The Voltcraft Power Monitor Pro is a digital instrument that measures voltage, current, frequency and phase angle on a single phase mains operated device. The measurements are supplemented with intelligent features to instantly display power and energy consumption, trace minimum and maximum values or to clock operating times.

Again this is a cross marked “out-of-the-plastic-box” unit and it lied around under my desk for at least a year until I remembered its existence a few days ago after having done my taxes. Tax declarations are anything but a favourite pastime, you know, and faulty measurement devices are a perfect compensation in my opinion.

The Power Monitor seemed to work properly as the display showed the default screen on power-up. However, it just always displayed 0 volts and 0 amperes and did not measure anything except from the 50Hz mains frequency. So I opened the plug unit first to see what was going on.

The plug unit contains a small PCB with a shunt wire, a 1µF MKP capacitor and a 100Ω resistor. The shunt wire is connected in series to the load under test and allows measuring the current. The series connection of capacitor and resistor and panel unit electronics is connected in parallel with the mains supply. This circuitry is a cheap and resonable alternative to using a mains transformer in case you both have only a small load to supply and do not need galvanic isolation. No defective parts in here.

Opened up the panel unit and checked the cable connections from panel to plug. One may think that broken cores should be too easy to be suspected but on the other hand there is nothing more obvious to be checked next.

Besides mostly SMD parts on the panel unit there are a few conventional through-hole components that, in combination with the already mentioned capacitor and resistor in the plug unit, form the voltage supply and stabilization circuit. In fact, the Power Monitor requires a symmetric supply voltage of +/-5V. This is achieved with the positive and negative half cycles running each through a 1N4001 rectifier diode in series with a BZX55C15 Z-diode to limit the peak input voltages on the following two 78L05 and 79L05 voltage regulators.

There is a rule of thumb saying that a malfunctioning power supply is the cause of about 90 percent of all failures. This rule prove true again when I found the Z-diode in the negative leg to show a voltage drop of 86mV in both directions. Replaced it with a standard ZPD15 and also replaced the 79L05 voltage regulator although, to be honest, I did not check if this was really necessary. Now the Power Monitor is fully operational again and meanwhile monitoring my fridge.

This entry was posted on Wednesday, August 24th, 2005 at 12:40 am and is filed under Electronics. You can follow any responses to this entry through the RSS 2.0 feed. You can leave a response, or trackback from your own site.