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
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