my supervisor ( a master electrician ) was checking a capacitor for an A/C unit. The capacior is rated in uf’s and he wanted to convert it to ohms. I didn’t think it was possible but he seems to think there is a formula to convert microfarads to ohms.

The capacitance value of a capacitor in Farads and it`s opposition to current flow in ohms are related to each other but they cannot be converted directly from one unit to another because they are the basic units of two different things.

The capacitive reactance of a capacitor in ohms (X of c) is given by (X of c) = 1 / (2 pi fc).

For example the reactance value in ohms of a .05uF capacitor operating at 500Hz is the same as the reactance value in ohms of a .025uF capacitor operating at 1000 Hz. From this you can see that a capacitor of a given Farrad value can not be assigned a unique ohmic value.

A capacitor such as the one in the air conditioner unit does not normally change enough in capacitance value to effect it`s function in the circuit. Normally when one fails it will first short out and then burn itself open. In either case your supervisor could detect this with an ohm meter. A gradually increasing high reading on the ohm meter indicates the capacitor is probably still good and of course a zero ohms reading indicates the cap is shorted. No defection on the ohmeter indicates the capacitor is open and is of no value in the circuit.

Ohms Conversion

From your question, it sounds like your supvr was wanting to check the start or run capacitor with his ohmmeter.

In the case of checking a capacitor with an ohmmeter, one watches for the meter to swing from 0 ohms when first connected to a very high reading after the capacitor has had time to charge.

When the leads are reversed, the meter should swing (if a dial type) violently to 0 then climb to a very high reading. A digital meter would likely read – ohms or possibly -voltage when the leads are reversed after the first check.

If he wants capacitive reactance, the formula is Xc=2pi*(f*C)

There is no direct conversion for µF to Ω. The two are expressions of different qualities.

You are right, maybe you should be supervising him.

Microfarads are a unit of capacitance and ohms are a unit of resistance. A capacitor has a reactance (= AC resistance) which is measured in ohms. It depends on the frequency of the AC and is given by:

Z = 1 / (2 x PI x f x C)

where Z is reactance in ohms, f is frequency in Hz and C is capacitance in farads.

He was clearly trying to find the reactance of the capacitor, not like converting inches to centimeters. So, Xc (reactance of a capacitor) = 2 x pi x frequency x capacitor value in farads

Answer 6

Inductive and capacitive reactances can be expressed in OHMS. Note these are reactive ohms, not resistive ohms.:

Capacitive reactance (in ohms):

XC = 1 / (2 * pi * f * C), where f is the frequency and C is the capacitance in Farads

Inductive reactance (in ohms):

XL = 2 * pi * f * L, where L is the inductance in Henrys

Answer 7

Maybe he was after the impedance. In that case:

Z = 1/jwC

Where j is the imaginary unit (sqrt(-1), you can kind of ignore this if you just want the number),

w is the angular frequency (or w = 2πf, where f is the frequency, e.g. 60Hz),

and C is the capacitance.

So for example, if the circuit was operating at 60Hz, the impedance would be (assuming around 1 uF):

Z = 1/j(2π*60)(1uF) ~ 2600 Ohms

You’re right, you’re talking apples and oranges here. Your supervisor is either testing you, or he knows “jack-squat.”

micro-farads is a unit of “capacitance” (C)

ohms is a unit of “resistance” (R)

You simply cannot convert one to the other.

Good Luck with this quack!!!

Farad Conversion

If what you are talking about is a motor start/ run capacitor, my guess is he is after the reactance, as its value will determine what current flows in the windings.

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