how to measure extremely high voltage?

[large_ghostman]

How do I measure very very high voltages? there are from a car ignition coil? no expensive answers no 1000x scope probes, but I have a x10 probe lol. I need to measure the output from a ignition coil with 18V going through it on the nice side, what I want to measure is the output. I need around 31,000V lowish Current (around 200mA) its for growing a crystal, well for kind of altering the shape as it grows.

 

[Miles Prower]

I need around 31,000V lowish Current (around 200mA) its for growing a crystal, well for kind of altering the shape as it grows.

Not. Gonna. Happen.

P= 31E3 X 0.2= 6.2KW

Your ignition coil will go Chernobyl on you. There really are no cheap solutions for such voltage measurement. If you could recover a high resistance, high voltage, color TV focus pot, and wire it in series with a 10K resistor to make a voltage divider, would probably be the best option. These focus pots are up to 90M in resistance. Measure the voltage across the 10K resistor, and back calculate from the voltage divider formula.

 

[JimB]

Hi there LG.

A couple of problems come to mind with this scheme, apart from the 6.2KW bit.

What are you using to drive the ignition coil?
In the normal application the coil is powered for a small fraction of the time and the collapsing magnetic field creates the HV pulse.
What the difference is with a "modern" ignition coil I dont know, I never had to fix the ignition on any of my cars in the last 25 years!

Using a voltage divider as suggested by Miles is the way to go, but, are you going to measure the AC or pulse voltage out of the coil, or are you going to rectify it to DC and measure that?
Your home made voltage divider will have a very poor frequency response, it will probably behave like a lowpass filter unless it is frequency compensated. Just like a 10:1 scope probe.

JimB

 

[large_ghostman]

Its the old style ignition coil, probably twice my age. a pulse just like a car spark plug would be fine, I can drop the current a bit and just run it longer. It needs to be on for around 15 mins a day. I didnt do a single calculation just thought up the voltage a bit from 12V and bingo pulse it like a distributor and away I go.
6.2KW is gonna need some heat sink , maybe a squirt of liquid nitrogen . Only joking. You would think by now I would know to at least do the basic calculations But No, I just thought car! easy peasy. Back to the drawing board

 

[large_ghostman]

I might try and get one of those pots though, sounds like something I would want in my might be handy one day box!

 

[KeepItSimpleStupid]

I worked on a 30 KW regulated 15 kV at 1A nominal, shunt regulated by a tube. Input voltage was 208 VAC 3 phase at 60 A. The transformer had triple primaries, was huge and painted red.

Then there are these probes: **broken link removed**

 

[large_ghostman]

They are way cheaper than I thought! HMmmmm, might have to apply for a bank of dad loan

 

[Dean Huster]

Fluke makes a high voltage probe for DMMs, but I don't remember if they're rated for 20, 30 or 50KV.

 

[dr pepper]

You can build an electrometer fairly easily, just a fet a few comps and a meter, but the measurement is fairly much an estimate, that might be good enough for you, a good point is that theres no connection to the circuit, it measures the electric field.
I use a professional electrometer often for checking antistatic bars in paper production.
I have a old automotive style meter that can measure the spark plug volatge, and yes I opened it up to see how, theres a stack of high value resistors in series with the meter movement, they are in a highly insulative tube.
Its probably 3 times your age.

 

[ronsimpson]

Because you voltage is a pulse a volt meter with a "TV" high voltage probe will not work.
Because you voltage is a pulse a capacitor divider will work.
Wish you were closer. I have some 100pF 40kv capacitors.
Series two caps to get 50pF 80kv, then a large low voltage cap (to ground) and you will see a much small pulse across the bottom large cap.

You don't have the pieces to make flyback transformers laying around. I have another plan.
I have taken copper tape and placed a piece around a high voltage wire. You can use tin foil.
This makes a capacitor to the inside conductor.
Measure the capacitor. 10pF. You probably can't so:
Add a 10nF 100V cap to ground. Now you have a 1000:1 divider. (but you really don't know the top cap's value)
Use a high frequency generator, sign or pulse, send a signal into the output of the ignition coil (what ever is making the high voltage).
If the generator is making 100khz at 10V then you should see 100khz at 10mV at the bottom of you divider.
You can add more metal in you high voltage cap or remove metal to get the divider ratio you want.
Or you can live with a 1357:1 divider and just do the math. (use the scope volts/div knob to set the ratio)

This will not measure DC but will give you a rough idea if the P-P AC on the high voltage wire.

 

[large_ghostman]

Because you voltage is a pulse a volt meter with a "TV" high voltage probe will not work.
Because you voltage is a pulse a capacitor divider will work.
Wish you were closer. I have some 100pF 40kv capacitors.
Series two caps to get 50pF 80kv, then a large low voltage cap (to ground) and you will see a much small pulse across the bottom large cap.

You don't have the pieces to make flyback transformers laying around. I have another plan.
I have taken copper tape and placed a piece around a high voltage wire. You can use tin foil.
This makes a capacitor to the inside conductor.
Measure the capacitor. 10pF. You probably can't so:
Add a 10nF 100V cap to ground. Now you have a 1000:1 divider. (but you really don't know the top cap's value)
Use a high frequency generator, sign or pulse, send a signal into the output of the ignition coil (what ever is making the high voltage).
If the generator is making 100khz at 10V then you should see 100khz at 10mV at the bottom of you divider.
You can add more metal in you high voltage cap or remove metal to get the divider ratio you want.
Or you can live with a 1357:1 divider and just do the math. (use the scope volts/div knob to set the ratio)

This will not measure DC but will give you a rough idea if the P-P AC on the high voltage wire.

Interesting stuff! I have a LCR meter that will read low pf caps, and I have a flyback from a TV, I will try all the above .
At last my LR meter is getting a good work out lol, so far all its done is mainly ESR readings

 

[dr pepper]

Keeps your fingers away, as soon as you put caps near a flyback it becomes very nasty.

 

[large_ghostman]

I will be careful! I follow dads golden rule 1. Never put your fingers where you wouldn't put your tackle .
Taking some old mobile phones apart at the moment so will get back to it soon as I have the motors out the phones. These are nokia ones with colour displays, fairly old so I was hoping to use the displays. Ive got the phone service manual but it dosnt tell me what display it actually is, i hope when I get it out there is a part number.
3 phones so I need 1 more for my quad copter! I wonder if dad will miss the one out of his new phone

 

[Nigel Goodwin]

Keeps your fingers away, as soon as you put caps near a flyback it becomes very nasty.

Most remotely modern LOPTX's already have internal capacitors, in the walton-cockcroft multiplier.

'Best' capacitor is the CRT it feeds of course, I've known those hold charge for years.

 

[tvtech]

Most remotely modern LOPTX's already have internal capacitors, in the walton-cockcroft multiplier.

'Best' capacitor is the CRT it feeds of course, I've known those hold charge for years.

Absolutely correct The shock from a "charged up" tube belts the living daylights out you and if you are unluckily enough to be the one told to dump it...and the Anode Cap belts you....and you drop it anyway...but far away from the Dump area... LOL.... Happened to me as a Trainee..

Thanks for bringing back all the memories Nigel.

Regards,
tvtech

 

[dr pepper]

OK then, so a lopty has a multiplier allready in it, I didnt know that.
So a telly that has a tripler would have a lopty without a multiplier?

I just got some B&W displays for nokia's, not from old phones, from china, I'm going to have a play and see if they are any good.

 

[ronsimpson]

Keeps your fingers away, as soon as you put caps near a flyback it becomes very nasty.

I am talking pF. Not much energy. and There is no diode so there will be no charged up cap.

 

[Nigel Goodwin]

I am talking pF. Not much energy. and There is no diode so there will be no charged up cap.

It would be VERY unusual to find an LOPTX without a built-in rectifier, generally only VERY old ones, where you had an external tripler and the transformer itself outputs 8KV AC, and the external tripler increases that to 24KV.

If you wanted to go back even further (to the valve days) the transformer provided full EHT, and used an external valve rectifier to get the DC, along with a stabiliser valve to regulate the EHT (absolutely horrible things! )

Only pF will still give you a good 'jolt' as it's at such a high voltage.

 

[4pyros]

Use a high voltage transformer from an old oil furnace.
They were used to make a spark and light the oil.
http://www.xn--kronjger-4za.de/hv/hv/src/obit/index.html

 

[dr pepper]

I have a small collection of valve radio's including a couple of tellys, and I've a couple of spare eht trannys, they look nothing like the later ones, and they are a pain to rewind, but they are usefull for high voltage projects.

If you just want a pulse how about a piezo ignition for a barbie.