Measuring in excess of "400kV" - Page 1
Forum section: electronics / general

author: MrQuan
author_rank: Contributor
timestamp: May 03, 2016, 12:24:58 pm
content: I have one of these little voltage boost modules from eBay:http://ebay.com/itm/131622594391It claims to output "400kV". I'm sceptical and curious.How would I go about measuring this? I searched for high voltage probes, but they only seem to go into the 10s of kV. I also don't want to spend a lot of money on an expensive probe, so I've been scouring eBay for cheaper options.Does anyone have an idea on how I could do measure this? I have DMMs and DSOs, so I guess some form of home made voltage divider may be a solution?

author: HackedFridgeMagnet
author_rank: Super Contributor
timestamp: May 03, 2016, 12:45:47 pm
content: Well it's not going to be 400kV, otherwise the cables would be a tiny bit thicker.At a guess it would be under 10kV. so you could put a lot of hi value resistors in series and measure the voltage across one of them.

author: HackedFridgeMagnet
author_rank: Super Contributor
timestamp: May 03, 2016, 11:16:41 pm
content: Ok I stand corrected. Its amazing what voltage you can get for $2.50 then.Also don't try the resistors, maybe just work off the distance of the air gap.

author: mmagin
author_rank: Frequent Contributor
timestamp: May 03, 2016, 11:52:05 pm
content: Quote from: blueskull on May 03, 2016, 03:34:54 pmUhh, it could initially breakdown 10 cm, or once the arc was started it could be drawn out to that distance?

author: alsetalokin4017
author_rank: Super Contributor
timestamp: May 04, 2016, 02:24:14 am
content: Spark gap breakdown voltages are commonly determined using large smooth spherical electrodes. The commonly cited "30kV/cm" is only valid for spheres that are approximately the same size or larger than the gap. Pointy electrodes or simple plain wire ends will break down at much lower voltages for a given gap. Between pointy electrodes or simple wire ends, a distance of 10 cm will break down at less than 80 kV.http://www.kronjaeger.com/hv/hv/msr/spk/and many other references.

author: mmagin
author_rank: Frequent Contributor
timestamp: May 04, 2016, 03:49:52 am
content: Now I want to build myself a big voltage divider.Maybe 20 * 5 G high voltage resistors (Ohmite SM103035007FE) and a more modest (but still 1kV rated) 10 M resistor (plus the 10 M input of a DMM) to give me a 20000:1 divider good to about 150 kV.Some pieces of teflon tubing sticking up from a board to suspend the mess 'above ground'It's totally reasonable to spend $100 to prove a point about a $3 device, right?

author: MrQuan
author_rank: Contributor
timestamp: May 04, 2016, 06:19:52 am
content: Thanks for the replies so far.That HV websitehttp://www.kronjaeger.com/hv/hv/msr/spk/is a good resource thanks.I think I'm going to build a HV probe. I have sketched up a design (see attachment) which should get my into the 100s of kV range. It'll output 10V with a 100kV input. It's designed to use 20 x 100M resistors in series, each will see 5kV and 50uA, so I think that's acceptable.I think the little device pulses the HV output, so the capacitance of the probe may be an issue. I'm really not sure about this, I don't often delve into HV stuff.

author: amspire
author_rank: Super Contributor
timestamp: May 04, 2016, 07:36:02 am
content: If the device outputs short pulses of high voltage, there is a good chance your divider will not have the bandwidth to see the true peak voltage. If it is a short peaked voltage, you might have better success with a capacitive divider but you can test the idea with a digital oscilloscope. Connect one of the outputs to ground and have the other wire suspended safely. bring a x10 oscilloscope probe to perhaps 10cm of the HV wire. Set the DSO to single shot and see if you can trigger the DSO from the HV pulses.Don't bring the scope probe too close to the HV wire - you do not want to risk damaging anything.If the idea works, you can look at making a capacitive divider.

author: mmagin
author_rank: Frequent Contributor
timestamp: May 04, 2016, 02:35:08 pm
content: Quote from: MrQuan on May 04, 2016, 06:19:52 amMake sure you're using 100M resistors rated at 5kV. These are readily available, but the cheap 1 and 2 watt resistors won't do it. Even if they don't visibly breakdown, the voltage coefficient of resistance will effectively be all over the place once you get beyond their datasheet rating. Most of the 5 kV or better rated resistors I've seen in Digikey or Mouser tend to be $3 and up (US) in single quantities.(I learned this first hand trying to make a divider long ago to measure the output of one of those electric flyswatters -- using 1/4W carbon film resistors)

author: mmagin
author_rank: Frequent Contributor
timestamp: May 04, 2016, 02:38:24 pm
content: Quote from: amspire on May 04, 2016, 07:36:02 amI assumed these things are a Cockcroft-Walton multiplier on the output. Do you think they might be a Marx generator?

author: amspire
author_rank: Super Contributor
timestamp: May 04, 2016, 03:01:14 pm
content: It is meant to be a Tesla coil with a pulsed HV side, I gather. That is why I suggested a a first step putting a scope probe somewhere near to see what is going on. It would give an idea of frequency and pulse length.A HV resistive probe may be totally useless with this device - it might drag the voltage down significantly, and if the pulse is short enough, you would not see the pulse properly. Find out what you are dealing with before spending money and building anything.

author: Marco
author_rank: Super Contributor
timestamp: May 04, 2016, 03:27:58 pm
content: A DC accurate probe is right out, the balancing of the RC sections is a huge pain when combined with the insulation requirements.So that leaves a capacitive divider probe.

author: alsetalokin4017
author_rank: Super Contributor
timestamp: May 04, 2016, 03:31:56 pm
content: http://www.coe.ufrj.br/~acmq/hvmeasurements/

author: mmagin
author_rank: Frequent Contributor
timestamp: May 04, 2016, 04:02:38 pm
content: Quote from: amspire on May 04, 2016, 03:01:14 pmOh, I see. These are exactly the thing that's found in cheap stunguns.

author: Skimask
author_rank: Super Contributor
timestamp: May 04, 2016, 11:36:56 pm
content: Starting at about 7:30 into it...

author: joeqsmith
author_rank: Super Contributor
timestamp: May 05, 2016, 01:47:13 am
content: Quote from: blueskull on May 04, 2016, 03:11:49 pmhttps://www.eevblog.com/forum/beginners/dangers-of-high-voltage-and-measurement/A different member had asked the same question.Quote from: Marco on May 04, 2016, 03:27:58 pmI believe this is going to be the best answer if they did not want to just measure the gap. Doing anything with a resistive divider would seem like a nightmare for a build it yourself. But again at 400KV, I have no clue. Post what you end up with.

author: Neilm
author_rank: Super Contributor
timestamp: May 05, 2016, 06:56:05 pm
content: I have not seen anything that indicated the output power. If you use a resistor divider chain you will (by definition) load the output. It is possible you will overload it and the voltage will therefore not rise so high.

author: mikerj
author_rank: Super Contributor
timestamp: May 05, 2016, 07:45:14 pm
content: No chance that this gives out 400kv, or anything like. Even the text on eBay hints at this:Quote2cm in air from pointy electrodes takes about 20-30kv I think? A factor of ten seems about right for Chinese eBay ratings