8mm Forum: Variac power supply

This is topic Variac power supply in forum General Yak at 8mm Forum.

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Posted by Terry Sills (Member # 3309) on November 07, 2017, 09:12 AM:

Some time ago I posted my interest in using a Variable Transformer to power projectors with the notion that it would save lamp life.
Luckily I acquired a Claude Lyons 10amp variac at a local car boot for very little money so I have been experimenting using it to gently run up some projectors to see if there were any benefits.
Obviously to save lamp life the lamp needs to be very under run, so I start a 240v machine on around 120v, which is just about enough to get the motor running, and then turn on the lamp and gently run it up to full voltage. It seems to work fine, but only time will tell if the lamps last longer, but it must be better than subjecting the lamp to the shock of putting the full voltage through it instantaneously.
I've been told however that some later transistorised amps do not like being underrun and that it can be detrimental. Is that true?
Anyway this surely has to be good practice for running up old projectors that require a voltage dropper (toaster types) and also old valve amp projectors.
Your thoughts gentlemen.

Posted by Steve Klare (Member # 12) on November 07, 2017, 10:04 AM:

I've used these professionally. I used to run prototype equipment up very slowly in case there were troubles. My friend had a different philosophy: "Plug it in, power it up and then fix the wreckage. Repeat as needed until it works." I won't say who was right, but I usually had eyebrows!

-so I'd definitely endorse one of these for first time power-ups of unknown stuff. It is damage limiting for sure. If I had something with a short the variac would give me a nasty sounding hum within the first couple of volts, and I'd know to stop.

The shock of turning on is of the biggest consequence when the lamp is old, the filament is already thin, and the tungsten is deposited on the envelope. This may actually buy you more time with a dim lamp.

My editor/viewer is extremely kind to lamps: sometimes the envelope turns black before I change them, rather than just have the lamp blow.

Posted by Terry Sills (Member # 3309) on November 07, 2017, 12:03 PM:

Thanks Steve for your input.
I just knew you would be the one to have knowledge and experience of them. Is there any disadvantage or damage that may result to motors/transistors or other components, by using it. I am a complete novice in this field, so any advice is most welcome.

Posted by Steve Klare (Member # 12) on November 07, 2017, 12:35 PM:

Fortunately it's an electrical power question!

(People also expect me to understand computers for some reason!)

I think a motor would just stall until you reach a certain voltage.

A transistor amplifier, especially a very complicated one, isn't as predictable. I know that under-voltage you would get clipping of the signal's peaks when you run out of supply voltage. This would give you distortion, but probably not damage anything.

I'd feel a little reluctant to do this on a day to day basis, though: not because of what I know it would do but more because of what I don't. As the circuitry becomes more complicated, it becomes capable of doing some amazing things in the wrong conditions.

Posted by Gary Schreffler (Member # 5945) on November 22, 2017, 07:01 AM:

The greatest use for a Variable Transformer in my case is when doing a 'real-time' transfer of a 16mm home movie (I usually do frame-by-frame/Stop Motion captures). No matter how much I sync up the camera, computer and projector, the shutter is still seen (that slow rolling artifact). I discovered that by using the Variable Transformer, I can make minute adjustments to the speed of the projector until the shutter artifact is completely removed.

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