Variac wiring. - UK Vintage Radio Repair and Restoration Discussion Forum

mickm3for · 23rd Mar 2023, 4:31 pm

Hi after many years of using this variac only today i found it does NOT ajust the voltage well it does but it does not. i will explain on full output the neutral side to earth is at 0v ac the live side is at 230v but when turned down the neutral side goes UP the live side remains at 230v so at 0v out neutral side 230 and live side 230 . only found this out when working on a ac/dc set with live chassis OUTCH but it did wake me up, This has been wired like this for 25 yrs or more not wired by me as obtained from radio rally, I should have checked it but it did vairy the output voltage ? Mick

GrimJosef · 23rd Mar 2023, 4:35 pm

Live and neutral crossed over on the input side, presumably. Precisely why the only way to be sure that the chassis on an AC/DC set isn't connected to mains live is to use an isolating transformer.

Cheers,

GJ

mickm3for · 23rd Mar 2023, 5:48 pm

Hi yes but on min voltage out both L and N have 230v to earth no N at all
so its bringing the N side up to match the L Mick edit just drawn it yes mains in would give this effect that means the L and N on the output socket are also wrong , the socket is wired red on the right this is indeed live reversing the input L and N (colour reversed) gives live on the LEFT on the output so its wrong input and output, yes i know isolating tx i have one on the shelf but didnt use it as i had made sure the chassis was N so N to earth 0v . only going live when variac turned down to 0v . i have a rcd trip on the bench this tripped when i connected myself between the case of the radio and the pl259 aerial plug the outer screen is earthed. This radio had been running for a while on full output from the variac , i disconnected the aerial turned variac to min put case on went to plug in the aerial and zap , alass i have not gained any super powers Mick

Silicon · 24th Mar 2023, 12:07 am

I bought a home-brew power supply which included a small panel mount variac.

Unfortunately it was wired up like a power rheostat and this probably caused the overheating at the counter clockwise end of the winding.

23rd Mar 2023, 4:31 pm	#1
mickm3for Heptode Join Date: May 2015 Location: Chatham, Kent, UK. Posts: 964	Variac wiring. Hi after many years of using this variac only today i found it does NOT ajust the voltage well it does but it does not. i will explain on full output the neutral side to earth is at 0v ac the live side is at 230v but when turned down the neutral side goes UP the live side remains at 230v so at 0v out neutral side 230 and live side 230 . only found this out when working on a ac/dc set with live chassis OUTCH but it did wake me up, This has been wired like this for 25 yrs or more not wired by me as obtained from radio rally, I should have checked it but it did vairy the output voltage ? Mick

23rd Mar 2023, 4:35 pm	#2
GrimJosef Dekatron Join Date: Sep 2007 Location: Oxfordshire, UK. Posts: 4,311	Re: variac wiring Live and neutral crossed over on the input side, presumably. Precisely why the only way to be sure that the chassis on an AC/DC set isn't connected to mains live is to use an isolating transformer. Cheers, GJ __________________ http://www.ampregen.com

23rd Mar 2023, 5:48 pm	#3
mickm3for Heptode Join Date: May 2015 Location: Chatham, Kent, UK. Posts: 964	Re: variac wiring Hi yes but on min voltage out both L and N have 230v to earth no N at all so its bringing the N side up to match the L Mick edit just drawn it yes mains in would give this effect that means the L and N on the output socket are also wrong , the socket is wired red on the right this is indeed live reversing the input L and N (colour reversed) gives live on the LEFT on the output so its wrong input and output, yes i know isolating tx i have one on the shelf but didnt use it as i had made sure the chassis was N so N to earth 0v . only going live when variac turned down to 0v . i have a rcd trip on the bench this tripped when i connected myself between the case of the radio and the pl259 aerial plug the outer screen is earthed. This radio had been running for a while on full output from the variac , i disconnected the aerial turned variac to min put case on went to plug in the aerial and zap , alass i have not gained any super powers Mick Last edited by mickm3for; 23rd Mar 2023 at 6:12 pm.

24th Mar 2023, 12:07 am	#4
Silicon Nonode Join Date: Jun 2007 Location: Coulsdon, London, UK. Posts: 2,163	Re: Variac wiring. I bought a home-brew power supply which included a small panel mount variac. Unfortunately it was wired up like a power rheostat and this probably caused the overheating at the counter clockwise end of the winding.