In theory, capacitors are not supposed to conduct direct current – apart from a small amount when a DC voltage is first applied to them and they need to ‘charge up’.
And with most practical capacitors using materials like ceramic, polyester or polystyrene or even waxed paper as their insulating dielectric, the only time they do conduct any DC is during charging.
That’s assuming they haven’t been damaged, either physically or electrically, or that their dielectric has not deteriorated with the passage of time. In that case they may well have a significant DC “leakage current” and need to be replaced.
Fig.1: block diagram of the Digital Capacitor Leakage Meter. It consists of two sections, a selectable DC voltage source based on IC1 and a digital current meter (it’s actually a voltmeter set up to read current), based on IC2, IC3 and the LCD module.
But as many SILICON CHIP readers will be aware, things are not this clear cut with electrolytic capacitors, whether they be aluminium or tantalum.
Even brand new electrolytic capacitors conduct a small but measurable DC current, even after they have been connected to a DC source for sufficient time to allow their dielectric oxide layer to “form”. In other words, all electrolytic capacitors have a significant leakage current even when they are “good”.
The range of acceptable leakage current tends to be proportional to both the capacitance and the capacitor’s rated voltage. Have a look at the figures in the Leakage Current Guide (Table 1). The current levels listed there are the maximum allowable before the capacitor would be regarded as faulty.
Commercially available capacitor leakage current meters are expensive (well over $1000), making this SILICON CHIP Capacitor Leakage Meter an attractive proposition since it will cost a great deal less.
It’s easy to build and provides seven different standard test voltages: 10V, 16V, 25V, 35V, 50V, 63V and 100V which will cover the majority of capacitors that most readers will be using. Built into a compact jiffy box, it’s battery powered (6 x 1.5V AA alkaline cells) and therefore fully portable. This makes it suitable not only for the workbench but also for the service technician’s toolbag.
The Capacitor Leakage Meter has a simple presentation in its plastic case. The lid carries the 2-line x 16-character backlit LCD module, as well as the test terminals, power and test switches, as well as the 7-position rotary selector switch.