When I passed the junk pile at my son's dental repair business today I noticed an old Ritter Meteor X-Ray unit sitting in parts. It was slated to go for scrap metal the next day. The one item that caught my eye immediately was a small Variac mounted to the front panel. I took a few minutes and removed it and brought it back to the electronics shop. In case you are not already familiar with variacs, they are a single winding transformer with some extra taps and a wiper that can slide across the full winding and tap at any point. I will draw a diagram of this one after I figure it out.
Variacs like this one were common in the dental x-ray units from the 1960s. They were used to adjust voltages to the electronics of the x-ray unit so that the x-rays would be more consistent. Another purpose was to adjust the filament voltage of the x-ray tube so that the intensity of the x-ray beam could be adjusted. Now days all these adjustments are done automatically with microprocessors. Being an old guy, pieces of electronic equipment from the 1960s get me all teared up. At slightly over 1 kg it felt like a brick of pure electronic gold in my hand.
This picture gives a clear view of the wiper mechanism. Turning the knob on the front rotates the wiper from one end of the winding to the other. The winding is set in a hard plastic and the enamel has been removed from the wires. The wiper is made of a spring loaded arm with a carbon point, very much like the carbon used in motor brushes.
This particular Variac had contacts for the two end points of the winding, numbered 1 and 2, the wiper which was numbered 3, and 3 additional taps numbered 4, 5, and 6. I began the process of determining the position of the 3 extra taps by putting AC voltage across the end winding points 1 and 2. For this I used an AC bench power supply that I built 50 years ago. This power supply is isolated from the mains and allows me to slowly increase the voltage while monitoring the current. When the voltage on the outer ends of the winding reached 120 VAC the current was reading 14.3 mA. This is the no load current of the Variac and was low enough so I was comfortable with continuing my investigation. The voltage from one end of the winding to the taps, numbered 4, 5, and 6 were now measured with a meter. Tap 6 had a voltage of 60 volts, tap 5 had a voltage of 100 volts and tap 4 had a voltage of 110 volts. It is common in the wiring of a variac to put the mains voltage between one end of the winding and one of the taps. This allows the wiper to cover a voltage range from 0 volts to a voltage 10 or 20 volts in excess of the input mains voltage. At this point it seemed that pin 1 should be the 0 point (Neutral) and either 5 or 6 should be the mains (Line) voltage. The two taps 4 and 5 were probably provided to adjust for locations having either 110 volts or 120 volts for mains voltage. For this experiment tap 5 was chosen and as noted in the diagram this created a 60 mA no load input current. Here is a diagram of the Variac with the mains hooked to pins 1 and 5.
As you can see this provides for the wiper to travel as high as 140 volts from a 120 volt mains supply. In the case of this variac, I decided that the load would most suitably be connected across pins 3, and 2. The reason that pin 2 was chosen as 0 volts for the load instead of pin 1 was that the direction of the wiper moves towards pin 1 when the knob is rotated in a clockwise direction. It is customary for me to expect an increase in voltage or volume with a clockwise rotation. This configuration gives an increase in voltage between 2 and 3 when the knob is rotated clockwise. Strangely, I expect just the opposite from water faucets where a clockwise rotation lowers the flow but that is another story. I have not been able to determine a purpose for pin 6 the center tap of the variac. As part of the experiment I put the line voltage across pins 1 and 6 but the current was too high for it to have been intended as line input.
Here are a couple pictures of my experimental set up. The picture on the left shows a Load Bulb attached from pin 2 to pin3 and dialed up to 100 volts. The AC ammeter is showing input current of 260 mA.
Before we had all the cool semiconductor devices that we have today the Variac was a simple solution to provide a variable voltage source. It is still the best way to have a variable AC source at the mains frequency but there are some important safety considerations. If you use a variac without also using an isolation transformer you will have live mains power on one of your power supply wires and it WILL eventually give you a shock.
After learning this lesson the hard way I have refused to use a Variac unless there is an isolation transformer between it and the mains power. Even with an isolation transformer it is important to remember that any voltage over 40 volts or so can be fatal. While an isolated supply will not give you a shock to ground there is still full voltage between the 2 output wires.
Caution! Working or experimenting with a variac can be dangerous unless you are experienced working with mains power.
John
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