TU56/TC11 restoration - VARIAC question

On Thu, 17 Mar 2005 10:20:10 -0800 "vrs" <vrs at msn.com> wrote: I doubt if a motor capacitor is made up out of a back-to-back electrolytic pair. Usually, motor capacitors are some form of 'oil filled' capacitor or modern variant, and are non-polarized. Usually not of a high enough value to need to be an electrolytic design. I have the 'perfect instrument' for testing and restoring capacitors. It's a US Navy LCR bridge that has built in charging power supply. So you can install a capacitor onto the 'unknown' terminals, charge it up to as high as 400 volts DC, while measuring the leakage current. You can then measure it's capacitance and quality while charged to said voltage. It's intended for tube-era parts, but will work fine for lower voltages. The best device to check out and 'form up' electrolytics of lower voltages (IMHO) is a regular constant-voltage/constant-current bench supply. You set it up to the working voltage for the capacitor, then turn the current limiting very low and attach the capacitor. If the capacitor is 'good' it should charge up to the supplie's set voltage and stop drawing current. If it draws current for awhile, you're not going to 'rush' current into it and cause damage. Important to remember when charging capacitors up is that they then have a fairly high 'instantaneous' current capability. If you charge, for instance, a 4700 uF capacitor up to 40 volts, care should be taken not to short it out. Discharge it slowly through a resistance, say 10K ohms or so. You might 'melt' whatever shorting item you use if you just crowbar it, and the hot metal can burn. Fooling around in the lab, we used to hang big 'computer grade' capacitors (I had a 470,000 uF one) across regular 'low current' bench supplies to 'arc weld.' You can melt copper wire with a part like that 'trickle' charging up to 40 volts on an ordinary supply. -Scott