Variac bypass wiring? - Page 1
Forum section: electronics / beginners

author: Chris Wilson
author_rank: Super Contributor
timestamp: October 16, 2012, 07:29:53 pm
content: I put a big 25 Amp Variac in a high voltage PSU cabinet for runningthe plate voltage on a linear RF amplifier. The idea being I canalso use the HV supply via the Variac to tame it a bit for otherapplications. I know a Variac can soften the supply so was planning onusing a multi pole change over switch to enable the HV PSU to runeither direct off the mains or via the Variac. I may have been overcomplicating things by planning to use a multi pole switch. Please seeattached, wouldn't a single pole on / off switch of about 30 amp workas I have shown in the schematic? Want to be sure I am not missingsomething daft...Thanks!

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: October 16, 2012, 07:33:38 pm
content: Consider what would happen if you closed the switch with the varoac set to miinimum.You need a changeover switch, and if possible an on-off-on type, to avoid arcing problems if switched on-load

author: Chris Wilson
author_rank: Super Contributor
timestamp: October 16, 2012, 07:40:16 pm
content: It would be an internal switch Mike, that I as the only only user of this thing, would know to change BEFORE the supply was turned on. In fact the door of the cabinet has an interlock switch killing the power altogether if it's opened. I see the potential issue though, thanks for pointing it out!

author: ejeffrey
author_rank: Super Contributor
timestamp: October 16, 2012, 08:28:22 pm
content: Quote from: Chris Wilson on October 16, 2012, 07:40:16 pmThat kind of thinking is generally what leads to letting the magic smoke out -- always assume you will forget and do something stupid. Also, I am not sure you see Mike's point: unless the variac wiper is set exactly to 1:1 engaging the bypass will create shorted turn(s) on the variac. This will lead to a large circulating current through the wiper. If you are only a couple of turns off, this will create unwanted heating of the core and the brush. If you are off by more than that, it will blow the secondary fuse.

author: SeanB
author_rank: Super Contributor
timestamp: October 16, 2012, 08:33:42 pm
content: Changover switch recommended. I have had to change variacs that were used only over a small range of control which wore out the windings at a point. I did use a 115VAC 400Hz 5A variac very successfully as a light dimmer, feeding it with 12VAC 50Hz from a toroid and dimming a 12V halogen lamp from the output. Still works perfectly, though I no longer use it, having upgraded to LED light.

author: Chris Wilson
author_rank: Super Contributor
timestamp: October 16, 2012, 08:56:00 pm
content: Quote from: ejeffrey on October 16, 2012, 08:28:22 pmYou are right, I was missing his point. I am glad I posted, I had a feeling it was all too easy...I will try and source a suitable changeover switch. The original single phase supply had a 40 amp main input trip switch, so I need something that will handle about 40 amps when it is feeding the HV transformer direct and it sees full load. If run via the Variac a much smaller current draw would be made, as it would be feeding a much lesser load, so the 25 Amp limit of the Variac would be respected.The infamous Ebay didn't show anything obviously suitable, especially if I try and find the ideal make / break / make type. I am in the UK, same as Mike, by the way. Thanks for stopping the magic smoke getting out