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Capacitor Leakage Adaptor For DMMs

Here's a cut-down version of the Digital Capacitor Leakage Meter we described in December 2009. Instead of using a PIC microcontroller and an LCD panel to display the leakage current, this version connects to your DMM to provide the readout. It provides the same range of seven different standard test voltages (from 10V to 100V) and can measure leakage currents down to 100 nanoamps!

By Jim Rowe

Specifications

Test voltages: 10V, 16V, 25V, 35V, 50V, 63V or 100V.

Leakage current: from 10mA down to less than 100nA (0.1μA), via two ranges:

0-10mA (default) and 0-100μA (manually selected).

Both ranges convert these current values into an output
voltage range of 0-1000mV DC, allowing all measurements
to be made on the DMM’s 0-1V or 0-2V range.

The Adaptor’s default 10mA range is current limited to
provide protection from damage due to shorted capacitors
or the charging current pulse of high-value capacitors.

Power: Internal 9V battery (6 x AA alkaline cells).

Current drain: Varies between 1mA and 125mA, depending on the test voltage
         and the current range in use.

Why would you need to measure capacitor leakage current? In case you missed the December 2009 article, here’s a summary of the introduction we provided there.

Click for larger image

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 have to ‘charge up’.

With most practical capacitors using materials like ceramic, glass, polyester or polystyrene - 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. In that case they may well conduct DC as a steady ‘leakage current’, showing that they are faulty.

But as many SILICON CHIP readers will be aware, things are not this clear cut with electrolytic capacitors, whether they be aluminium or tantalum. All 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 given in the Leakage Current Guide opposite. The current levels listed there are the maximum allowable before the capacitor is regarded as faulty.

So an instrument capable of measuring the leakage current of capacitors can be very handy in many areas of electronics.

Commercially available capacitor leakage current meters are expensive (ie, over $1000) and even the Capacitor Leakage Meter we described in the December 2009 issue will probably cost you over $100 to build. That’s why we’ve developed a cut-down version described in this article, which lets you make all of the same measurements with your existing digital multimeter (DMM).

Click for larger image
Fig.1: block diagram of the adaptor shows it has two elements: a selectable DC voltage source and a simple current-to-voltage converter.

The Adaptor is easy to build and will have a much lower cost than the December 2009 meter while still providing the same choice of seven different standard test voltages: 10V, 16V, 25V, 35V, 50V, 63V or 100V. It is also able to make current measurements from 10mA down to a fraction of a microamp. So it’s capable of making leakage current tests on the vast majority of capacitors in current use.

It’s built into a compact UB1 size jiffy box and is battery powered (6 x AA alkaline cells). This makes it suitable for the workbench or the service technician’s tool kit.

How it works

The Adaptor’s operation is straightforward, as you can see from the block diagram of Fig.1. There are two functional circuit sections, one being a selectable DC voltage source (on the left) which generates one of seven different preset voltages when the TEST button is pressed and held down.

The second section is a simple current to voltage converter (on the right) which is used to generate a voltage proportional to the direct current passed by the capacitor under test, so that it can be measured easily using your DMM.

Any direct current passed by the capacitor being tested flows down to ground via resistor R2, which therefore acts as a current shunt. The voltage drop across R2 is then passed through an output buffer which feeds your DMM. The DMM is set to its 0-2.0V DC voltage range, which allows its readings to be easily converted into equivalent current levels.

So that’s the basic arrangement. The reason for resistor R1, in series with the output of the test voltage source, is to limit the maximum current that can be drawn from the source, in any circumstances. This prevents damage to either the voltage source or the current-to-voltage converter sections, in the event of the capacitor under test having an internal short circuit. It also protects R2 and the output buffer from overload when a capacitor (especially one of high value) is initially charging up to one of the higher test voltages.

R1 has a value of 10kΩ, which was chosen to limit the maximum charging and/or short circuit current to 9.9mA even on the highest test voltage range (100V).

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