how do i wire up this variac?

[danz409]

ok. next step on my build. i want higher voltage DC for things like transmitters and such.

so i planned on re-useing the variac that was in the power supply. it seems to be one of the very few reuseable parts in it.

anyone know where i plug in what on this thing? can i even use 12V DC on this thing?

[amspire]

120V active goes to pin 2
120V neutral goes to pin 1

Output Active to pin 3
Output neutral to pin 1

Pin 4 disconnected.

You should then be able to get 0V to 132V out with 120V in.

Richard.

[danz409]

so i can't use DC on this at all?

[w2aew]

so i can't use DC on this at all?

No you can't.  AC in and AC out only.  You can follow it up with a bridge rectifier and filter cap to get adjustable DC output, but you will be limited to the stated current.

[onewatt]

I agree completely with w2aew. A VARIAC stands for Variable AC and is an AC Auto-transformer.  It's constructed just like a huge wirewound potentiometer, however, it's used for varying AC voltage whether boosting or bucking.  Looking at my illustrations, starting at the top, the wiper is at the 1:1 primary to seconday ratio position; move the wiper to the right, and it's boosting the AC to 140Volt AC; when the wiper is rotated to the left, the VARIAC bucks the voltage down in a smooth continuous manner to 70 Volt and eventually down to 20 Volt and then 0 volt AC.

My VARIAC has a primary input resistance or 2.4-Ohms in all positions of the dial. The seconday output resistance is 34-Ohms at 0 Volt AC and 98-Ohms at 140 Volt AC.

I'm adding that my secondary Ohms readings seem a little odd, however, my VARIAC works nicely.  I took it apart and I did notice that the secondary resistance depends heavily on the pressure the large carbon wiper contact has on the primary coil.  Besides that, right now I don't know why the secondary resistance seems a little odd.

The primary and secondary are designed to act as inductors not resistors; that means the VARIAC is usually designed to work at a specific frequency, either 60 Hz 120VAC or 50Hz 220 VAC.  However, I can see on Danz409 picture that the VARIAC label seems to indicate either 50/60 HZ, though I can't read it too well.

This artwork is my own design that I used in a presentation on this subject.

Ratio 1:1


Boosting


Bucking

[alm]

Note that a variac, being an autotransformer, is usually not isolated from the mains. This means that it's not very safe to make a power supply from a variac and a rectifier. Just get a proper mains to 12V transformer or a regulated 12V plug pack if you need 12 VDC.

[mobbarley]

Note that a variac, being an autotransformer, is usually not isolated from the mains. This means that it's not very safe to make a power supply from a variac and a rectifier.

I think this should be in bold.

[Psi]

Also be aware that if you have a 500W variac you can't extract 500W at low output voltages.

[ciccio]

My VARIAC has a primary input resistance or 2.4-Ohms in all positions of the dial. The seconday output resistance is 34-Ohms at 0 Volt AC and 98-Ohms at 140 Volt AC.

I'm adding that my secondary Ohms readings seem a little odd, however, my VARIAC works nicely.  I took it apart and I did notice that the secondary resistance depends heavily on the pressure the large carbon wiper contact has on the primary coil.  Besides that, right now I don't know why the secondary resistance seems a little odd.

You surely must clean the (usually carbon) wiper and the exposed part of the winding.
The resistance you measure on the "secondary" is too high.
In fact you should not call it secondary or primary: a variac is an autotransformer: it has one and only winding....

[onewatt]

A VARIAC is an autotransformer because it only has a primary winding that operates by self inductance; there may be several physical taps on the primary, or a wiper on the primary, that creates the secondary voltages. When I first came across an autotransformer, it was confusing for me to think that a section of the primary winding was simultaneously acting as the secondary winding depending on the tap location, which is only a solder connection to the primary. There is no physical separation between the primary and secondary windings.  Consequently, the secondary AC return will still seek ground just like the primary AC side - and is an electrocution  hazard.

In contrast, an isolation transformer operates by mutual inductance where the primary and secondary windings are physically separated, and where the only connection between primary and secondary is the magnetic flux linkage.  Consequently, the secondary will not seek ground like the primary AC, instead, the secondary AC return is from one secondary leg to the other secondary leg, not between a secondary leg and ground.  To get shocked you would have to touch both secondary legs at the same time. Because the secondary AC return is no longer to ground, you will not get shocked by touching either secondary leg by itself.  In essence, your body becomes the same potential as the secondary leg that you are touching and no AC current flows.

I have done a lot of experimenting with transformer operation, but I an concerned that someone might try to duplicate my demonstrations who are not experienced and may hurt themselves, so I'm refraining from talking at great length about electronics techbench isolation transformers.

I am not suggesting that you try touching the secondary legs of transformers.  I am only describing why an isolation transformer is sometimes called a safety transformer.  But we all know that electricity kills if you don't know what you're doing.



Autotransformer gets its name from AUTO, which is Greek for SELF; like AUTO-Navigation that means self-navigating; AUTO-motive that means self-propelled; and AUTO-transformer that means self-excited transformer.

[Zero999]

Yes, for safety reasons you should use an isolation transformer too but beware that it's still not 100% safe, as higher voltages can be present if the Variac develops a broken winding.

You can also use another transformer to get other voltages. For example, by adding a 0-120-240V autotransforer you can get 120V to 240V for testing European appliances and have selectable 0 to 120V and 120V to 240 ranges.

My VARIAC has a primary input resistance or 2.4-Ohms in all positions of the dial. The seconday output resistance is 34-Ohms at 0 Volt AC and 98-Ohms at 140 Volt AC.

I'm adding that my secondary Ohms readings seem a little odd, however, my VARIAC works nicely.  I took it apart and I did notice that the secondary resistance depends heavily on the pressure the large carbon wiper contact has on the primary coil.  Besides that, right now I don't know why the secondary resistance seems a little odd.

You surely must clean the (usually carbon) wiper and the exposed part of the winding.
The resistance you measure on the "secondary" is too high.
In fact you should not call it secondary or primary: a variac is an autotransformer: it has one and only winding....

My guess is the open circuit test voltage on the meter is too low to ensure a good connection. I suspect the resistance would be more consistant, if he tested it with a higher voltage.