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Re: [TCML] Measuring Voltage?
Voltmeters off X ray machines are NOT rated to the voltage indicated
on the scale. They are a low voltage meter that is fed from a voltage
divider circuit.
You can apply low voltage DC and determine what the actual rating of
the meter is and also the resistance and then make a voltage divider
to match the scale.
If you think about it, IF the meter actually was rated at the voltage
indicated, it would have a thick case and highly insulated
connections to withstand the rating.
Normally the voltmeter is tapped off a bleed resistor to ground.
Depending on the X ray machine, the HV transformer may be center
tapped to ground with the ammeter connected to one leg of the taps
and the voltmeter connected to a series, tapped dropping resistor off
the HV side and the meter is on the resistance tap closest to ground
so a failure will not present a serious shock hazard to the meter itself.
If you want to use the meter on DC, you need a divider circuit like
the old TV HV probes had, usually around 100 meg and 1 meg, just
depends on the ohms per volt of your meter.
Rectifying the voltage will give you a peak reading and not the
actual rms value. You basically would have to divide the meter
reading by .707 to obtain a correct AC reading.
On the coil side, that is RF energy and you need entirely different
measuring equipment. Normally some sort of hot wire or thermocouple
type meter is needed.
Frank
At 08:52 AM 5/23/2010 -0500, you wrote:
I wanted to ask if whether or not the primary, or secondary,
voltages can be measured with a meter. Here's what I did, and I
don't know if my meter is that far off, or if there is something
else affecting the readings that I'm not aware of. I have an old KV
meter from an x-ray machine with a range of 40kv to 140kv dc. I
attached a couple hv diodes to the meter and attached it to the
secondary electrodes on a bi-pole coil. The needle was pegged almost
immediately when I started to apply power. Then I switched it over
to the output on the transformer (9kv, 30ma nst). With the variac
at about 1/4 power, the meter was registering 100kv. Not what I expected.
So I have a couple questions here; first, why is the primary voltage
reading so high with this meter? I'm thinking that the meter is
showing the voltage rise due to resonance, perhaps? If that's the
case, is there any way to accurately measure the voltage? I'm
actually more interested in knowing secondary output. How can one go
about measuring, even "guesstimating" the approximate output on a
coil? I'd like to have a somewhat more accurate answer for people
when they ask what the voltage is of the arcs. Can my bi-pole coil
be putting out more than 140ish kv at such low power? Thanks in
advance for your help with this!
Neal N.
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