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Old 21st Sep 2018, 4:13 pm   #7
Philips210
Nonode
 
Join Date: May 2007
Location: Redruth, Cornwall, UK.
Posts: 2,562
Default Re: Electrolytic capacitor reformer meter circuits

Hi.

Thanks everyone for the positive feedback, it's much appreciated. I hope the notes will inspire others to build something similar.

A few other points about the reformer:

The mains transformer came from a German valve radio, I think it may have been a Telefunken model. A 100mAT fuse protects the HT winding. The heater winding not being used, though I guess it could be a useful 6.3V ac supply. I may well add this feature by way of a couple of 4mm sockets.

The selenium rectifiers in the original circuit were replaced by BY133 silicon diodes with a series 560 Ohm 11W wirewound resistor. This value will depend on the transformer's HT winding and will need to be experimented with to obtain the correct HT voltage across the divider chain. I placed a 2n2 Y ceramic cap in parallel with the rectifier in both cases to help reduce RFI to other equipment, (ie my AM radios) not that it really matters that much.

To keep reasonably close to the specified capacitor values in the voltage doubler circuit, I used two 33uF 400V electrolytics in series to achieve the quoted 16uF values. I should have included voltage sharing resistors across each cap to avoid the possibility of exceeding the working voltage of the cap. In practice, the Panasonic caps were well matched so there have been no problems. It's not difficult to calculate the required sharing resistor values, typically 10M Ohm will be suitable in this case.

The 2k Ohm wirewound resistors in the potential divider generate a fair amount of heat if the unit is on for extended periods. For each, I used two 1k Ohm Vitreous wirewound types in series from Welwyn, type W23. They're all laid out on the tagboard but standing clear of the base to allow air circulation, so good ventilation in the case is preferable. The wiring to the divider tagboard has silicone coated fibre glass sleeving.

The main difficulty with building this reformer will be sourcing a decent 2 pole 12 way rotary switch. I was lucky to have something suitable in my junk box of spares. The mains transformer will probably be best taken from a write off valve radio.

Having the meters fitted gives you the reassurance that reforming process is going to plan or not.

Quite a lot of the time, the reformer is used for checking paper caps for leakage and it certainly has weeded out any below par examples. It could be regarded as being an insulation tester but with more ranges. It shows up problems with those Hunts paper types and those now grotty mixed dielectric caps. Most of the latter are swollen/cracked in appearance. It's also worth seeing how much those cracked Rifa class X types leak. Some examples appear fine while others are terrible. Not that I'd want to risk using any across the mains in any case!

Regards
Symon.
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