ESR Project Build - Part 1 - Designs

I wasn't planning to blog this project, but it has turned into quite a bit fun, even though at the time, it seemed to be small project. As is so often the case, even the small projects provide challenges.

Why?

Many things today stop working due to switching power supply problems. Let's face it, switching supplies work hard at converting power and the capacitors involved usually fail at some point. Monitors and TVs etc. can often be fixed by just replacing the faulty caps.

But identifying the failed caps is not always easy. Many times you have a bulged or other obviously marked cap. But at other times, you may have parts that are border line, which are difficult to identify. 

No one wants to remove caps to test them on an ESR meter. So when I went hunting for a design, I wanted the following:

1. Must work in-circuit
2. Be relatively simple to build

So in my research, I came across the following two designs:

• VE7IT ESR Capacitor Tester
• W2AEW ESR Meter

I believe the W2AEW design is based upon the VE7IT design. 

Designs

The VE7IT tester produces a small 156 kHz signal of about 250 mVpp, to stay under the silicon diode conduction voltage of about 0.6V. While a Schottky diode conducts between 150 to 450 mV, am hoping that this doesn't apply often or interfere too much when it does. The VE7IT design says the cap being tested must be greater than 1 uF. Fair enough.

The signal returned from the DUT pair of probes in the VE7IT design, is then AC amplified by a 2N2222 stage and rectified by a bridge rectifier.

After the rectification, the circuit drives a 50 uA meter with a potentiometer for zero adjustment.

A note on the VE7IT schematic indicates that:

"There is no DC output until approx 75-100 ohms of ESR is seen at the test terminals (like a bad cap). Mid scale is approx 10 ohms. Full scale is 0 ohms."

As a rough measurement this is ok I guess, but I wanted to be able to measure down to the ohm if possible.

The W2AEW circuit substitutes a precision rectifier instead, so that greater precision can be had:

This should permit a finer resolution of readings from zero to 75 ohms that the VE7IT design is unable to read.

The other aspect of this design that I liked is that due to the opamps, it is no longer confined to using a 50 uA meter movement.

In the next part, I'll describe my own experiences with the build of the W2AEW circuit starting with the 10 kHz oscillator.