Interesting Variac Idea- Toroidal? Rackmount?

[homebrewer99]

Attention all electrical engineers, techies, builders, etc etc...

The Problem: I am interested in reducing the source voltage in my home or at local gigs (up to 128V at times) to a vintage amp friendly 115V or so. The ideal situation would be to reduce voltage to a safe level, then rebias my amps if needed. This would put my vintage and vintage-style amps at ease. I am NOT interested in the elusive EVH "brown sound" at 90V or other unsafe levels. I understand that heater voltages could be too low with source voltage below 110v.

My idea: A rackmounted or "low profile" Variac using a toroidal transformer. I could safely reduce voltage with a smaller and easier to transport toroidal model, and then feed this voltage into a Furman or other regulator. Perhaps the packaging could be made rack-friendly so as to sit under the Furman in my rig. Many Furmans have an LCD display that could be used to monitor source voltage and allow for adjustment of the variac to a consistent 110-115V.

Does anybody know of a lightweight Variac?

Would it be simple to build? I know toroidal transformers are more expensive, but the decreased weight and size would be beneficial to many who want the benefits of a variac without the size/weight hassle. Also, high end voltage regulators are $$$$$$. A Variac + Furman setup would be much less expensive, though larger. It seems like a good idea....

Any thoughts?

[pgissi]

My idea: A rackmounted or "low profile" Variac using a toroidal transformer


Technically a Variac is already Toroidal but the low profile design is something that would be needed. Not sure Variac and lightweight and low profile go together, we are talking about dissipating excess current with a Variac, one big mutha rheostat.

But I will undertake a google search and post what I find

Check out this guys Variac 101 page-
http://www.nmr.mgh.harvard.edu/~reese/VariacPage/


http://www.nmr.mgh.harvard.edu/~reese/VariacPage/variac_under.jpeg

[homebrewer99]

pgissi-

Thanks for the link!

I now understand that the traditional format of a Variac IS toroidal, but I seem to remember seeing lower-profile toroidal transformer designs than the typical huge Variac. Check out high end PA amplifiers (Crown etc) for examples of rackmount-sized gear with toroidal trannies.

Perhaps a dumbed-down version is needed, as I see no need for voltage boost or rotation reversal features. All I want is a 10-15% reduction in source voltage. I wonder about the use of zener diodes in place of a transformer. In theory a network of zener diodes/low profile tranformer(s) would bring down the size/weight of such a device.

cheers,
-homebrewer

[SatelliteAmps]

You'd be better off building a browning device. You can wire up a 12v transformer to remove 12v from the ac line so you would always be reducing roughly 10 percent. I can draw a picture if you need one. Much more foolproof, and a heck of a lot smaller. A 10a 12v transformer is roughly 2 inches cubed. Would be very easy to rackmount. And would handle more current than zener diodes. You could get all fancy and attach a light, fuse, switch, and even a power meter and still have tons of space in a rack.

[homebrewer99]

Satellite Amps-

I like your idea of a constant voltage reduction, say -12V. Not as versatile as a variable voltage reduction, but smaller and cheaper.

Any hints as to the type of transformer needed? I suppose I would need a 1.1:1 winding ratio to decrease voltage by 10%. Also, do I really need a 10 A rating as you discussed in your example? By my calculations, a 100W Marshall, which in reality could output 150W or more running on 128V, would consume 1.25A. Seems like a 5A tranny would be fine for up to 2 amplifiers, including some headroom for error/safety.

150W = 128V * 1.17A

It seems as though every transformer manufacturer advertises different specs, which makes things difficult for a newbie like me.

Thanks to all who are contributing!

cheers,
homebrewer

[SatelliteAmps]

Sounds like you have good math for a newbie. Here's what to look for. You want a transformer with a 120v primary (117v is what some will say), and a 12v (or 12.6v) secondary. If you are purchasing a new one, don't worry about a center tap on the secondary. You won't use it. If there is one that is the same specs, but has a center tap, that is fine too. Just cap it off.

The next spec for the transformer is the amps. While it is true that your math works for 1.17A, you should try and AT LEAST double that figure for safety. So, shoot for 3A minimum. Higher amperage will let you run more than one amp on this supply if you want to, and the price difference is only about one or two dollars more.

Once you have the transformer you wire one side of the primary to the negative of the electrical. The positive goes to the diagonally opposite secondary. The other part of the secondary goes to the positive AC out. Make sense? If you wire it backwards it will add 12v instead of removing it.

[scottl]

Why not just get the Furman Ar1215 or the ETA sys regulator? They are toroidal and have steps to keep the voltage in a sweet range. I usally vary between 117 and 120 with them. No matter the wall. Even if the wall is 110!! Much more neat and tidy than using a variac and regulator. I understand you want a few less volts, but couldn't you just bias the amps up to take advantage of the narrow operating range with just the regulator?

[reaiken]

Once you have the transformer you wire one side of the primary to the negative of the electrical. The positive goes to the diagonally opposite secondary. The other part of the secondary goes to the positive AC out. Make sense? If you wire it backwards it will add 12v instead of removing it.

Here's a couple of pages with wiring diagrams - go down to the part on making a "bucking" transformer...

http://www.allaboutcircuits.com/vol_2/chpt_9/5.html

http://www.triode-systems.com/?q=node/4

Be sure to use a transformer with a secondary current rating greater than the current required by your amp.

RA

[SatelliteAmps]

I like the triode-systems one's drawing better, but those are both what I was talking about.

As for why use it instead of a power regulator, because it would be inexpensive, and a cool project to make. Which is what this thread was started about.