BF2S Forums Calling all Electrical Engineers

Ask your dad?

EE sux

AE ftw

Scorpion0x17 wrote:

SE pwns all

sex engineering?

OrangeHound wrote:

Connect a high resistance path using multiple resistors in parallel with the source and sink (on the order of 10^8 ohm-meters, the resistance of air) .... make sure one of your resistors is small, and connect your meter across this resistor.  It might take some experimentation with the chain to prevent arcing across the resistors ... you might need to use 1 watt resistors to handle the voltage.

Measure the voltage across this resistor when arcing begins, and then doing standard calculations will give you the total voltage.

Well, at least in theory ...

This was my original idea, but I think my dad is right when he says this won't work because I don't know the amperage.

I can't use the voltage of the source because really what I want to know is the inefficiency in the system. I am building a Marx generator, basically a simple voltage multiplier. I know what the voltage across the gap should be, but because I don't know what it is I don't have enough information to use this method right?

OrangeHound wrote:

Another approach is to make an electroscope ... just search Google for instructions.

Well there's an interesting idea.

I can't seem to think of a way to convert a relative measure to a definite measure though. Surely there it's not a direct (or even measurable?) relationship between the voltage and the effect on the electroscope?

OrangeHound wrote:

Flaming_Maniac wrote:

This was my original idea, but I think my dad is right when he says this won't work because I don't know the amperage.

You don't need to know the amperage* ... you know the resistor value, so just measure voltage drop across a single resistor and then multiply.  This is EE 101 ... and I'm assuming you have a good multimeter.

Your biggest problem might be arcing across your resistor chain.  If you used this technique, you would need to do some work to ensure this doesn't happen.



*Unless you are concerned about frying a resistor, but that shouldn't be a problem if you use 1 watt resistors and a total high resistance ... and, anyway, smoking a resistor occasionally is part of the fun of EE.

Oh right. Well you don't multiply (unless all the resistors are the same value) but you can use a proportion.

I am pretty sure I walked through this in my head though, and I thought there was something wrong with it...ah well, maybe I was just being dumb.

Flaming_Maniac wrote:

Oh, how would I be able to get a reading off of the voltmeter when the gap sparks? I don't believe the reading is instantaneous?

Oscilloscope trigger if you just have a single instantaneous spark ... but, I would assume that the voltage would be slowly building up and you can watch the voltage increasing across the resistor.  Just record the voltage once the spark emits.

Flaming_Maniac wrote:

I need a way to measure extremely high (ideal max of  60kV) voltage across a gap that will be producing sparks.

My dad has a multimeter, obviously it's not going to handle something like this. He is an electrical engineer and he has a lot of interesting stuff laying around, so semi-exotic equipment might be on hand.

My current best bet is to try to measure the energy output of the spark by looking at the color spectrum given off, and then go from energy to voltage by using the properties of air...obviously this isn't very exact and won't be incredibly easy to pull off either. If anyone has a better idea I would be very happy to hear it.

big resistor, measure amps???

Im not really sure what your trying to do. If you know the voltage, you can calculate the rest without experimenting, Air has a given resistance. So, in essence, your "gap" is a resistor. No need to blow up your dads multimeter btw, how are you getting 60,000 Volts?

Last edited by Locoloki (2008-12-20 23:50:47)

Im_Dooomed wrote:

wtf are u building Tesla? I mean, Flaming?

http://en.wikipedia.org/wiki/Marx_generator

CrazeD wrote:

Im_Dooomed wrote:

wtf are u building Tesla? I mean, Flaming?

http://en.wikipedia.org/wiki/Marx_generator

It can also be used as an ignition switch for thermonuclear devices.

I thought I heard FM mention he'd converted to Islam recently...

Yeah the problem was the multimeter can't take instantaneous readings, my dad had said you need current flow, not that I need to know that amperage. Poor memory on my part.

The oscillator idea is good, thank you OH.

Locoloki wrote:

Flaming_Maniac wrote:

I need a way to measure extremely high (ideal max of  60kV) voltage across a gap that will be producing sparks.

My dad has a multimeter, obviously it's not going to handle something like this. He is an electrical engineer and he has a lot of interesting stuff laying around, so semi-exotic equipment might be on hand.

My current best bet is to try to measure the energy output of the spark by looking at the color spectrum given off, and then go from energy to voltage by using the properties of air...obviously this isn't very exact and won't be incredibly easy to pull off either. If anyone has a better idea I would be very happy to hear it.

big resistor, measure amps???

Im not really sure what your trying to do. If you know the voltage, you can calculate the rest without experimenting, Air has a given resistance. So, in essence, your "gap" is a resistor. No need to blow up your dads multimeter btw, how are you getting 60,000 Volts?

That.

Find out the resistance of air, then just Ohm's law it.

Or use a big-ass industrial amp-meter. Car battery testers can usually take 2-3kA.

You can find out the minimum value it could by using the breakdown voltage of air - 3 x 10^6 v/M - multiply that by the separation in meters (i.e. multiply by 0.5 if it's a separation of 0.5m, multiply by 0.001 if it's 1 mm apart to get the minimum voltage

then adjust the separation so that it just doesn't spark and then adjust so it just does spark, and calculate the voltage for both of those and take an average?

might that work?

although it does depend on the humidity of air. Slightly more accurate than the other methods though i would think, the separation could be measured pretty accurately (provided you are able to adjust the separation of the 2 objects sparking)