December 2012

Bucking Transformer
Reduce line voltage to achieve happy circuits :)
Article By Jeff Poth

  "What's the matter with the transformer baby?" asked my wife.

"Huh?"

"You came in, yelled ‘Fucking Transformer' and slammed the door!"

"Oh! Nothing!" sez I, "I said Bucking Transformer, and I'm happy it's working properly."

"Oh! Good!" says wifey as she disappears back down the hall, shaking her lovely head and wondering why her lunatic husband gets so excited about a hunk of iron and wire.

It is not surprising that she wouldn't "get it". She didn't find herself stuck using a stack of power resistors and an extra switch in her amp because the transformer voltage was just slightly too high for the circuit. When I first built my "Herniator" amplifier.

Lucky for me, there is a solution that does not require the added resistance in the power supply, nor the extra heat in my amp chassis. That solution is the bucking transformer. In simplest terms, a bucking transformer is a way of reducing line voltage. A transformer's secondary is used in opposite phase to the power line input, and the result is a reduction in line voltage. In cases like mine, where a few volts of reduction is all that's needed, they can be an invaluable tool. If you're completely befuddled, this article may not be for you. Use of a bucking transformer in an application like mine requires working carefully with 120V in some countries and 220+ in others. Bucking transformers should be properly built into grounded metal enclosures and fused, as well as the transformers safely sized to the load required. Those of you who have worked with the basics of power supply iron may be wondering "Why wouldn't you just use a step-down transformer? Or re-spec the iron?" Entirely valid questions.

A step-down transformer is a great way to lower excessive line voltage. The problem with this is that a step-down must have the same power capability of the transformer in the device you're powering. This means I would need an even larger transformer than the already huge toroidal in my Herniator. (Transformers have internal loss so you have to oversize step-downs somewhat relative to the devices they're feeding). If you haven't read the Herniator article, this massive hunk of iron is a pricey jobber. Another one would be a difficult burden on your poor author's sad little project budget (donations of 15" woofers gladly accepted- anyone with an extra pair of Magnum 15HO?).

The cost and size of the iron answers the second question- for a one-off, replacement might be expensive or difficult to source. Certainly at the size and quality of my pricey custom trafo, replacement would have been a poor option for me. Thus we began with resistors, initially intended to be manually switched out as part of the power up cycle for "soft start". By including sufficient resistance to the power supply primary, I was able to get the voltage where I needed it. But such a solution is inefficient, requires big resistors, and wouldn't be needed if I had closer to 115V instead of 125V on my outlets. Upon considering my options, I chose to pursue the bucking transformer route. I already had some nice fat EI transformers that would bring my line voltage down sufficiently and had plenty of power capability (VA rating) to do the job safely and efficiently, without too much excess heat. Another plus about bucking transformers (also true of step-downs) is that if I had a more normal line voltage like 115, I could easily just pull it out of the circuit and use as-is.

Many transformers are not labeled for absolute phase, as it is not strictly relevant in normal transformer supply hookups. But with a bucking transformer, the phase of the wiring determines whether the voltage of the secondary is subtracted from the primary... Or added! That's right party people; a bucking transformer has a flipside arrangement called a boosting transformer. The best way to make the determination is by wiring up the assembly with wire nuts or other removable connection methods. This assembly is then tested at low voltage, using a Variac, to ensure proper operation. Variacs are an indispensable tool in a typical workshop and are useful both for safety, and for testing. It's a lot easier to test these things when you're operating with low voltages and don't have to worry about a shock hazard. You could also use a normal transformer with a lower-voltage secondary, but typical Variacs already have an outlet built-in, and are variable and thus quite easy to use for this purpose.

Below are a few images of how to wire bucking, boosting and "Bucking Autoformer" arrangements.

The bucking autoformer arrangement is one I've only seen one place, and that's at this link . Rod Elliott of Elliott Sound Products does a great job of describing bucking transformer arrangements, and goes into a little more depth than this article, which is really more of an introduction to the concept, and how one might look assembled, etc.

Safety is very important here, you're working with full mains voltage. You want a power switch, a properly grounded metal box, the transformer body properly grounded if you're using a metal-framed device, and you want to fuse the AC input. This is all redundant to what's already in your component, but you need the protections at the moment the AC enters any device, not just the "end user".

As you can see, my build uses captive cords; where feasible I prefer these to using additional connectors. They do require extra care with strain relief, though. In this instance the strain reliefs are the types as used in wet location outlet boxes. I had to drill through the holes and manually tap for the screw threads, and secured with JB Weld (metal bearing epoxy) to ensure that my strain reliefs were nice and solid.

The enclosure is switched, fused, and properly grounded, including the transformer body, which is done using lock washers. The orange stuff you see is self-fusing silicone tape, a very handy material, which is capable of withstanding high voltages, very flexible Radio Shack sells it for about $3 a roll as I recall, but there's also versions of self-sealing tape available at large hardware stores, and online.

You can use just about any reasonable power cable, you don't have to go super-thick like I did, but having been a wiremonger, I care about cordage so used some of "The good stuff". For inexpensive decent, widely available (and safety-appropriate) power cordage, Carol brand "SJOOW 14/3" is my preferred cable. Many Home Depots will carry this by the foot, though if you're like me, you can go through a 50ft mini-spool pretty darned quickly.

Strain reliefs can be done a variety of ways, I like compressible connections like those shown as I've found them to be very reliable and have a nice look. Likewise fuses- I used an inline fuse holder, but one could use a solder-terminal fuse holder mounted to the chassis, a fuse on an IEC inlet, or even a circuit breaker. You'll want to size this for slightly larger than the max "normal operation" load of your device being powered.

In practice, it's worked out to be a great solution. A pretty high level of effort and expense, since it had to be capable of 1200+VA operation, and I wanted a nice looking enclosure, but it allows my amp to do its thang unhindered by the problem I faced with line voltage. That's all folks. The reason wifey got confused is the reason I can use my big honkin' amp in conjunction with an overzealous power line voltage.