Caps in Ohms?

[CuttingJesse]

I know a guy who owns a music store and they used to do TV and radio repairs in the 70s. They have a drawer FULL of capacitors. But they have them all organized in terms of ohms, which I've never heard of because they don't know the exact voltage? So what would a 0.022uF cap be in ohms if anyone has a clue?

[dynabite]

Red-Red-Orange Bumblebee. Voltage doesn't matter.

[guitarEd]

if I remeber correct from my electronics classes, way back when, you cant measure capacitors with an ohm meter. you would get infinate ohms....
a capacitor is 2 metal plates separated by a dielectric, or something like that.
there is no physical contact of the plates inside. It is kind of like a quick charge battery of sorts....
could be wrong, it has been a while.
you need to sort through them to find .022uf printed on the cap...

ed

[CuttingJesse]

Originally Posted by guitarEd

if I remeber correct from my electronics classes, way back when, you cant measure capacitors with an ohm meter. you would get infinate ohms....
a capacitor is 2 metal plates separated by a dielectric, or something like that.
there is no physical contact of the plates inside. It is kind of like a quick charge battery of sorts....
could be wrong, it has been a while.
you need to sort through them to find .022uf printed on the cap...

ed

Yeah I'm taking electrical engineering in college right now Thats why I was confused by this way of ordering. The formula in the Impedance which is measued in ohms for caps is 1/(2*pi*frequency*capacitance) which makes no sense in a guitar because the frequency is always changing.

[guitarEd]

you are correct jesse, impedence is the resistance of an ac circuit.
if I remember, a cap in a circut,(sp) like a guitar, which is producing an ac current from the pickups, is used, along with the pots, which are variable resistors, is just a filter circiut, for filtering out certain frequencies.
but then again, you probably know all that...lol

There must be a standard frequency used to determine the farads of the capacitor, along with the formula. Your college instructor may be of help here.

but it is most likely easier, go to radio shack, or order online, stew mac, warmoth etc, all have caps.
Some guys it seems like to use the big ol' caps from old tvs and such, but remember, caps deteriorate over time and change values or dont work at all.
More so the old caps that have had a lot of high voltage run through them...

ed

ps, good luck with your college coarse, it is a rewarding area to get into...

[dynabite]

The tolerances on older caps can be quite off, I suggest one of these.

[stadams]

My hope would be that the units were mislabeled on his storage bins.

Originally Posted by CuttingJesse

Yeah I'm taking electrical engineering in college right now Thats why I was confused by this way of ordering. The formula in the Impedance which is measued in ohms for caps is 1/(2*pi*frequency*capacitance) which makes no sense in a guitar because the frequency is always changing.

Actually, you can only call this impedance if you neglect the Equivalent Series Resistance (ESR), the Real resistance of the capacitor which is typically comprised of lead resistance and is also a product of capacitor geometry and composition. Impedance is a vector implying magnitude and direction hence when seen in rectangular form is written Z = R + jXc. When calculating using 1/(2*pi*frequency*capacitance) you are only calculating the imaginary quantity.

It does make sense in a guitar. As the frequency varies so does the reactance of the capacitor. As can be seen from the formula you provided, a capacitor's reactance which is the majority of its impedance is inversely proportional to frequency therefore, the lower the frequency the more the capacitor resists the current passing through it. The higher the frequency, the more readily current flows. So when applied in a circuit in different ways, it can act as a low-pass or high-pass filter.

Check out this link:

A couple of typical pickup simulations.

It's just a few electrical characterizations of Les Paul pickup/volume/tone circuits that I was analyzing.

Originally Posted by guitarEd

you are correct jesse, impedence is the resistance of an ac circuit.
if I remember, a cap in a circut,(sp) like a guitar, which is producing an ac current from the pickups, is used, along with the pots, which are variable resistors, is just a filter circiut, for filtering out certain frequencies.
but then again, you probably know all that...lol

There must be a standard frequency used to determine the farads of the capacitor, along with the formula. Your college instructor may be of help here.

but it is most likely easier, go to radio shack, or order online, stew mac, warmoth etc, all have caps.
Some guys it seems like to use the big ol' caps from old tvs and such, but remember, caps deteriorate over time and change values or dont work at all.
More so the old caps that have had a lot of high voltage run through them...

ed

ps, good luck with your college coarse, it is a rewarding area to get into...

Capacitance is charge versus potential. This measurement is independent of frequency. When using an LCR bridge or a capacitance meter as suggested by dynabite, the selection of frequencies is usually used for determining the ESR of capacitor as different capacitors exhibit their maximum ESR's at higher or lower frequencies.

I very much agree with vintage caps being suspect. After years of usage these caps typically dry out, change capacitance and/or tolerance, and become physically brittle and difficult to work with as in the picture below:



Broken leads on expensive caps.

Jesse, keep up with the studies, it will pay off.

Later,

[Armitage]

Capacitors also break down faster when NOT in use, then when in use.