What is ESR?
What exactly is “ESR”?
Electrolytic capacitors differ from other types. They have two aluminium foil plates rolled into a cylinder, but the dielectric consists of a very thin layer of aluminium oxide formed on the anode plate. One plate in contact with this oxide layer is of
course the anode plate it is formed on. The other “plate” actually consists of a layer of porous material moistened with a solvent-based liquid (the “electrolyte”) which is highly electrically conductive when the capacitor is new. It completes the electrical circuit between the metal negative plate and the oxide dielectric. As you can see, the electrical resistance of the electrolyte is in series with the capacitance of the dielectric, so it is known as the Equivalent Series Resistance, abbreviated to “ESR”.
Why increased ESR is a problem.
Electrolytic capacitors have the job of blocking the flow of DC current in a circuit while presenting a low impedance to AC, also known as “Ripple Current”. Especially when an electrolytic capacitor is exposed to high temperatures because it is close
to heat-generating components or has a large ripple current flowing through it, the electrolyte can dry out or be chemically altered, making its electrical resistance, the ESR, increase. This has exactly the same effect as putting an external resistor in series with a good capacitor: AC current flowing through the capacitor will cause a higher than normal AC ripple voltage to appear across the capacitor. This will eventually lead to abnormal operation of the circuit the capacitor is in. Faults caused by increased electrolytic capacitor ESR are often quite strange. ESR increases rapidly as the temperature drops, and this type of problem often shows up in winter and before the equipment has reached normal operating temperature. Attempts to identify defective electrolytic capacitors by using a capacitance meter are usually unsuccessful. These kind of meters are designed to ignore the ESR which is causing the fault, to measure only the capacitance which hasn’t really changed. They will produce a low reading only if the capacitor is open circuit or close to it. Also, they usually can’t measure a capacitor while it’s in circuit. This meter has been designed to measure the ESR of capacitors in circuit, made possible by keeping the test signal voltage low enough
(<100mV P-P) not to cause any semiconductors in the circuit to conduct. As well as saving time, the capacitors won’t be
heated by unsoldering them, causing the ESR to fall and possibly hiding their real condition.
What are typical ESR readings?
Unlike other electrical characteristics, there is no such thing as a “normal” ESR value for an electrolytic capacitor of a given capacitance and operating voltage. The ESR to some extent depends on the physical size of the capacitor. It also varies between manufacturers, and whether it a low-ESR or high temperature-rated type. The chart on the front of the meter contains
sample ESR values for a range of common electrolytic capacitor values and voltage ratings. These have been taken from a capacitor manufacturer’s data sheets and a collection of actual capacitors. It is provided only as a rough guide for new
users, to give an idea of what readings to expect until they become familiar with using the ESR meter in actual repair situations.
Defective capacitors stand out clearly!
Electrolytic capacitors differ from other types. They have two aluminium foil plates rolled into a cylinder, but the dielectric consists of a very thin layer of aluminium oxide formed on the anode plate. One plate in contact with this oxide layer is of
course the anode plate it is formed on. The other “plate” actually consists of a layer of porous material moistened with a solvent-based liquid (the “electrolyte”) which is highly electrically conductive when the capacitor is new. It completes the electrical circuit between the metal negative plate and the oxide dielectric. As you can see, the electrical resistance of the electrolyte is in series with the capacitance of the dielectric, so it is known as the Equivalent Series Resistance, abbreviated to “ESR”.
Why increased ESR is a problem.
Electrolytic capacitors have the job of blocking the flow of DC current in a circuit while presenting a low impedance to AC, also known as “Ripple Current”. Especially when an electrolytic capacitor is exposed to high temperatures because it is close
to heat-generating components or has a large ripple current flowing through it, the electrolyte can dry out or be chemically altered, making its electrical resistance, the ESR, increase. This has exactly the same effect as putting an external resistor in series with a good capacitor: AC current flowing through the capacitor will cause a higher than normal AC ripple voltage to appear across the capacitor. This will eventually lead to abnormal operation of the circuit the capacitor is in. Faults caused by increased electrolytic capacitor ESR are often quite strange. ESR increases rapidly as the temperature drops, and this type of problem often shows up in winter and before the equipment has reached normal operating temperature. Attempts to identify defective electrolytic capacitors by using a capacitance meter are usually unsuccessful. These kind of meters are designed to ignore the ESR which is causing the fault, to measure only the capacitance which hasn’t really changed. They will produce a low reading only if the capacitor is open circuit or close to it. Also, they usually can’t measure a capacitor while it’s in circuit. This meter has been designed to measure the ESR of capacitors in circuit, made possible by keeping the test signal voltage low enough
(<100mV P-P) not to cause any semiconductors in the circuit to conduct. As well as saving time, the capacitors won’t be
heated by unsoldering them, causing the ESR to fall and possibly hiding their real condition.
What are typical ESR readings?
Unlike other electrical characteristics, there is no such thing as a “normal” ESR value for an electrolytic capacitor of a given capacitance and operating voltage. The ESR to some extent depends on the physical size of the capacitor. It also varies between manufacturers, and whether it a low-ESR or high temperature-rated type. The chart on the front of the meter contains
sample ESR values for a range of common electrolytic capacitor values and voltage ratings. These have been taken from a capacitor manufacturer’s data sheets and a collection of actual capacitors. It is provided only as a rough guide for new
users, to give an idea of what readings to expect until they become familiar with using the ESR meter in actual repair situations.
Defective capacitors stand out clearly!
Years of experience have shown that in almost every case, a capacitor’s ESR needs to rise to at least 10 times its normal value to cause a circuit malfunction. Very often it will be found to have risen to >30 times, or even so high that the meter will not give a reading (>99 W). It is for this reason that the front panel figures don’t need to be extremely accurate or complete. When you encounter a capacitor whose value or voltage is missing from the chart, it’s sufficient to assume that its ESR should be similar to that of a capacitor adjacent to it on the chart. If you have any doubts, it’s best to compare the meter’s reading on a suspect capacitor with that of a new capacitor of the same value and voltage rating.
publicada por vitor manuel às
09:18
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