All capacitors have a certain amount of resistance to the passage of AC current. ESR (Equivalent Series Resistance) is the sum of all internal resistances of a capacitor measured in Ohms. An ideal capacitor has ZERO ohms ESR. From the diagram below you can see a resistor in series with a PERFECT CAPACITOR. The value of the resistor is called ESR. Electrolytic Capacitors have a tendency to increase its ESR overtime due to drying or corrosion. High ESR is a frequent problem in todays electronic circuit. Even a 1 or 2 ohms rise in ESR can cause complicated problems. Under normal conditions the ESR has a very low value, which stays that way for many years unless the rubber seal is defective, in which case the electrolytes water component gradually dries out and the ESR creeps up with time. The increase in ESR increases both voltage drops within the capacitor and the heat, produced in the capacitors due to resistive heating. If you dont check for ESR, you may be in for a TOUGH DOG repair. High ESR is the first sign of a capacitor failure. High ESR will cause complete circuit failure, over heating of capacitor, loading of the circuit, overstressing of other circuit components and other undesirable effects.

Why should I use an ESR?

Because measuring an electrolytic capacitor with an analog or digital capacitance meter can MISLEAD a technician into believing that a defective capacitor is good. This can waste your precious time and you are unable to repair the equipment. Which means you cant charge your customer! If you dont test for ESR on the capacitor, you will always miss the bad capacitor. Normally, these bad capacitors have high ESR reading which your normal multimeter or digital capacitance meter cant measure. Only by using the ESR meter, you will be able to measure the ESR on the capacitor and you will not be fooled by capacitors with bad ESR. ESR meter can even work IN-CIRCUIT, which means you dont have to solder out the capacitor in order to measure it using normal capacitor tester, which would not be accurate anyway.

Do you know that ESR meters can perform other wonders besides checking the ESR of a capacitor? Some digital ESR meter have the features of checking:

Low ohms resistor such as 0.22 ohm, 0.33 ohm and etc.

Horizontal yoke coil winding of a picture tube, inductor and primary winding of a transformer If a shorted turn is present, the inductance drops dramatically and the meter will show the ohms reading.

Horizontal output transistor (HOT) in a monitor or TV If an open-circuit reading is obtained, the short-circuit is elsewhere. If the short-circuit remains, the transistor is faulty.

Short-circuit in the track, usually trace to the components - You can use the ESR meter to locate short-circuits on printed circuit boards by measuring the actual track resistance. If the reading increases as you probe further along the track, you know youre going in the wrong direction!

Speakers, amplifier etc - The pulses have a fast rise/fall time, so it would probably make a crude RF signal injector as well.

The condition of both the normal and rechargeable batteries Flat and faulty battery will have high ESR.