Midwest Devices LLC
16657 East 23rd St. Suite 353
Independence Mo 64055
How it Works

What is ESR? ESR ESR is the abbreviation for “Equivalent Series Resistance”, sometimes referred to as “Effective Series Resistance”. ESR is a DYNAMIC (AC) property of a capacitor and cannot be measured with a DC ohm meter. ESR can be considered to be the total series NON-REACTIVE AC resistance of a capacitor. ESR includes the DC resistance of the leads, DC resistance of the connections to the dielectric, capacitor plate resistance, and the IN-PHASE AC resistance of the dielectric material. The combination of components that make up ESR are symbolized by a resistor in series with a capacitor as shown. The series resistor in the symbolic representation of “ESR” does not exist as a physical entity. Direct measurements across the ESR resistor are not possible! The Capacitor Wizard® overcomes this limitation by measuring only the NON-REACTIVE AC resistance(ESR).


ESR: A Phenomenon or a component Property? A property of a component is static and exists at all times. A phenomenon in Physics is an observable event that appears only in the presence of the observer under certain conditions. Since the representative ESR resistor doesn't really exist could ESR be considered a phenomenon? Just something to think about when you have nothing else to do! We will refer to ESR as a capacitor "property".


Why check ESR? An increase in ESR is the first indicator of a bad capacitor. As a capacitor fails its’ ESR increases until ESR reaches infinity, total failure. By measuring ESR one can locate failing capacitors before they quit completely. Often a capacitor will measure GOOD with a capacitance reading meter yet it will actually be BAD due to increased ESR! Todays' modern circuits are designed with low ESR parts. Checking ESR guarantees a reliable test of a capacitors health.


A capacitor checks good but by substitution I know it's bad. Why? Every repair shop has run across this problem: Caps that are known to be bad yet present correct capacitance! These are likely defective due to high ESR. Capacitance meters can not read ESR. They only measure capacitance. Capacitance is important but it is not the only important capacitor property. ESR is probably more important. A BAD capacitor can have the correct capacitance yet have HIGH ESR. High ESR upsets time constants, causes part heating, disrupts current flow. Increased ESR frequently causes complete circuit failure, even when a capacitor measures correct capacitance. Without the ESR test you may accidentally declare a BAD capacitor GOOD. A mistake like that can add hours of troubleshooting time to what would otherwise be a simple repair. Don't be fooled by a standard capacitance meter!! Use the Capacitor Wizard® In-Cicuit ESR Meter.


How does the Capacitor Wizard® ignore other in-circuit parts? There are four EXTERNAL parallel parts that could interfere with in-circuit ESR measurements: inductors, resistors, solid state junctions, capacitors. Parallel inductance: The test frequency is high enough ( 100khz ) to force expected inductive reactance (power transformers, coils) many orders of magnitude higher than the ESR scale. Parallel Resistors: Resistors found around capacitors 1uf and up will be large values compared to the Capacitor Wizard®'s low input impedance (2.5ohm). Parallel resistors have little effect on the reading and can be ignored. Solid State Junctions: The test voltage is less than 20 mv pp, too low to turn on any GOOD solid state devices. (You can use the Capacitor Wizard® to find leaky or shorted diodes, transistors IN-CIRCUIT!) Parallel Capacitors: Occasionally will you encounter a circuit design using parallel capacitors. One leg of a parallel cap must be lifted. Both caps can then be individually tested.


Why must I discharge Capacitors? The input resistance of the Capacitor Wizard® is held very low by two one ohm resistors (R51 & R52). If you would measure DC resistance between the Capacitor Wizard® probes you would measure 2.5ohms. This low impedance insures a proper ESR measurement discarding transients. If you attempt to test a charged cap these resistors will often just discharge the capacitor, no damage done. However if the unit under test is turned on or if the capacitor has a lot of stored energy you will damage one or both of these resistors. You may also damage other discrete parts. ALL PARTS NEEDED TO RECOVER FROM DAMAGE DUE TO A CHARGED CAP ARE INCLUDED IN OUR INEXPENSIVE "REPAIR KIT". Find it on our Websites' PARTS Page.


What happens if I accidentally test a CHARGED CAP? Often no damage is done. If you do damage the meter never fear, we have an inexpensive repair kit. Just replace all parts in the kit and you will be back in operation in no time. If you want to try to troubleshoot it yourself download my repair procedure.


How do I know my Wizard is working? Measure some low value resistors (1ohm, 2ohm 3ohm etc). Verify agreement with the Capacitor Wizard®. If the Capacitor Wizard® closely agrees then it is working properly and in calibration.


Can I protect my Capacitor Wizard®? Yes, we have an add-on optional protection kit, the CapSVR. We have sold the Capacitor Wizard® for over 20 years and protection has not been an issue. Most technicians know to discharge caps before testing. The Capacitor Wizard® will absorb a pretty good hit without protection. The CapSVR provides the ultimate protection for the absent minded tech.

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