bridge circuit composed of Schottky diodes D2-D5, and
through current-sensing resistors R20 and R21 to ground.
The voltage developed across these resistors is fed back to
the inverting input of U3, thus completing the feedback
loop.
Within the diode bridge, the AC is rectified and
passed through the front panel meter, which responds
only to the average (i.e., DC) component. By enclosing the
bridge within the op-amp feedback loop, most of the non-linearities inherent when a bridge is used to drive a
moving coil meter are removed.
Switch SW1 puts R20 in parallel with R21, reducing
the value of the current-sense resistor combination, thus
increasing the sensitivity of the meter. With SW1 closed,
the full scale sensitivity of the ESR meter is one ohm. With
it open, an ESR of five ohms is required to drive the meter
to full scale.
The gain of this stage is set by R17, R18, and R19.
The latter is a 10K ohm trimmer potentiometer used to set
the calibration of the ESR meter after the circuit is built.
If the ESR instrument is powered up with no CUT
connected, R24 limits the average current through the
panel meter to a maximum value of about 2 mA, thereby
making life a bit easier for the meter.
The Power Conversion
Section
In this design, I chose to provide both +5V and -5V
power buses for the op-amps. This simplifies the circuit
design and makes it easier to follow, in my opinion. A
single-supply approach would require the additional
complication of providing a virtual ground reference
throughout the ESR meter. A conventional three-terminal
voltage regulator at the input U5 supplies the +5V bus.
The -5V bus is easily supplied by U4 — a dandy
component from Texas Instruments (TI) that conveniently
puts out a DC voltage equal in magnitude to its input, but
with a reversed polarity.
Construction
I used the services of ExpressPCB
( www.expresspcb.com) to lay out and fabricate the
printed circuit board (PCB) for this project. Their standard
low cost MiniBoard fits very nicely into a 3 x 4 x 5 inch
aluminum enclosure, with plenty of room for a 0-1 mA
meter and two binding posts to be mounted on the front
panel. The PCB (shown in Figure 3) is laid out with J1 (the
external source connector), SW1 (the meter range switch),
and D7 (the power-on LED) along one edge. The PCB is
January 2016 27
■ FIGURE 3. Printed circuit board. ■ FIGURE 4. ESR meter after calibration. The meter is displaying the value of the one ohm test resistor.
■ FIGURE 5. The ESR meter in action, reading the
ESR of an old (date code 1966) 100 µF tantalum
capacitor as 0.3 ohms.
