How to measure laser tube voltage with a normal tester

[irresistiblecam]

I still didn't solve my problem with my broken k40.
I open a new thread just to well understand how I could check the voltage that power supply produce to fire the laser co2 tube.
This voltage would be >10kV, and to measure it, I should have a specific high voltage tester, that is so expensive.
So like aj suggest I could measure it with a classic tester making a voltage divider.
But I don't know how I can do it.
Please some one could help me?
aj suggest me  this blog:
http://imajeenyus.com/optical/20131104_co2_laser_current_voltage/index.shtml

but I don't like the 460000:1 divider, because at the end he test a very few volts (about 38 mV), but my tester can test high voltage until 1000V.
So I think I could use  a minor divider, that produce a voltage of about 10-30V so it is better to measure and still not dangerous if I touch it.

aj suggest to buy this resistor:
http://www.aliexpress.com/item/1Pc-50K-Ohm-50W-Watt-Aluminum-Power-Metal-Shell-Case-Wirewound-Resistor-Resistance-Free-shipping-High/1864214120.html

But is is just 50K Ohm and it is so different from 470M Ohm.

What is the best value for the resistor, and what is the scheme of the voltage divider?
Thank You.

[John Dickinson]

The voltage produced by the supply is variable and controlled by the tube itself.

If you put the two wires close together then a spark will jump the gap, it takes a certain voltage to make a spark and the voltage is obviously controlled by the distance apart of the two electrodes (red wire and earth).

If you hold the wires 10 cm apart then a much bigger spark wll happen or, if the distance is too much you will get no spark at all.

If you want to look at the voltage produced then you need an oscilloscope, not a meter. The voltage waveform shape is important adn controlled by the power supply. The big resistor you showed a picture of si not useful at all to make a potential divider.

Some arithmetic.

Let's assume that the voltage is around 30,000 volts. You want to measure it with a meter that can stand 1000 volts so you need about 1/30th of the actual voltage at your measuring point.

We need to take the 30,000 and divide it by 30 and we can do this easily with two resistors.

30,000 > resistor > test point > resistor > ground

We also have a limitation of about 20 ma because if we try to take more than that we will damage the power supply but we do need to be somewhere near that in order to have the power supply operating in its normal operating area so let's assume 15 ma.

30000 volts, 15 ma that gives a resistance of 2 million ohms using ohms law. That means that R1 + R2 = 2000000

R1 and R2 need splitting in a ration to give us 1/30th of hte voltage at the measuring point so if we make R1 1.9 Mohm and R2 66 kohm then the voltage at the test point will be 1/30 of the voltage that is being put out by the power supply.

Having set those resistor values we need to be sure that we do not overload them so with a 1.9 Mohm resistor dropping 29,,000 volts we have a power dissipated of  44 watts so it needs to be a pretty damned powerful resistor.

That is the theory of it but the practise of it is that you look how big a spark it will make to see if the pwoer supply is OK.

[ajf]

I didn't recommend the 50K ohm wirewound resistor for making the divider, you are confusing two separate posts.  You don't need to test the voltage to know if the PSU is bad or if it's the tube. And, I specifically said that I did NOT recommend making your own probe.

The 50K ohm resistor was for use in one of the tests noted in this video (at about 1:44) where she disconnects the tube and replaces it with the resistor. That video has everything you need to know to determine if the problem is the tube or PSU.

regards,

aj

[Gareth McClean]

This video demonstrates how to to it.

https://m.youtube.com/watch?v=Qz6J4Rs7VCw

Personally I would be very careful to insulate and not touch any of this during the test but I have been shocked by HT in the past and have no intention of ever repeating the experience!

[ajf]

Hi Gareth,

Yeah, that the same video as on the blog mentioned earlier.  It's interesting and I like the homemade probe a lot, but there really is no need to test the voltage to determine if the PSU or tube is bad. There are some other good videos at the G.WEIKE site, where the video I recommended earlier is sourced.

regards,

aj

[Gareth McClean]

Sorry I should have read everything !