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[help plz] Measuring HIGH DC Pulse Voltage with DMM

ztkraptor

May 3, 2011
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I have searched the internet, and this forum, and can't quite find what I need.

What I have:
I have a 14/20 Holiday Detector made by DEsterns
http://www.destearns.com/hd.html#14

Basically what this device does is sends out a high voltage, low amp spark between a ground and a positive node. This is used to detect holes in coatings on pipes and various other things.

What I need:
What I need to do, is find a way to measure the peak voltage generated from this device.
It has 20 settings ranging from 1kV to 14kV.
My DMM ofcourse maxes at 600volts.
I have been trying to find a voltage network or something like that to use. But I can't quite find the math I need.

I'm assuming that the current given off by this spark is very very small.

The power source for the device is a 6 or 12 volt battery, so it isn't powered from an AC source or anything very high powered.

Please Advise. Thank you.
Steve


EDIT: Also, I don't have a scope to check it on. And we tried an older kV meter(40kV max) but we are guessing that it has too high of a resistance for the device to throw a spark.
 
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duke37

Jan 9, 2011
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I have repaired electric fence energisers which give a high voltage pulse of about 6kV each second. I originally used a spark gap to estimate the voltage using a value of 3kV/mm. A 6mm metric or 0BA bolt has a pitch of 1mm. I have now been given a Gallagher digital meter for the same job and they seem to agree. It depends on the accuracy you require. I do not know whether the Gallager would be happy with pulses at 14kV or more often than one/sec.
You could get a high voltage probe for a scope or make a voltage divider. You will need many resistors in series since about 1kV is the maximum you should put across each resistor. If your pulse is very short, it may be difficult to see if it does not repeat often.
 

duke37

Jan 9, 2011
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If your kV meter used electrostatic attraction to deflect the pointer, then a steady voltage is required, not a pulse.
 

ztkraptor

May 3, 2011
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Thanks, yeah we actually are ordering a meter and probe... That we hope will do the job.

I was going to do one of those "voltage networks" using Vout = Vin(R2/(R1+R2))
But, You still need a source to calibrate it. And I also wasn't sure if the current would be enough with the resistance values there.
 

poor mystic

Apr 8, 2011
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The voltages you're talking about are so nasty.
I hope you don't plan to hold your meter and read the potential while it's connected... that's how people get killed.
If you use any kind of capacitive charge storage or capacitive potential divider, please don't forget to put bleeder resistors on the caps! Bleeder resistors make measurements so much safer.

All that said, you *could* use a diode from a TV HV power supply, build charge on a cap until you get equilibrium, and use a DC electrostatic voltmeter.
 

ztkraptor

May 3, 2011
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The voltages you're talking about are so nasty.
I hope you don't plan to hold your meter and read the potential while it's connected... that's how people get killed.
If you use any kind of capacitive charge storage or capacitive potential divider, please don't forget to put bleeder resistors on the caps! Bleeder resistors make measurements so much safer.

All that said, you *could* use a diode from a TV HV power supply, build charge on a cap until you get equilibrium, and use a DC electrostatic voltmeter.


Well, I don't think its really possible to be killed by this device. Its basically like a spark from a spark plug. The Amperage is so incredibly low, were talking milliamps.
The voltage is only that high because it is needed to make the spark jump to the wall of the rubber sheet lined steel tank.

My boss actually has been shocked with it on the highest setting. He said you "feel it" but its nothing crazy.
 

poor mystic

Apr 8, 2011
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In Australia, an average of about 6 people are killed every year by heart attacks triggered by taste-testing 9V batteries. Those kV potentials can definitely kill. Getting away with something isn't the same thing as demonstrating it to be safe.

(help me, guys!)
 

ztkraptor

May 3, 2011
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In Australia, an average of about 6 people are killed every year by heart attacks triggered by taste-testing 9V batteries. Those kV potentials can definitely kill. Getting away with something isn't the same thing as demonstrating it to be safe.

(help me, guys!)

I don't mean to start a war here. But taste testing a 9v battery can't kill you, unless you have a heart regulation device and that device is basically broken.
Even then its very unlikely. As the current doesn't travel though your circulatory system and though your heart. It travels the distance between the terminals, which is conducted by the saliva in your mouth.
 
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poor mystic

Apr 8, 2011
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(My last word)
No war, just look it up and work it out for yourself. "Common sense" is no guide, how about "ask a doctor"? 6 people per annum die in this country of about 20 million. True.
 

ztkraptor

May 3, 2011
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(My last word)
No war, just look it up and work it out for yourself. "Common sense" is no guide, how about "ask a doctor"? 6 people per annum die in this country of about 20 million. True.

I looked it up, and it said it was an urban legend.
The Logic involved just baffles me that someone on this forum would believe something so crazy.

http://msgboard.snopes.com/message/ultimatebb.php?/ubb/get_topic/f/104/t/000779/p/1.html
Batteries are rated in terms of voltage and their energy output in mAh - milliamp hours (how many milliamps they can supply for how long). A 9-volt battery supplies 9 volts under its "rated" load - of 200mAh. That means a 9-volt can supply 200mA for an hour, or 100mA for 2 hours, etc. (In practice, as the battery weakens its voltage drops and it dies more quickly, but that isn't important here.) Put the 9V battery across a wet surface, like your tongue, and there will be sensation from the current that flows *between the terminals*. (Keep your eyes on this phrase, it will be important.) For a new battery, the current will be 9V divided by the contact resistance of the human tongue. How much this is, I don't know, but it's not really important.
The energy rating (mAh) of the battery isn't important either because the duration isn't long enough to deplete the battery either.

Usually people get electrocuted and die because they have about 90mA or higher passing through their heart, which causes arrhythmia. Higher currents can stop the heart completely, which is in theory preferred, because you can be kept alive with CPR. (CPR cannot save a person from an irregularly beating heart.) People usually get electrocuted by grabbing a live wire and the current flow between their hands and across their heart, or from their hand to their feet. People can also be electrocuted when standing near a lightning strike, because the lightning will set up a high voltage between their feet (there's a long explanation about ground current flow and gradients here which I won't talk about.) In any case, flow of electricity needs two terminals - in and out.

If you put the battery on your tongue, current will flow between the terminals, and be restricted only to your tongue. You can't die from this - the current flow isn't anywhere near enough and it's local. A car battery would have similar results - the voltage is 12V and while it is much bigger, the current it supplies is limited by the resistance and not the size of the battery. Not that car batteries are harmless - putting a wrench across the terminals will weld it into place.

These numbers vary, but my rule of thumb is that voltages in excess of 25V DC and 50V AC are potentially unsafe. Typically people (like me) work on 24V DC systems live without any risk - you can't even feel the sensation of 24V DC on your fingers. Some electrical meters announce "unsafe voltage" at 30V or higher.

As for the web site, there are other ways to be killed while testing or using batteries besides sticking them on your tongue. Lead-acid car batteries give off hydrogen gas - a spark above a charging battery has had disastrous results. Battery chargers with high input voltages can also be dangerous.
 

ztkraptor

May 3, 2011
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Back to the main point.

I would like to make a voltage divider.


I have the basic Idea

voltdivider.jpg


I really just need to know what the input impedance of the DMM i have has to do with anything. its 10Mohms. Where does it go in the calcs i need to do.

I suppose R2 would be a variable Resistor to fine tune what i need.
 

duke37

Jan 9, 2011
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Electric fence energisers are rated in energy/pulse, 5J is about the maximum and I keep well away from that. The pulses occur once/sec so you have time to withdraw. I assume that your device is also pulsed so measuring with a DVM would not be possible. Adding a rectifier and capacitor to give a measurable voltage would risk exceeding the permitted energy. To measure the voltage dynamically you will need to use an scope. Your divider could consist of nineteen 1M resistors for R1 and one 1M resistor for R2, You do not need anything variable and if the resistors are 1% you will get an output of 5% of input. You will need ensure a comon ground to C. I think high voltage scope probes are available to measure TV voltages. Do you need high accuracy or will a spark gap do?
 

duke37

Jan 9, 2011
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Electric fence energisers are normally rated by the energy in a pulse. 5J is the recommended maximum but I keep well away from one this powerful. The pulses occur about once per second so you have time or are forced to withdraw. I could not find what the energy in each pulse is or the repetition rate.
I assume that the output is pulsed and, if so, a DMM will not measure voltage correctly, you will need an oscilloscope. The voltage divider could have R1 as a chain of nineteen 1M resistors and R2 a 1M resistor giving a division ratio of about 20:1. These should be placed in a plastic tube and a good connection to ground made to C.
I believe that such probes are available to monitor the high voltages in TVs.
 

ztkraptor

May 3, 2011
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Thanks for your response.

Well. We had tried a spark test, but we don't have the proper balls to use in order to provide accurate information. We basically just used the Ground Cable that came with the item, and the "positive" cable, and slowly moved them together, But, Since the ground was a type of weaved wire, and the positive cable was a thicker cable with a rounded end, it really isn't going to give us accurate readings.
We were getting some 40kV readings.

I'm hoping that the device we ordered, will work even with the short burst. The period of bursts seems to be about 1/4 of a second. So we shall see.
 
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