ESR Meter is not a new thing. Some are commercial and many DIY project too in the Internet. I have ever heard my friend said "If electronics equipment still have bad caps, there is still many technician.
I think ESR Meter really useful when you are troubleshooting some faulty electronics equipment, you can't tell just from capacitor looks good, because capacitor couldn't judge just by capacitance value, they have gone high in their "inner" resistor and make your electronics equipment failed.

As Wikipedia said ESR Meter is 
"An ESR meter is a two-terminal electronic measuring instrument designed and used primarily to measure the equivalent series resistance (ESR) of real capacitors; usually without the need to disconnect the capacitor from the circuit it is connected to. Most ESR meters work by applying voltage pulses to the capacitor under test which are too short to appreciably charge it; any voltage appearing across the capacitor is due to ohmic drop across the ESR. Measuring ESR can also be done by applying an alternating voltage at a frequency at which the capacitor's reactance is negligible, in a voltage divider configuration.
Other types of meter, including normal capacitance meters, cannot be used to measure a capacitor's ESR, although a few combined meters are available which measure both ESR and out-of-circuit capacitance. A standard (DC) milliohmmeter cannot be used to measure ESR, because a steady direct current cannot be passed through the capacitor."

ESR or Equivalent Series Resistance of real capacitors. Resistance??
Yup !! capacitor have a resistance, identacally ESR like as resistor that attached or added to capacitor capacitance. Below picture is the analogy example.



I take from a Wikipedia quote :
"Other types of meter, including normal capacitance meters, cannot be used to measure a capacitor's ESR, although a few combined meters are available which measure both ESR and out-of-circuit capacitance. A standard (DC) milliohmmeter cannot be used to measure ESR, because a steady direct current cannot be passed through the capacitor."
So it means we as repairer needs this tool after AVO meter or digital meter. I want to share my experience about building this tool. Actually I get the schematic from another website.
So, I prepare the components to make this ESR Meter from here
ESR METER: Equivalent Series Resistance Capacitor Tester
Finally, my ESR Meter was done and I can't wait to test it. I found my wall wart adapter power supply have failed because one of capacitor gone high in ESR value, regardless no sign of bulging.  I replace the new capacitor and my wall wart adapter brought back to live. I have very happy had this tools on my repair bench because it's very useful. I conclude this ESR Meter really needed by technician to search for bad capacitor.

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