Capacitors are used in a wide range of applications in today’s electrical installations – including, for example, spike and harmonic suppression, motor starting for single-phase motors and smoothing in power supplies.
The ability to measure capacitance with a good degree of accuracy is therefore very useful and is provided by many modern multimeters.
Capacitance measurement
So what’s to be done? Discharge the capacitor, of course! But how? Definitely NOT by using the age-old trick of shorting the terminals with a screwdriver.
Capacitors can store a lot of energy and releasing this into a short circuit can create a dangerous arc leading to blobs of molten metal flying out from your screwdriver blade. The capacitor may also be damaged by such a violent discharge.
It’s essential to use a safe form of discharge device, which need be no more than a resistor of a few kΩ connected to two well-insulated probes. The problem is, how many of us carry around such a device? In fact, wouldn’t it be convenient if the multimeter itself automatically and safely discharged the capacitor?
With a standard digital multimeter, this is not a practical solution. The input impedance is likely to be around 10 MΩ, so you’ll wait a very long time for even a relatively small capacitor to discharge. But with the new Megger AVO830 and AVO835 multimeters, it’s a different story.
Just as you’d expect, these produce an alarm and inhibit testing if voltage is detected when they’re switched to the capacitance range. However, they then automatically switch to an input impedance of just 10 kΩ, so the capacitor will be discharged safely in a realistically short time. During the discharge, they continue to display the remaining voltage on the capacitor, so you can be absolutely sure when it’s safe to proceed.
This automatic discharge feature makes testing capacitors safer, easier and more convenient. It also means that there’s no need for you to carry around a separate discharge device – it’s right there whenever you need it, built into your multimeter!