The post discusses a simple ESR meter circuit which can be used for identifying bad capacitors in an electronic circuit without removing them practically from the circuit board. The idea was requested by Manual Sofian
Do you have a schematic about ESR meter. Technicians recommend me to check the electrolytic first every time I come up with a dead circuit, But I don't know how to measure it.
Thank you in advance for your answer.
What is ESR
ESR which stands for Equivalent Series Resistance is a negligibly small resistance value that normally becomes a part of all capacitors and inductors and appear in series with their actual unit values, however in electrolytic capacitors especially, due to aging, the ESR value could go on increasing to abnormal levels adversely affecting the overall quality and response of the involved circuit.
The developing ESR in a particular capacitor may gradually increase from as low as a few milliohms to as high as 10 ohms, affecting the circuit response severely.
However the above explained ESR may not necessarily mean that the capacitor's capacitance would also be affected, in fact the capacitance value could remain intact and good, yet have the capacitor's performance deteriorating.
It is due to this scenario a normal capacitance meter entirely fails to detect a bad capacitor affected with high ESR value and a technician finds the capacitors to be OK in terms of its capacitance value which in turn makes troubleshooting extremely difficult.
Where normal capacitance meters and Ohm meters become totally ineffective in measuring or detecting abnormal ESR in faulty capacitors, an ESR meter becomes extremely handy for identifying such misleading devices.
UPDATING a Simpler Alternative
The op amp based circuit given below looks complex, no doubt, therefore after some thinking I could come up with this simple idea for assessing the ESR of any capacitor quickly.
However for this you will have to first calculate the how much resistance the particular capacitor possesses ideally, using the following formula:
Xc = 1/2(pi) fC
- where Xc = reactance (resistance in Ohms),
- pi = 22/7
- f = frequency (take 100 Hz for this application)
- C = capacitor value in Farads
The Xc value will give you the equivalent resistance (ideal value) of the capacitor.
Once you find the ideal internal resistance of the capacitor, you can then use the following practical circuit to compare the result with the above calculated value.
For this you will need the following materials:
- 0-12V/220V transformer
- 4 diodes 1N4007
- 0-1 amp FSD moving coil meter, or any standard ammeter
The above circuit will provide a direct reading regarding how much current the capacitor is able to deliver through it.
To derive the internal resistance value, we can now apply Ohm's law formula as shown below:
R = V/I, where will be 12 V, and current as indicated on the meter.
Once you get the resistance value through the practical method, you can compare it with the calculated method. The difference will be the effective ESR of the capacitor.
Obtaining Ideal Value of a Capacitor Quickly
In the above example if you don't wish to go through the calculations, you can use the following benchmark value for getting the ideal reactance of a capacitor, for the comparison.
As per the formula, the ideal reactance of a 1 uF capacitor is around 1600 Ohms at 100 Hz. We can take this value as the yardstick, and evaluate the value of any desired capacitor through a simple inverse cross multiplication as shown below.
Suppose we want to get the ideal value of a 10uF capacitor, quite simply it would be:
1/10 = x/1600
x = 1600/10 = 160 ohms
Now we can compare this result, with the result obtained by solving the ammeter current in Ohms law. The difference will tell us regarding the effective ESR of the capacitor.
NOTE: The voltage and the frequency used in the formula and the practical method must be identical.
Using an Op Amp for Making a Simple ESR Meter
An ESR meter can be used to determine the health of a doubtful capacitor while troubleshooting an old electronic circuit or unit.
Moreover the good thing about these measuring instruments is that it can be used to measure the ESR of a capacitor without the need of removing or isolating the capacitor from the circuit board making things pretty easy for the user.
The following figure shows a simple ESR meter circuit which can be built and used for the proposed measurements.
How it Works
The circuit may be understood in the following manner:
TR1 along with the attached NPN transistor forms a simple feed back triggered blocking oscillator which oscillates at some very high frequency.
The oscillations induce a proportionate magnitude of voltage across the 5 turns secondary of the transformer, and this induced high frequency voltage is applied across the capacitor in question.
An opamp can also be seen attached with the above low voltage high frequency feed and is configured as a current amplifier.
With no ESR or in case of a new good capacitor the meter is set to indicate a full scale deflection indicating a minimum ESR across the capacitor which proportionately comes down toward zero for different capacitors having different amounts of ESR levels.
Lower ESR causes relatively higher current to develop across the inverting sensing input of the opamp which is correspondingly displayed in the meter with a higher degree of deflection and vice versa.
The upper BC547 transistor is introduced as a common collector voltage regulator stage in order to operate the oscillator stage with a lower 1.5 V so that the other electronic device in the circuit board around the capacitor under test is kept under zero stress from the test frequency from the ESR meter.
The calibration process of the meter is easy. Keeping the test leads shorted together the 100k preset near the uA meter is adjusted until a full scale deflection is achieved on the meter dial.
After this, different capacitors with high ESR values could be verified in the meter with correspondingly lower degrees of deflection as explained in the previous section of this article.
The transformer is built over any ferrite ring, using any thin magnet wire with the shown number of turns.