A capacitor is a two terminal electrical component used to store electrical energy temporarily in an electric field. The types of practical capacitors vary widely, but all contain at least two electrical conductors (plates) separated by a di-electric. The conductors can be in the form of thin film, foils or sintered beads of metal or conductive electrolyte etc.
The non conducting dielectric helpd in increasing the capacitors charge capacity. A dielectric can be glass, ceramic, plastic films, air, vaccum, paper, mica, oxide layer etc.
Capacitance C = Q(charge)/V(voltage)
Why capacitor induces A.C. but not D.C.
- The capacitive reactance (Xc) is given by
Xc = 1/(2πfc)
Where f = frequency
From basic principle it’s clear that for DC, the frequency is zero. Therefore the capacitor offers infinite resistance to DC and allows AC to pass freely because AC has a fixed or defined frequency.
- The capacitor plates are separated by a dielectric and hence when a DC supply is increased across it the plate connected across the +ve terminal of the battery and that connected across the –ve terminal of the battery is charged to the potential of the battery and hence it starts blocking the charge entering it from a DC source. Where as from an AC source, the voltage changes continuously in magnitude and direction and hence the capacitor plate does not get a fixed charge to block any of the voltage and current that is alternating in nature, hence capacitor blocks only DC and acts as a normal conducting element for AC supply.
- By default, current can’t cross the dielectric, so that’s simply why DC can’t pass, but even in AC the current doesn’t actually pass through the capacitor (because of dielectric).
- As AC periodically reverses the direction of current flow, as indicated by the sine wave.
- In the +ve cycle , the plate 1 (say) depletes some electron when its opposite plate (plate 2) accumulates electrons because of the polarity of that half cycle.
- Now when –ve half cycle appears, a polarity reversal happens so the plate 1 will accumulate electrons and plate 2 will consequently rid itself of electrons. This will continue for every cycle and this accumulation and depletion cycle is what we see as “current flow”.
In case of DC, the plate connected to the +ve terminal will deplete an equal amount of electrons, and that places a +ve charge on that plate. So actually there is no crossing over. Just a depletion in one plate as reaction to the accumulation in the other. Since there is no gain of charge, the NET charge is zero.
How to test Capacitor
Test a capacitor with an ohm-meter or a multimeter
- By taking the capacitors resistance, we can decide whether the capacitor is good or bad.
- To do this, we take the ohmmeter and place the probes across the leads of the capacitor. The orientation doesn’t matter because resistance isn’t polarized.
- If we read a very low resistance (near 0 ohm) across the capacitor the capacitor is defective. Its is reading as if there is a short across it.
- If we read a very high resistance across the capacitor, this is a sign that the capacitor likely defective as well. It is reading as if there is on open across the capacitor.
- A normal capacitor would have a resistance reading up some where in between these 2 extremes say, anywhere in the tens of thousands or hundreds of thousands of ohms.
- This is a simple and effective method for finding out if a capacitor is defective or not.
Test a capacitor with the help of multimeter in the capacitance setting
Another check we can do is check the capacitance with a multimeter, if you have a capacitance meter on your multimeter.
You have to read the capacitance that is on the exterior of the capacitor and take the multimeter probes and place them on the leads of the capacitor. Polarity doesn’t matter.
This is same as before, only now the multimeter is set to the capacitance setting. Value should be near the capacitance rating of the capacitor. Due to tolerance and the fact that may dry up, the reading obtained may be a little less than the capacitance of the rating. However, if you read a significantly lower capacitance or none at all, then surely the capacitor is defective.
Checking the capacitance of a capacitor is a great test for finding whether a capacitor is good or not.
Test a capacitor with voltmeter
- Capacitors are energy storing devices. They store a potential difference of charges across their plate, which are voltages. The anode has a +ve voltage and cathode has negative voltage.
- A test that we can do is to see if a capacitor is working as normal is to charge it up with a voltage and then read the voltage across the terminals.
- If the voltage that we charged is to, then the capacitor is doing its job and can retain voltage across its terminals. If it is not charging up and reading voltage, this is sign the capacitor is defective.
- To charge the capacitor with voltage, supply DC voltage to the capacitor leads. Now polarity is very important for polarized capacitors.
- If we are working on a polarized capacitor, then you must observe polarity and the correct lead assignment. Positive voltages goes to the anode (the longer lead) of the capacitor and –ve or ground goes to the cathode (the shorter lead) of the capacitor. Supply a voltage which is less than the voltage rating of the capacitor for a few seconds.