HV transformer current.

Discussion in 'General Electronics Chat' started by Xray, May 31, 2009.

  1. Xray

    Thread Starter Well-Known Member

    Nov 21, 2004
    58
    1
    Hey guys,

    I've been in electronics most of my life (which by the way is a long time!) and I still occasionally come across something that confuses or baffles me. One recent thing that I can't quite get a grip on is reading the current in the HV circuit of a Dental X-ray tube head. The circuit is very simple. A high voltage transformer with a 65KV secondary winding supplies power to an X-ray tube. The tube operates in "self rectification" mode, which means that the tube is connected directly across the HV winding (very common practice for "cheap" Dental X-ray heads). Therefore, the tube conducts every other half-cycle when its anode is positive in relation to its cathode. The "low" end of the HV secondary winding is connected to ground through a 1000 Ohm resistor. This allows a technician to measure the tube current by measuring the voltage drop across the resistor. In this particular tube head, 7 Volts = 7mA tube current. Okay so far. Now here is the point of my confusion. I find that I can also measure the current by placing a milliammeter across that resistor. When I do, I read 7mA on my Fluke 77 digital meter. Now, my question is, when I use a milliammeter across that resistor to measure the tube current, there is still 7mA flowing through the resistor, AND there is 7mA now flowing through my meter while it's connected across the resistor. So, how can this be? The X-ray tube is still drawing 7mA from the transformer, but how can there be 7mA flowing through the resistor AND 7mA flowing through my meter? Does that mean there is 14mA flowing through the circuit? I think not, but what am I missing here?

    Thanks
     
  2. bertus

    Administrator

    Apr 5, 2008
    15,648
    2,348
    Hello,

    When you measure the current in the Ampere range of the multimeter, you are placing a low value resistor accross the 1K.
    Almost all the current will flow through the ampmeter.
    If you use the multimeter in voltage range, you will place a very high resistence (most times 10 MOhm) accross the 1K and almost all current will flow through the 1K resistor as you are measuring the voltage.

    Greetings,
    Bertus
     
  3. Xray

    Thread Starter Well-Known Member

    Nov 21, 2004
    58
    1
    Ah... Okay, now that makes sense! When I place a milliammeter across the resistor, the internal resistance of the meter is very small compared to the 1K resistor, and so most of the 7mA current drawn by the X-ray tube will flow through the meter, and only a very small current will flow through the resistor. That makes sense why I get a slight difference in the measured value when I use a voltmeter and then use a milliammeter. The current is dividing between the two, so that the total current is still only 7mA. THAT'S IT!!!

    Thanks very much for your reply. :)
     
  4. Mike Mandaville

    Active Member

    May 27, 2009
    81
    1
    This is a little off-topic, but I still remember when some shoe stores had x-ray machines for "fitting" shoes. I remember sticking my foot in one, and watching my bones move when I was a boy. It seems that most of the hobby projects which interest me involve potential safety hazards.
     
  5. Xray

    Thread Starter Well-Known Member

    Nov 21, 2004
    58
    1
    Oh sure... I remember those shoe fitting X-ray machines too. I thought they were so cool when I was a kid. Maybe that's what sparked my interest in X-rays (?). I've seen those machines being offered for sale on Ebay every once in a while. Some of them were total junk, and some were in pretty decent condition. If I had nothing better to do, I probably would buy one of them and restore it to working condition. Of course I would not recommend anyone stick their feet into it. I understand that shielding in those machines was poor at best, and in some cases is totally non-existant!

    Fun stuff!
     
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