How to test CABLE capacitance using DMM

[rogeryu_ph]

Guys,
Hi, I'm searching for a guitar cable with the least capaitance which is not the same price as Klotz or George L. I read several review to look for least capacitance to avoid tone suck or treble loss. Here in our electronic district there are many shop selling sealed and or customized guitar cable which tag as "LOW NOISE" etc. How or can I test these cable with just using DMM in the shop? or do I need something else?

Thanks,
Roger

[darron]

most DMMs don't test capacitance, so you are out of luck on that one. make your own cable, hit the websites of your local electronics shops and they should give you the technical info on the cable.

here's the one that i like to use:
http://www.electusdistribution.com.au/productView.asp?ID=3546&CATID=22&keywords=&SPECIAL=&form=CAT&SUBCATID=225

it's double shielded with 100% coverage from micar, then copper shielded braid around that. reistance is VERY low, capacitance is okay.

[rogeryu_ph]

Is there another way beside using multimeter? this is my only tool

Thanks,
Roger

[darron]

i've never seen or used one, but i think that you need a capacitance meter. and they cost quite a bit... maybe someone can step in here with some better advice and save the day though.

[R.G.]

Is there another way beside using multimeter? this is my only tool

It's like they say, necessity is not the mother of invention - DESPERATION is the mother of invention.

Let's think - what do capacitors do? They store electrical energy, so you could charge it up and discharge it, and they have AC impedance, so you could measure the AC impedance somehow with an AC signal.

The problem is that we're talking about a capacitance in the range of 25 pF per foot of cable. So a two meter cable will only have a capacitance of 150pF. That is so small that you can't read the charge up/down times on the capacitor with a meter. DC testing is out. But if you used a 555 timer to make it charge and discharge all the time, you could convert your problem reading a capacitor into a problem of reading a frequency. There is some promise there.

Hmmm... let's look at AC testing. I bet you have a source of maybe 12Vac to 24Vac - a wall wart. If you hooked up a cable as a capacitor, and put another capacitor in series with it, then put 12Vac across it, the voltage across the capacitors will divide by their impedances, just like resistors divide a DC source.

So if you have a 150pF ordinary cap and a 150pF cable capacitor, then putting 12Vac across the two of them in series will cause 6Vac to appear at the junction. HOWEVER(!!!!!!!) the impedance of a 150pF cap at 50Hz is Z = 1/(2*pi*150R-12) = 1,060 Mohm. Your 1M to 10M meter can't read that - it will short it out.

But there is another trick - bridge measurements. If you have two 150pF capacitors across an AC source, and two 1M resistors across an AC source, and you put the meter between the two middle junctions, the meter reads the difference in the AC voltage at the junctions. If you carefully adjust the resistors, at some point the two resistances will have the same ratio as the caps, and the voltage at the junction of the resistors will be exactly equal to the voltage at the junction of the caps. If you put a meter across the two junctions, the voltages on each end of the meter will be equal, so no current will flow in the meter. Since no current flows the meter cannot load down the capacitive divider and you can measure the two resistors and use simple algebra to calculate the value of the unknown cap.

This is a simple application of bridge measurements. When a bridge is balanced, the measuring meter does not load it. Bridge measurements are very powerful indeed.

Note that there are many difficult issues with doing this practically, but it can be made to work.

[rogeryu_ph]

RG,
Hi, pls give me a simple schematic for the bridge measuring idea, maybe this can be handy to make. I have to test my cables even also my several customized pedal cable.This could or may also help our comrade with your idea.

Thanks
Roger

[Paul Perry (Frostwave)]

I have had some success measuring cable capacitance using a cheap DMM that happened to have a capacitance function built in.
Successful enough to be able to tell which end of the cable had the break in it, anyway!

If I were on a desert island with a bunch of 10 foot cables and a few components, I'd make a 555 oscillator & use the cable as the frequency determining cap.
Then, the cable with the highest pitch wins

[darron]

I have had some success measuring cable capacitance using a cheap DMM that happened to have a capacitance function built in.
Successful enough to be able to tell which end of the cable had the break in it, anyway!

If I were on a desert island with a bunch of 10 foot cables and a few components, I'd make a 555 oscillator & use the cable as the frequency determining cap.
Then, the cable with the highest pitch wins

and use the 9 volt battery from your oscillator on some steel wool to start a fire....

[rogeryu_ph]

Guys,
Sometimes we need these sort of handy circuit tools like impedance tester, current draw measuring, cable capacitance measuring, audio probe tester, transistor germanium Hfe tester, inductance measurment for our  search wah inductor etc is'nt it guys. Hope someday we could buy this just in one package or someone could design the whole in one circuit project that's would be wonderful and call it  DIYERS tester.

Roger.