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Re: [TCML] measuring frequencies and high voltages

To measure High voltage (1 Million Volts) when your meter only goes to 500 or 5000 or even 5volts is simple... (there are of course more accurate means, and other such things but a rough guide I'm sure will do you fine)

As you are probably aware a voltage applied across a resister will cause current flow through the resistor and if the resistor is uniform then the voltage will drop uniformally across the resistor.... that being said if you replace the resistor (say a 1 meg ohm) with 10 that are each 1/10 the size (ie 100k) then each resistor will drop 1/10 of the total voltage.... so if your meter only goes to 500 and you want to measure a full voltage swing of 5000 then you would string together 10 resistors in series across the load and then put the probes across only one of the resistors which if they are all exactly the same will measure 500volts.... Of course you will want to have a safetly margin so you could use 20 resistors and then each will measure 1/20th of the total voltage...or even 1000:1 would be better - ie for 10,000Volts you would only measure 100 on your meter...

The above is perfectly fine for measuring DC, but the principle is similar with AC - sometimes really fancy multimeter divider probes have carefully chosen values of capacitors and inductors and resistors in a series bridge type arrangement - ie all elements in series but each element consisting of an individual resistor, capacitor and inductor.

To measure Voltages accurately you don't want to load the supply too much other wise you will cause a voltage drop and will not get an accurate votlage.... ie use the highest values of resistance, and lowest of capacitance etc... also your multimeter will have a characteristic input impedance as well and you want to have your probe within a sensible range to get accurate results....

here is a diagram of what the probe should be.....
R1 should be as high as possible and R2 should be as low as possible
R3 is the input impedance of the meter - usually 10MegOhms.
VOM: Z-in is the voltage range (full scale) times the ohms/volt rating of the meter. which means that R1 should be in the range of several hundred to several thousand MegOhms..and R2 should be roughly equal to the input impedance of the multimeter on the scale you want to use.......

High Voltage <------/\/\/\/\/\---------+-------------> + to DMM/VOM
                          R1            |                      |
                                        \                      \
                                     R2 /                   R3 /
                                        \                      \
                                        /                      /
                                        |                      |
 Ground Clip  <-------------------------+-------------> - to DMM/VOM

Now if R2 = R3 then in parallel this equals 5Megohms on most meters..
then R1 will equal 5000MegOhms...

Put R1 and R2 in a long probe (ie is a tube of 1/2 inch PVC Pipe) and you will need
to make r1 VERY LONG
(ie make it from lots and lots of individual resistors so that arc over will not be an issue...
Or you can order special high voltage resistors or take an old Tv apart....
Hope this helps......

P.s there are lots of high voltage probe designs on the internet....
(this is were I got the diagram above from)

John Forcina wrote:
How high Frequency is it?  You can try hooking up a small speaker to the
outputs and if it's in the audible range and if it's working properly you
will hear a tone at the frequency it's set at.  You also might want to add a
DC blocking cap on one of the speaker wires also but it's not 100%

On Wed, Oct 14, 2009 at 12:19 AM, Rhys Sage <rhys_sage@xxxxxxxxx> wrote:

I've built my DC pulse generator. I have a nice green LED coming off it. My
big question now is, how do I know if it is a pulsed or constant signal? I
don't have an oscilloscope and only a very basic meter.

Similarly, how do I measure high voltages when my meter only reaches 500v?

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