I have the same variac with the same loose flaky paint problem and worn brushes (plus two slightly burnt track points - but not fatally so) and I found your video in a search for replacement brushes. Lo and behold you not only gave a source but also the part number which as of Sept 2016 is still correct, cost £4.32 inc post. Well done!

While the fused plug is a good idea, it is essential to fuse the output at (or below) the rated current to protect the brush and any portion of the coil from excessive load current. Consider for example, how much current could be drawn from the output of a 2 Kva Variac with the output set at a mere 10 volts without blowing the mains fuse. No part of the winding could handle that amount of current (~200 amps) without quickly overheating while the localized heating from the brush losses (I2R) will burn the coil at the point of contact. The carefully controlled resistance of the carbon brush is the key to the design and longevity of any variable autotransformer. It's what allows the brush to bridge 2 adjacent turns of the winding without creating a "shorted" turn. The brush is the hottest operating part of the entire device. The disk that it is fastened to on larger Variacs serves as a heat sink. If you want to properly protect your Variac investment, fuse the output.

Just done one of these. Thanks for the brushes part number. Thought I'd chip in that I filed the brush holder not the brush. Keeps it at 2.5mm thick so less likely to break, less messy and ready for next time. A word of warning on this....file the holders opposite ways 'cos they go in back to back and will be out of line if you do them both the same. Thanks for the video John....it gave me the b--ls to try it! Dave.

Hi. Nice restoration project. Isn't an output fuse a good idea to protect the variac from high current overdrive if the output is not monitored (low voltage high current situations /applications)? Thanks for sharing your knowledge.

Another great video, really enjoyed this series. BA threads are actually metric by the way.

@Graham_Langley

8 жыл бұрын

BA is indeed based on the millimetre. Although the sizes are usually given in imperial, if you convert them to metric the origin is very obvious. 2BA as used here is 0.1850" or 4.7mm o/d. The thread pitch is based on a 1mm starting point for 0BA.

My earlier question apparently did not have a reply button but John did respond and so I'm posting it here in case others have the same question. Thanks John. "I could not find any brushes which were the exact size. The ones used were from uk.rs-online.com/ stock number 3473849 These are the correct length and width (10x6mm), but are slightly too thick - they need to be filed or sanded down to fit into the brush holders."

Hi, good job, can you tell me were i can get new brushes? Thank you.

When you reassembled it, the bridge contact to the slider seemed to be very close to the frame. I would have added a piece of "fish"paper as a insulator to prevent any possible contact between them.

Hi John, I just bought this exact model off eBay, and before plugging in I just buzzed the plug that was fitted and goes what dead short, i took the cove r of and found the short is in the variac i.e. i disconnected the mains in cable and there is a short across the input terminals. If you any ideas I'd be greatfull, otherwise it will be going back. Bob

@jwflame

7 жыл бұрын

Depends what you mean by dead short - as it's a transformer the DC resistance will be very low, mine is only 2 ohms including the mains cable. Unless physically damaged, faults with the primary winding are unlikely.

@MrBobwells48

7 жыл бұрын

Hi John, yes the meter reads 1.75 ohms across the live & neutral, but as i say it trips the MCB in my house. Thanks Bob

@MrBobwells48

7 жыл бұрын

Hi John, me again, it seems the variac is ok, the problem is inrush current, I replaced my MCB to the bench from a 16A one to a 32A and all ok now. It seems I could use a delay (D Type) MCB so i'll look into that in the near future. Thanks again mate, Bob

can u tell me waer can i get new brushes for the Variac wot u hav i got the same 1as u

@jwflame

8 жыл бұрын

+mg42cammyhigh Mine came from RS Components, they don't have the exact size so it's a case of selecting the nearest larger size and filing them down to fit.

Setting a bad example there John - a wire stripper is not a spanner.