ESR Project Build - Part 4 - Precision Rectifier + Meter

We've so far looked at the ~100 kHz oscillator, driver and gain stages. After the gain stage comes a precision rectifier and driver, which looks like this (Figure 1):

Figure 1. The Precision Rectifier and meter driver

I decided to breadboard this portion since I am substituting the LM358 for the AD8032, which the W2AEW design called for. The AD8032 is a bit pricey and harder to obtain (Mouser price for DIP was $8.11 US in single quantities). The AD8032 is a rail-to-rail opamp but this circuit only requires the bottom rail to function correctly.

The LM358 is a jelly bean part and operates at the V- rail (ground in this case). This part is targeted as a single supply solution. As a bonus, it uses the same pinout as the AD8032. The one other deviation from the W2AEW design is that my selected meter has a 100 uA movement instead of 200 uA. So instead of the 4.7k + 2.2k with 10k adjustment pot, I simply ran the output signal into a 100k trimpot (for this breadboard test), with the other leg of the pot going to ground. Then the meter went to the wiper arm of the pot with its negative lead going to ground. The full deflection is adjusted with the DUT leads shorted. In the final soldered up build, I'll arrange a protective limiting resistor for the meter in case the pot gets cranked.

The signal coming out of the LM358 precision rectifier and going into the driver (pin 5) appears as shown in Figure 2:

Figure 2. Input signal at LM385 pin 5

It is interesting to see that the signal actually does dip below ground by -590 mV. The positive swing of the signal goes up to 650 mV. This signal is generated when the DUT leads are shorted (zero ohms).

With the 1N4148 diode and the 0.1 uF filter cap connected, a DC voltage is established, which is then delivered to the meter. To calibrate, short the DUT leads and adjust the pot to give the meter full deflection like the old fashioned ohm meters.

Early testing has shown that low ESR values show very close to the shorted leads point on my meter. Any resistance over an ohm shows as a reading near the 10% deflection point. So presently, I found this meter to be extremely ESR sensitive.

Once you have a hammer, everything looks like a nail. So I went around the lab looking for caps to measure. All but one measured 100%. This failed cap was pulled out of an '80s power supply along with three others just like it. I had been debating whether or not to use these or to replace them in a power supply project. One of the four showed no deflection at all, while the remaining three all showed 100% good.

I also in-circuit tested this circuit on Intel Atom motherboards with caps and measured a few other boards. All indications look very good so far. Now I just need to find some time to solder this up in its final form.

In the next part, I'll present some pictures of my build into an old Sencore transistor tester case.