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Author Topic: Measurung capacitance of wire  (Read 10464 times)

Offline joojoo

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Measurung capacitance of wire
« on: 11/07/2004 17:20:11 »

Hey guys, this may be a bit of a weird question, but i bought a lighting control gear box and it says that "For the use of indipendent ballasts with incorporated ignitor, the lamp must be placed as close as possible to the indipendent ballast, taking care that the distributed capacity of cable is never exceeding the value of 100pF (with standard cables the max. distance is about 1.5-2.0m)"

My question is: how does one measure the distributed capacity of the cable?

Hope you can help,

Joojoo


 

Offline tweener

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Re: Measurung capacitance of wire
« Reply #1 on: 11/07/2004 17:58:24 »
Welcome to the forum!

You can measure it with and oscilloscope and a pulse generator.  That's expensive instrumentation for a one time measurement.  

Or, I believe there are instruments that do just that, mainly used for tuning antennas and matching them to their transmitters.  You might be able to take your gear to an electronics shop and get someone to measure it for you.  I don't know what they would charge for that sort of thing - not much probably, or nothing if you buy something.  If you are a student at a university, you might be able to get access to an electronics lab in the physics or engineering departments.

Good luck, and let us know how you come out with this.

----
John - The Eternal Pessimist.
 

Offline gsmollin

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Re: Measurung capacitance of wire
« Reply #2 on: 13/07/2004 19:15:01 »
Some hand-held voltmeters also have a capacitance meter built-in. Take the cable limit seriously; I would have said 1 m length for 100 pF capacitance.
 

Offline Pumblechook

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Measurung capacitance of wire
« Reply #3 on: 15/01/2008 14:40:13 »
RG59 Coax 75 ohm cable is 53pF/metre.   Twisted wires would be similar maybe??   In the coax the inner is surrounded by the outer but there will be a bigger gap.

I suppose if you ran two lengths of 75 ohn coax abd didn't connect the outers (braid) it will be quite a bit less than  26pF/metre.

Maybe ordinary twin and earth cable will be low C per metre as the  conductors are held apart.

100pf sounds very low to bother about???




 

Offline Pumblechook

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Measurung capacitance of wire
« Reply #4 on: 15/01/2008 14:44:32 »
Capacitance of twisted pair (F):


http://www.sigcon.com/lib/htm/TWIST.htm

Example cable is 1.3 pF per inch. 
« Last Edit: 15/01/2008 14:48:02 by Pumblechook »
 

Offline syhprum

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Measurung capacitance of wire
« Reply #5 on: 15/01/2008 17:23:23 »
I think that the capacitance limitation is due to the design of the igniter I expect a high voltage source charges a small capacitor that then discharges a short pulse via a spark gap to the lamp.
Similar devices were used on the argon-neon lasers that the Hell Scanner/Recorder's that I used to work on used and on the xenon arc lights that are popular on top grade cars.
I think capacitance of a cable can be derived from the SI defination of capacity    
« Last Edit: 15/01/2008 17:25:54 by syhprum »
 

Offline syhprum

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Measurung capacitance of wire
« Reply #6 on: 15/01/2008 17:33:16 »
If you have say a 100 meter length of cable and a high input resistance voltmeter (they are typically 10 meg Ohm) you can feed in a current via a 10 meg resistor and note the speed at which the voltage rises and do a little arithmetic.
 

Offline Bored chemist

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Measurung capacitance of wire
« Reply #7 on: 15/01/2008 19:54:36 »
You could do that, but you would need fast reflexes; with 100pF and 10MOhms the rise time would be less than a milisecond. The easy answer is to use a short cable.
 

Offline syhprum

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Measurung capacitance of wire
« Reply #8 on: 15/01/2008 21:41:59 »
I did suggest using 100 meters of cable.
I did in fact use this technique to estimate where the break was when I lost my telephone line, it was also used by the engineers back in the 1860,s when they had problems with the first transatlantic cables
« Last Edit: 15/01/2008 21:48:32 by syhprum »
 

lyner

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Measurung capacitance of wire
« Reply #9 on: 15/01/2008 21:49:49 »
Many very modest multimeters will measure capacitance for you these days.
When they say 'distributed capacitance' I think all they mean is the capacitance for a length of cable which is much shorter than the wavelength of the signal you are using it for. At 50Hz  (60Hz?) this is not a problem.  Transmission lines behave differently when your line approaches, say 1/8 wavelength but this effect is only relevant for long power distribution cables.
If you really want to have your ballast a long way from the lights you can space your wires a few cm apart, instead of having them inside the same sleeve; this will reduce the  capacitance a bit.
btw, am I showing my age if I show surprise that no one has suggested a 'capacitance bridge'?
Wayne-Kerr (yes, that was the name) used to make a whole range of component bridges; I cut my teeth on things like that.
 

Offline Bored chemist

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Measurung capacitance of wire
« Reply #10 on: 16/01/2008 20:47:21 »
"I did suggest using 100 meters of cable."
I know you did, and I did the arithmetic with 100pF because that's the capacitance the guy is interested in.
Typical coax cables are about 100pF to the metre so you would still have well under a second to make the measurement with 100M (10nF and 10MOhms gives an RC time constant of 0.1sec). My DVM only updates the display a few times a second. I think mains cables would have lower capacitance which would make matters worse.
 

Offline syhprum

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Measurung capacitance of wire
« Reply #11 on: 17/01/2008 17:35:11 »


Here is a DIY Bridge , after obtaining a balance use the resistance measuring facility of the DVM to measure the ratio set by the pot and multiply accordingly.
« Last Edit: 17/01/2008 17:40:56 by syhprum »
 

lyner

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Measurung capacitance of wire
« Reply #12 on: 17/01/2008 18:39:12 »
The only trouble with that design for measuring100pF (a low value) is that actual layout is important and stray capacitance can account for quite a few pF error, in total. OK for a useful ball-park figure, though.
I still say the best way (value for money) is to use a DVM with Capacitance measuring facility - I have had at least three cheapos that could do it.
 

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Measurung capacitance of wire
« Reply #12 on: 17/01/2008 18:39:12 »

 

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