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Electrolytic Capacitor Reformer & Tester

Got a bunch of old electrolytic capacitors you'd like to use . . . but don't know if they are any good? Or do you need to re-form the electrolytics in an old valve amplifier or vintage radio set? This Electrolytic Capacitor Reformer and Tester will do the job for you, at any of 11 different standard voltages from 10V to 630V.

Part 1: by Jim Rowe

In addition, it provides the ability to apply the selected test voltage for any of seven periods ranging from 10 seconds to 60 minutes.

Thus you can use it for ‘reforming’ electrolytic capacitors that have developed high leakage and high impedance due to years of inactivity. As well, it can be used to test the leakage of virtually all capacitors at or near their rated voltage.

Click for larger image
Fig.1: block diagram of the Electrolytic Capacitor Reformer and Tester. Not shown here is the safety interlock microswitch and discharge resistors.

Of course, we have to state that not all old electrolytics can be restored – they can’t. Some will have very high leakage due to contamination of the can seal or breakdown of the electrolyte and some will have just dried out. In those cases, you cannot do anything to resurrect them but in many cases you will be able to restore and re-use capacitors that have not been used for many years if not decades.

Some very old caps (1960s vintage!) we had took several hours to come good while others, made in more recent years, were good within a few minutes.

Most high voltage (ie, 250V and above) capacitors should be capable of being reformed to the extent that their leakage current drops to around 3mA or less.

The Reformer circuit is designed so that no damage can occur if the capacitor connected to it is short circuit or has very high leakage, or is even connected back-to-front (ie, with reverse polarity). Furthermore, even if the capacitor leakage is very high, the output current is limited so that the maximum dissipation in the capacitor is no more than 2W.

Click for larger image

This means that some capacitors might get warm while they are being reformed but none will get so hot that they are in danger of swelling up and “letting the smoke out”.

That’s a good thing because electrolytic capacitor smoke is particularly foul-smelling! And as any serviceman will tell you, the gunk (electrolyte) inside is particularly nasty if it escapes with the smoke.

The Electrolytic Capacitor Reformer and Tester is housed not in a traditional instrument case or box but in a standard plastic storage organiser case which, together with a microswitch interlock, provides a safe compartment for the capacitor when it has high voltage applied.

Another compartment provides handy storage for the switchmode 12V plugpack.

Opening the lid of the case means that no voltage is applied to the capacitor – until the lid is closed – but perhaps even more importantly, opening the lid safely and quickly discharges the capacitor so there is no chance of a nasty electric shock – for you or anyone else. A charged 630V capacitor with its leads exposed is not something to be trifled with!

With the transparent lid closed you can select the test voltage and the period of reform/testing and view the 2-line LCD which shows the capacitor voltage, its leakage current and the time elapsed.

The design

This Electrolytic Capacitor Reformer and Tester is based on the smaller and simpler unit described in the December 2009 issue of SILICON CHIP but with a much bigger selection of test voltages plus the inbuilt test timer which allows the test voltage to be applied for as long as 60 minutes.

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