Power Monitor Pro repair

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.

18 Responses to “Power Monitor Pro repair”

  1. Ricardo sagt:

    I bought one in Conrad some years ago and use it many time.
    I received the user manual in German only, but I can`t reed. Is it possible to find a manual in english or spanish and where ? I would also find the schematic, because yesterday a resistor was burn and it works anymore.

    Thank for your answer,
    Ricardo Santini

  2. Geert sagt:

    Great article for me. I had the same issues and the same problem. I replaced only the 15V zener of the negative supply voltage and everything is fine again. Thanks

  3. Alex sagt:

    Please help me where can i find display for the Power Monitor ?


  4. Wilfried sagt:

    Manu thanks. Had the same problem with my power monitor. Your article is very clear. Replaced diode only. All functions oké now!

  5. Uwe sagt:

    Thank you, Hajo. I had the exact same failure – except that the voltage was at 2,3V. I only changed the Z-diode and voila.

    I wonder why it is only this diode that fails so often. It was worse if the diodes would regularly „open“ instead of „short-circuiting“, because then the mains voltage would probably kill the other parts.

    • Hajo sagt:

      Hi Uwe,

      either a bad batch of Z-Diodes or thermal/electrical loads near the limit. I have got another Power Monitor Pro in my toy box that is getting fairly warm at the display unit. It is fully operational though, so I did not need to open it yet. Maybe some day I will do and then write a short post about the findings.

      All the best,

  6. Otte sagt:

    Ich benötige für oben abgebildetes Energie Messgerät
    eine Bedienungsanleitung in Deutsch.
    Wer kann mir dabei helfen?
    Im vorraus herzlichen Dank.

    Gruß Horst

  7. Rob sagt:

    I too, left my power manager for some years in a drawer. After plugging it in again to check my fancy Miele coffeemaker and possibly power consumer of industrial size, the display of the Power Manager showed total rubbish. After plugging it in with both Energie and Messen pressed, it reset itself and did the job. For 10 minutes or so. Next this show repeated itself, and so on and so forth.

    I checked my zeners (both a drop of about 547 mV in the good direction), seems all OK. Checked the other diodes; the 1N00x’s OK too, Diode D3 (SMD) in one direction 543mV in the other, the supposedly blocking direction, 653mV), possibly due to some other part of the circuitry. I noticed after 10 minutes of use that the PCB in the region of D12, D2 and the negative 5V regulator was handwarm. Not really hot, but seen the history use 10 minutes, reset, use, etc, it wasn’t used for a very long time.

    Not quite sure what to do. Hate throwing away good stuf with a possible small & cheap failure. Of course, if the logic custom-IC’s kicked the bucket, I have a free and good display for Alex ( :) and :( ).

    You can answer, if you like, of coures, in English, German, Dutch or French, what is the easiest for you. (Für mich ist es völlig egal).

    • Hajo sagt:

      Hi Rob,

      you already tested the most critical parts. It is ok that the voltage regulator gets warm. Some additional ideas from my side (but no quick fix, sorry): Does the Power Manager Pro (PMP) recover automatically after having cooled down — could it be a thermal error? What about the supply voltages when the PMP starts to show rubbish? Does your PMP always need a manual reset after showing rubbish — maybe it reads/writes rubbish from/to the EEPROM?

      All the best,

  8. Klaus sagt:

    Hi Hajo
    have 3x Power Monitor Pro here in New Zealand. Lost the manual and don’t remember to set the trigger for „on time“ current. Do you still remember?
    The manual isn’t anymore on Conrad’s document server. Appreciate your feedback and maybe a copy of the manual via email.
    Thanks, Klaus