post by Phil Nelson
Any help?
Phil Nelson
Phil's Old Radios
http://antiqueradio.org/index.html
to contact me via email, kindly go to http://antiqueradio.org/contact.htm

[Phil Nelson]

Thanks, guys, that's exactly what I was looking for.

Now I just need to make a box big enough to mount all that stuff. The local
Radio Shack didn't have anything suitable.

Phil Nelson

[gb]

Post by Phil Nelson
Thanks, guys, that's exactly what I was looking for.
Now I just need to make a box big enough to mount all that stuff. The local
Radio Shack didn't have anything suitable.
Phil Nelson

Phil -

Here is the PERFECT cabinet for your project (nice price).
You can mount the voltage and amp meters side my side at top.
Variac with outlets on the bottom of cabinet.

eBay Item number: 7576260403

gb

[Engineer]

Phil, I don't know the model you have but I mounted a surplus "Variac"
(but different brand) in an upside-down large coffee can, using three
bolts though the bottom with another centre hole punched for the
spindle. I mounted a two hole 115 VAC domestic socket in the side by
clipping out a rectangle with tin snips and bolting it on using a metal
(not plastic) plate, the metal plate being slightly bent to conform
with the radius of the coffee can. The 115 VAC input is an via IEC
plug and socket from a surplus PC power supply on the other side of the
can (needs a small rectangular hole - chop it out by hammering on a
sharp screwdriver on a thick piece of wood backing.) Use a surplus PC
power cable, of course.

The voltmeter and ammeter are external for this one. My other variac
is in a proper box with a built-in voltmeter (only.).

BTW, I have connected both my variacs for 120 VAC maximum to avoid any
inadvertent over voltage - I've never had use for 140 VAC in tube
radios and amplifiers.

Cheers,
Roger

[Phil Nelson]

Well, the coffee-can solution is definitely on par with my metalworking
skills and equipment! I'm pretty sure I can dig up a tin snips, various
screwdrivers, and a few worn-out files, but that's about it. I really would
like to put everything into one case, however. Maybe I'll have to pick up a
couple of new tools if I decide to use one of the prefab aluminum or steel
enclosures.

Thx.

Phil Nelson

[Tim Mullen]

Post by Phil Nelson
Well, the coffee-can solution is definitely on par with my metalworking
skills and equipment! I'm pretty sure I can dig up a tin snips, various
screwdrivers, and a few worn-out files, but that's about it. I really would
like to put everything into one case, however. Maybe I'll have to pick up a
couple of new tools if I decide to use one of the prefab aluminum or steel
enclosures.

I hear Eddie's got a milling machine. :)

[Tom Adkins]

Post by Phil Nelson
Well, the coffee-can solution is definitely on par with my metalworking
skills and equipment! I'm pretty sure I can dig up a tin snips, various
screwdrivers, and a few worn-out files, but that's about it. I really would
like to put everything into one case, however. Maybe I'll have to pick up a
couple of new tools if I decide to use one of the prefab aluminum or steel
enclosures.
Thx.
Phil Nelson

Just letting my mind wander a bit Phil,
How about a generic 20's battery set cabinet of appropriate size using period
looking meters and switches. I would personally use a wattmeter instead of an ammeter.
Mount the receptacle on the front panel, or use cloth line cord in and out of the back
with a plug on one and outlet on the other. A black plastic front panel, or a metal
front panel finished in black or brown wrinkle paint with brass (or gold painted)
fasteners. Add some gold lettering for the controls. The look i'm imagining is like if
Atwater Kent or Chas.Freshman made a metered variac in the late 20's.
I saw one similar to this at Estes once and tried for it. Sadly, I was outbid. It
really looked cool.
I later bought one from Gary Schneider in a metal cabinet with black wrinkle finish.
It looked about as neat, only in a Frankensteins lab sort of way.

[Phil Nelson]

Post by Tom Adkins
I would personally use a wattmeter instead of an ammeter.

What would be the advantage? You look at the radio's wattage rating and
compare to the meter reading while slowly powering up?

Phil Nelson

[Tom Adkins]

Post by Phil Nelson

Post by Tom Adkins
I would personally use a wattmeter instead of an ammeter.

What would be the advantage? You look at the radio's wattage rating and
compare to the meter reading while slowly powering up?
Phil Nelson

Strictly personal preference Phil. Most radios have the current draw listed in Watts
on the ID tag and I use that for a base line. If a radio reaches that rated current
before the rated voltage, there's a problem.. and so on. Both metered variac setups
I've had used wattmeters, so it's what I'm comfortable with.
To be honest, if I saw a radio rated at 60 Watts drawing, say, .75 amp at 95 volts I
would have no idea if that was good or bad. I've never learned the correlation(sp)
between Watts and Amps at varying voltages. Laziness on my part as it's something I
should probably know.

[Jeffrey D Angus]

Post by Tom Adkins
To be honest, if I saw a radio rated at 60 Watts drawing, say, .75 amp
at 95 volts I would have no idea if that was good or bad. I've never
learned the correlation(sp) between Watts and Amps at varying voltages.
Laziness on my part as it's something I should probably know.

Volts X Amps = Watts.

From you're above example 67.5 watts.

I'm lazy... I put a chart next to the amp meter for when the
Variac is set to 125 VAC.
0.1 Amp = 12.5 watts
0.2 Amp = 25 watts
And so forth.

And easier (also lazy) method is to look at the tag on the back of
the radio. If it reads 60 Watts at 110 Volts, then that means it
shouldn't draw more than 0.545 amps. So, as you crank up the Variac
if it goes up over 0.6 amps, you know you have a problem.

Jeff

[Bill]

Post by Jeffrey D Angus
And easier (also lazy) method is to look at the tag on the back of
the radio. If it reads 60 Watts at 110 Volts, then that means it
shouldn't draw more than 0.545 amps. So, as you crank up the Variac
if it goes up over 0.6 amps, you know you have a problem.
Jeff

Tube radios will generally draw less amperage at lower voltages so you
shouldn't reach that tag rating (watts or amps) until you hit full voltage.

-Bill

[Tom Adkins]

Post by Jeffrey D Angus

Post by Tom Adkins
To be honest, if I saw a radio rated at 60 Watts drawing, say, .75
amp at 95 volts I would have no idea if that was good or bad. I've
never learned the correlation(sp) between Watts and Amps at varying
voltages. Laziness on my part as it's something I should probably know.

Volts X Amps = Watts.
From you're above example 67.5 watts.
I'm lazy... I put a chart next to the amp meter for when the
Variac is set to 125 VAC.
0.1 Amp = 12.5 watts
0.2 Amp = 25 watts
And so forth.
And easier (also lazy) method is to look at the tag on the back of
the radio. If it reads 60 Watts at 110 Volts, then that means it
shouldn't draw more than 0.545 amps. So, as you crank up the Variac
if it goes up over 0.6 amps, you know you have a problem.
Jeff

Thanks for the lesson Jeff.

[Peter Wieck]

If a radio reaches that rated current before the rated voltage, there's a problem..

If it reaches even 90% of that rated current, there is a problem. Just
my experience. Usually the rated current is peak + some small allowance
rounded UP to the nearest even 10 (sometimes 5).

So, take the nameplate rating with a grain of salt.

Peter Wieck
Wyncote, PA

[d***@hotmail.com]

Post by Peter Wieck

If a radio reaches that rated current before the rated voltage, there's a problem..

If it reaches even 90% of that rated current, there is a problem. Just
my experience. Usually the rated current is peak + some small allowance
rounded UP to the nearest even 10 (sometimes 5).
So, take the nameplate rating with a grain of salt.
Peter Wieck
Wyncote, PA

Hi
Has anyone mentioned that a transformer set may actually show
more current than the rated value because of inductive current?
Many power meters measure true power while having a separate
voltage and current meter you may not be able to properly interpret
the actual power level.
Dwight

[Peter Wieck]

Either way, there is trouble. In the realm of vintage radios,
excessive current inductive or otherwise suggests something unwanted is
happening. It is important to have separate meters so as to determine
threshold voltages for rectifiers, and to track current draw separately
from voltage... especially in the case of older radios where the
difference between the design voltage (110V, mostly) and actual
wallplate voltage (up to 130V it seems these days) can be significant.

My Heath iso-variac has a dual-scale switchable AC ammeter (0-1, 0-3)
as well as a 0-140 AC voltmeter, and does the job nicely.

Peter Wieck
Wyncote, PA

[gb]

Post by Phil Nelson
Secret Santa generously sent me a NIB General RadioW2 variac (thanks,
man!). I would like to wire it up with a voltmeter to show output voltage
and an ammeter to show current draw.
http://antiqueradio.org/art/temp/Variac.jpg
I guess terminal 3 is the variable output, and I would put the ammeter in
series with that terminal and the line going to the radio. I suppose the
voltmeter would be wired across that terminal and one of the others.
My feeble brain doesn't quite grasp why there are four other terminals . .
. Any help?
Phil Nelson
Phil's Old Radios
http://antiqueradio.org/index.html
to contact me via email, kindly go to http://antiqueradio.org/contact.htm

Now how are you going to "package this" variac?

Enclosure ideas ... built into the workbench or luggable?

gb

[Phil Nelson]

Post by gb
Now how are you going to "package this" variac?
Enclosure ideas ... built into the workbench or luggable?

I'd like to build it in a case of some sort. My workbench is too
disorganized and ever-changing to have built-in instruments :-)

The variac is compact (maybe 5x3x3 inches) but heavy. I was thinking of a
vaguely cube-shaped case, with variac mounted from the top and meters plus
output plug on the front. I started pawing through my Mouser and Allied
catalogs and promptly got a headache from too many choices. I suppose the
best idea is to assemble all the parts & pieces and then pick something of
the right dimensions.

Regards,

Phil Nelson

[Peter Wieck]

Coffee can.....
Variac (Variable Auto-Transformer)....
Gounded receptacle....
Two meters....
& associated wiring....

Why am I getting cold shivers contemplating this design?

OR, was the coffee-can in question one of the new plastic ones?

Domino Sugar, BTW, now has very nice pretty solid sugar-boxes (round)
made of stiff plastic. I am very gradually collecting them for parts
management. We barely use sugar so it will be a while. But this might
be an option.

Peter Wieck
Wyncote, PA

[Phil Nelson]

No worries, I think I can manage to spring loose a coupla bucks for a new
enclosure :-) Might even look for something in heavy plastic -- easier to
work -- possibly with a metal plate on top to support the variac.

My ancient homebrew variac was poorly wired/mounted in an all-metal case
(complete with hand-lettered and inaccurate voltage scale), so I well
understand the excitement of brushing one's elbow against it while grappling
for three different things on the workbench!

Regards,

Phil Nelson

[Ron in Radio Heaven]

I have a friend that has his built in an small old battery set cabinet.
Black panel, black knobs, old style meters.
Looks really cool.

Ron

[DaveM]

Post by Phil Nelson
No worries, I think I can manage to spring loose a coupla bucks for a new
enclosure :-) Might even look for something in heavy plastic -- easier to
work -- possibly with a metal plate on top to support the variac.
My ancient homebrew variac was poorly wired/mounted in an all-metal case
(complete with hand-lettered and inaccurate voltage scale), so I well
understand the excitement of brushing one's elbow against it while
grappling for three different things on the workbench!
Regards,
Phil Nelson

Just a suggestion, and not trying to deter you from any of the other
suggestions...
Build a nice little box out of 1/2" plywood or MDF, and mount the Variac and
meters on a sheet of acrylic or Lexan from your nearest Home Depot or Lowes.
You can paint the acrylic and use rub-on lettering to label the components.
Everything is sturdy and well-insulated.
I've built a couple for friends like this... turned out quite well.

[Pete_O]

What ever happened to simplicity?

I use a simple outlet box that has the black wire broken out in a short
half turn loop and connected back in the box to a duplex outlet. Current
is digitally measured with a clip-on AC ammeter on the loop.

If I really need to know the voltage accurately I will measure it with a
voltmeter. Otherwise I use the simple hand scribed marks that I put on the
Variac. Do we really need to know if the DUT fails at 24, 57 or 83 volts?
-Pete O

[Peter Wieck]

Current is digitally measured with a clip-on AC ammeter on the loop.

Does your clamp-on unit give you accurate measurements within 5%
(total) and read meaningful values between 0 and 100 watts? Can you
watch it track as the rectifier kicks in? And can you watch it track in
tiny fractions over time and as the unit under test experiences various
loads? If not, it is useless for the requirement. This is one of the
very few times that an analog meter has a major advantage over a
digital meter as it is infinitely easier to track trends and tiny
movements without distracting last-digit chatter.

Peter Wieck
Wyncote, PA

[Pete_O]

Does your clamp-on unit give you accurate measurements within 5% (total)
and read meaningful values between 0 and 100 watts? Can you watch it track
as the rectifier kicks in?

**Generally I don't care about exact current when using a Variac; all I
want is an indication of potential smoke. If I make an accurate current
measurement at 50 or 80 volts it serves little purpose since those are not
the currents that will be seen at 115v- they tell me only that I might
expect smoke when the rectifier conducts and that I should do some more
tests with my ohmmeter before continuing.

I abhor digital autoranging also. It can be somewhat eased with an
"analog" bar meter (better DVM's have it) and not using autorange. -Pete

[Jeffrey D Angus]

By the way, for those of you that don't to be bothered with
flailing around making a cabinet and so forth.

http://www.mpja.com/productview.asp?product=15163+TR

$89 for a complete 2KVA variac with a volt meter.
(2 KVA = approximately 15 amp rating.)

Jeff