What kind of VARIAC?


2002-06-14 12:08 am
I am looking for a variac to help in initially testing my projects. Could anyone tell me what to get or look for? A local surplus store has one 10A 110V variac for a pretty resonable price. It looks in good shape and even has a meter that reads the actual voltage output as opposed to just trusting the gauge on the dial. Is that enough amperage?
Variac Test Jig

I have used the following variac/isolation transformer setup for years and it works very well.
The lamp limits the maximum load current and is very usefull for running up unknown equipment.
An amplifier drawing excess bias current will cause the lamp to light and dissipate most of the power thereby saving burnups if you have a fault condition.
The isolation transformer is for safety when i am working on live chassis equipment like tvs.
On three wire equipment the switch in series with the earth wire provides isolation from earth for the chassis which can be usefull too.
Fit a lamp holder socket and have a selection of lamps powers (40W, 60W, 100W etc).
With this jig you can safely run up almost any device, especially prototypes with less fear of costly and inconvenient burnups.



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I do not have a symbol for a variac so I used a pot symbol instead.
Always remember that a variac is an autotransformer - ie you have a single winding with a moving contact to pick off the voltage desired and there is no DC isolation from the mains, hence the 1:1 isolation transformer.
The secondary of the isolation transformer is not switched to enable DC continuity at all times so that any suppresion capacitors in the Device Under Test will discharge.

The lamps are standard 240V incandescant lamps, available in a range of wattages, of course.
If you mount the Variac, lampholder and Iso transformer in a box with a carry handle on top you have a very useful workshop item.

This setup is fine for all normal equipment including quite high power class A/B amplifiers, but probably limited usefullness for running up a high power class A amp.
Salvage yards and scrap metal dealers can be a source of industrial high power variacs for low cost - I have another 5 kVa variac that I picked up for $10.00 this way.