Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Confusion open circuit transformer detector

  1. Jan 12, 2010 #1
    Hello to you all,

    I have seen something at a friends work place and did not understand how it worked? after asking I didn’t get an answer that satisfied me.

    The company produce voltage detectors and test them on a single phase 50kV transformer. One output post is connected to earth and the other is producing up to 50kV depending on the input voltage. They then offer the voltage detector to the high voltage output terminal.

    If the voltage detector reaches a pre determined voltage then the detector will light up and whistle, indicating that the line is live, my question is if this detector is put onto a 50kV line then why does it not blow up, the unit goes straight to a PCB so surly the transistor would pop???

    The detector is battery operated if this helps? surly the detector must only be taking 1-12v as not to blow the transistor up, I’m confused and have not been told why this wouldnt blow the voltage detector up???

    thanks all, I hope to hear from someone soon.
  2. jcsd
  3. Jan 12, 2010 #2


    User Avatar
    Science Advisor
    Gold Member

    It will work as a Potential Divider. When there are two resistors in series, the voltage is shared by the two resistors in proportion to their values. In this case, there will be an enormous resistance, connected to the 50kV end, in series with a small resistor, connected to Earth. Most of your 50kV will appear across the big resistor.The voltage appearing across the small resistor is a scaled down version of the high voltage. All that is necessary is for the high value resistor to actually work at the high voltage and not break down, as an 'ordinary' resistor, out of the drawer, would. All voltmeters with multiple scales do the same thing; a sensitive voltmeter reads up to, say 0.01V and you just 'Pot-down' bigger voltages and adjust the scale accordingly as you turn the knob.
  4. Jan 12, 2010 #3


    User Avatar
    Science Advisor
    Gold Member

    Do you ask yourself why all the transistors in a television don't blow up when it is plugged into a 120 V outlet (or 220 if you are outside the U.S.A.)? Just because a high voltage is introduced onto a PC board does not mean it is hooked right to a semiconductor device immediately. sophie explained it quite well, I just wonder why it hadn't ocurred to you with other electronic devices.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook