This I did not know about variacs...

[Externet]

Wondering about if a variac could be used as an attenuator in speaker level circuits ; focused attention to the wiper contact. It always touches about three turns at any position.

Well... that should be shorting those windings, implying an unhappy variac behavior. Just like any transformer if had a few internal turns shorted.

It is surprising why it does not happen, from a clever way to avoid the shorting problem.

Miguel

[Bigglez]

Wondering about if a variac could be used as an attenuator in speaker level circuits.

Probably not, or at least not well. The audio signal covers
about four decades of bandwidth, while a variac operates
on a single frequency (or limited range if spec'd for 50/60Hz).

In PA systems for large buildings (warehouses, train stations,
factories) it is common to distribute the audio at 100V line
level, and have small transformers to drive the speakers.
By changing taps (turns ratios) the amount of power at
each speaker can be changed (ot suit local conditions).

Therefore you could employ a speaker matching transformer
as part of your attenuator experiments.

[Janitor Tzap]

I'm wondering........

Are you looking at something like an L-Pad?

These are quite common in speakers, and speaker systems.


Signed: Janitor Tzap

[MrAl]

Hi Miguel,


I have to agree in that a variac probably wont work too well on audio unless it is
low frequency like 40 to 400Hz or thereabouts. One of the problems is that in
the design of a variac it is known beforehand what frequency it will be operating
at and that frequency is usually either 50Hz, 60Hz, or 400Hz (the most common
being 50 and 60Hz), and so a magnetic core material that works well at these
low frequencies is used to keep cost within reason. The problem with the core
material is that it absorbs more energy from the signal as the frequency goes
up, so at some point you get way too much attenuation which steals much
of the power. With that extra power loss comes extra heat too, which actually
could overheat the core with enough input power applied.

If you really want to try this you might look for a 400Hz model and see how
high you can get before problems come up. You may be able to push it
to 1kHz or so depending on other things like rms voltage applied.

I guess there is a chance they make models that work up to higher frequencies
so maybe do a search, but for this thread i assumed you were talking about
a 'standard' variac that might be found in the home workshop or something.

[Dean Huster]

Miguel, are you actually looking at a true Variac (tradename of General Radio) or a Chinese knockoff? The brushes I've seen in high-end variable autotransformers (Variacs) have been "pointy" rather than flat on the winding contact end so that not so many turns are shorted. However, you must realize that if NO turns were shorted by the brush, the voltage would run like this as you turned the knob: 50v; 0v; 50.5v; 0v; 51v; 0v; 51.5v; 0v; 52v; 0v; etc.

By the way, the standard level for driving audio lines (at least here in the U.S.) is 70.7 v, not 100v, and there are other, lower voltage standards as well that are rarely used, such as 25v.

Dean

[Externet]

Yes, a variac probably would not be applicable for speaker lines, I was NOT trying to use them for audio, just wandering about applicability; but the windings shorting is what caught my curiosity.

The brush does touch about 3 windings at any position, that is shorting them; and from something I read there is an interesting approach which I do not understand.
It is the use of anisotropic graphite which has directional conductivity for the brush ?

http://sci.tech-archive.net/Archive/sci ... 02749.html

If such thing is the key; how can it work if at the opposite (upper) end of the brush all the graphite layers are joined !

Trying a graphic...

______-----------------------------wiper terminal that holds the brush below it
|||||||||| -------- layered graphite, conducts vertically and very poorly horizontally
nnnnn ------- windings apex were the brush contacts the toroidal coil

Edited: added---> Found a "schematic" of the clever special graphite brush; explains it better. So it is not a plain whatever piece of graphite... and cannot be metal ! , that would toast the thing... interesting.
http://sound.westhost.com/articles/variac.htm

(My variacs are Powerstat, no chinese)
(Yes, the transformer taps are the correct way for audio distribution)

Miguel

[Lenp]

Dean,

The 25 volt speaker distribution system is most exclusively used in commercial intercom systems. Bogen, Rauland, Dukane, Telecor and most other big system use it. I have box full of these transformers, Need some?

Len

[Dean Huster]

Len,

70.7 systems are the only ones I've found myself, but that's just me, isn't it? Interesting that the intercoms use the 25 v systems. Most PA systems that I've seen have provision for all the standard speaker impedances from 4 to 16 ohms and then three line outputs.

In general, the longer the audio run, the higher the system voltage that should be in use to minimize losses through IR drops.

Nope. I don't need any transformers.* Need a Radio Shack PA amp?

Dean

*Actually, I should qualify that comment. I DO need several audio output transformers if they were designed for push-pull vacuum tube amps in the 10 watt or less category for repairing a few antique radios I have lying around. When those audio interstage paper dielectric coupling caps get leaky or short, they nearly always wreck the primary of the output transformer.

[Lenp]

Dean,

The line losses are only part of the reason the intercoms use the 25V system. Since every remote station returns to a selector switch panel as a home run, and there can be hundreds, the switches are small (25 in a 1 RU space) and can cary only low currents. Additionally, these systems can be configured to 'broadcast' program material to rooms and groups of rooms by setting the individual selector switches to a program channel. In the 'all call' or 'emergency' modes, a bus gathering relay operates and connects all the speaker lines, whatever buss they are on, together. Think about 8 ohm speaker line circuits X 100 all connected in parallel!

Most intercoms have a 'privacy' switch position at the remote station. This actually shorts a tap on the speaker transformer so it's efficiency goes down the drain with the small signal from the loudspeaker when used as a microphone, but, has little effect on the larger signal coming from the console amplifier. The switch must be placed in 'normal' for two way comunications.
This scheme has been used since the 50's and is still used if there are remote speaker/call switch stations, even if the console is digitally switched.

Len

[MrAl]

Yes, a variac probably would not be applicable for speaker lines, I was NOT trying to use them for audio, just wandering about applicability; but the windings shorting is what caught my curiosity.

The brush does touch about 3 windings at any position, that is shorting them; and from something I read there is an interesting approach which I do not understand.
It is the use of anisotropic graphite which has directional conductivity for the brush ?

http://sci.tech-archive.net/Archive/sci ... 02749.html

If such thing is the key; how can it work if at the opposite (upper) end of the brush all the graphite layers are joined !

Trying a graphic...

______-----------------------------wiper terminal that holds the brush below it
|||||||||| -------- layered graphite, conducts vertically and very poorly horizontally
nnnnn ------- windings apex were the brush contacts the toroidal coil

Edited: added---> Found a "schematic" of the clever special graphite brush; explains it better. So it is not a plain whatever piece of graphite... and cannot be metal ! , that would toast the thing... interesting.
http://sound.westhost.com/articles/variac.htm

(My variacs are Powerstat, no chinese)
(Yes, the transformer taps are the correct way for audio distribution)

Miguel

Hi Miguel,


Anisotropic conduction would mean that the material conducts differently
depending on the (3d) direction of current flow. Holding a cube of copper
on the top of an ordinary road map: connecting leads on the west and east
sides the copper might have 0.001 ohms resistance, and connecting leads on
the north and south sides the copper would have that same resistance, and
connecting leads on the bottom and top of the copper the resistance would
still be the same because that material is isotropic. It conducts the same
in all three directions.

Given a block of anisotropic material that is plane oriented, connecting
the leads east and west might provide 0.001 ohms, but when connecting
the leads on the top and bottom or north and south we would read a totally
different resistance like perhaps 0.010 ohms depending on the material.

The brush on the variac could be oriented so that north and south provides
little resistance, but east and west causes a higher resistance. What
this does in effect is puts one resistor in series with each winding that
happens to be touching the brush with a common node at the brush
mounting metal arm where the brush connects to it. This means any current
flowing has to flow through those resistors, and any two windings see
at least two resistors in series between them, not a direct short. For three
windings that would mean three resistors, four windings four resistors, etc.
Add to this the winding resistance itself and you've got a current limited
short, not a direct short.

If i remember right the brush was one of the key factors in rating the
variacs ability to handle power.


Here is a little illustration...

[reloadron]

Hi All

OK, I can tell you that I am sitting here looking at one of THESE.

The one I have here is a Type 236BU
Input Volts: 240 Freq: 50/60 Hz.
Output Volts: 0 to 280 10 Amps 2.8 KVA

I measured the coil end to end DC resistance and it is 1.7 Ohms. I just measured with my Fluke 87 and nulled the lead resistance out. I didn't bother to use the bench DMM. The brush is beveled and looks to bridge 2 turns (maybe 3). The measured resistance matches the resistance called out in the specification PDF linked to above.

I took a piece of kapton tape and laid it over a section of the windings. I rotated the brush over the tape while observing the resistance. There was absolutely no noticiable change. Now granted resolving 0.1 Ohm doesn't say much but my best guess as to the effect of the brush bridging two or three turns is neglible. Considering the number of turns on this beast, it really shouldn't matter.

Somewhere around here I have some smaller units and I thing I have a 120 Volt 20 Amp unit. I used the one I could grab.

Ron

[CeaSaR]

Dean and LenP,

IIRC, RatShack used to sell a 25v PA amp, probably around 10 watts or so, a long time ago.
On occasion, I still see references to them or others like them.

Dean,

I have a few older tube stereos that I am never going to fix. I'll take a look and see what they
are and let you know, if you want them.

Everyone,

I was out on a job where they were razing an old campground and came across a huge variac.
I couldn't let them bury the darn thing, so I lugged it home (it's freakin heavy!). Confirmed, it
is a General Radio Variac, mounted on a panel with what looks like a mains type switch and a
smaller toggle switch. I wonder what the darn thing was used for. I'll upload some pictures later.

CeaSaR

[Lenp]

Ceasar,
Likely it was used for light dimming. These were plentiful years ago for stage and studio lighting control since they were way more efficient than series resistors, or salt baths or tanks that were used eons ago. There always wasn't solid state! It's also possible that it was also used for a ventillation fan speed control.

There were many companies that made these variable autotransformer. Superior, General Radio, Variac, Ward Leonard, Hub electric and a host of others.

FYI:
If the line voltage is fed to the ends of the coil, the output is zero to the line voltage. But, if the voltage is fed from one end of the coil to a tap near the other end, there is a boost and the voltage output will be zero to more than the line input.

Len

[MrAl]

Hi again,


Years ago we used three phase variacs that were almost 4 feet tall for testing synthesized
sine wave converters. One of them was huge and it was motor driven.
The three phase units were made out of three single phase units with
a common shaft and wired in either delta or wye. Dont remember the rating, probably 50 kilowatts.

[reloadron]

Hi again,


Years ago we used three phase variacs that were almost 4 feet tall for testing synthesized
sine wave converters. One of them was huge and it was motor driven.
The three phase units were made out of three single phase units with
a common shaft and wired in either delta or wye. Dont remember the rating, probably 50 kilowatts.

About 6 months ago I had a stash of several 3 phase motor driven units like that. They were 480 Volt 100 Amp units. I was out of town for three days and they gutted my stash for space. They tossed about $30K in a dumpster. I had a good cry. That happens when an electrical type is surrounded by mechanical types. and

Ron