supply rails. Even though the op amps
are rail-to-rail types, they can’t supply
much current at voltages right near
their rails.
Similarly, IC2 has a 33V positive
supply so that the output from IC2a
can deliver a voltage to indicate the
air/fuel ratio at lean values, where the
required voltage is well above 12V.
Negative supply generation
The -3V supply is derived using a
voltage inverter that inverts the +5V
supply, while the +33V supply is from
a voltage tripler that increases the 12V
supply by almost a factor of three.
The -3V supply is generated by transistors Q2 & Q3, diodes D2-D4 and
their associated capacitors. This circuit is driven by a pulse width modulated output of IC1 (PWM3) that delivers a 31.25kHz 5V peak-to-peak square
wave signal. Q2 & Q3 buffer this signal and drive an inverting diode pump
circuit consisting of D2 & D3 and two
10μF capacitors.
The square wave at the emitters of
Q2 and Q3 ranges from about 0.6V
to 4.4V; it is not the full 0-5V swing
due to the base-emitter voltage drop
of each transistor. When Q2 is on, the
10μF capacitor connected to it charges
via diode D2 to ground. The total voltage across the capacitor is 3.8V (4.4V
– 0.6V).
When the PWM3 output goes low
(0V), transistor Q3 switches on, pulling the positive side of the capacitor
to about 0.6V. The opposite side of the
capacitor is pulled negative, causing
diode D3 to conduct and charge the
second 10μF capacitor to a negative
voltage.
This produces a negative supply rail
of around -3V. We don’t get a full -5V
SC6721 Kit ($120 + postage)
Includes the PCB and all the parts
that mount directly on it; the microcontroller comes pre-programmed
(the Bluetooth module is also
included). You need to separately purchase the oxygen sensor, case, wiring,
fuse holder, off-board connectors
(including those for the O2 sensor)
and optional parts like the pressure
sensor and LED display.
because of the transistor and diode
voltage drops. Diode D4 clamps the
negative rail, preventing it from going
positive by +0.6V when the negative
supply generator is not working, such
as when the power is first applied
and IC1 hasn’t started generating the
square wave. Zener diode ZD3 limits the total voltage across IC3 to 15V.
The 15V voltage limit is needed as
the LM6482 has a total supply limit
of 16V. So when the positive supply
is 12V, as supplied by REG2, the negative supply is clamped at -3V. The
alternative recommended IC for IC3
is the OPA2171, which can handle
supply rails up to 36V in total. In that
case, ZD3 could be left out.
33V supply generation
The 33V supply for IC2 is from the
voltage tripler driven from the PWM4
output of IC1 (pin 8). This produces
a 31.25kHz square wave that drives a
buffer comprising transistor Q4, Q5
and diode D5.
When the PWM4 output is low,
transistor Q4 is off, so its collector is
pulled toward the 12V supply via the
1kW resistor. As this point also connects to the base of Q5, Q5 is on and its
emitter is pulled up to around 11.4V.
When the PWM4 output goes high,
Q4 switches on and pulls its collector
(and thus Q5’s base) down to around
0V. This means that Q5 is off, but D5
conducts, so its anode voltage drops to
about 0.3V. Diodes D5-D9 are schottky
types that have lower forward voltages
than standard diodes.
The resulting 11.3V to 0.3V swing
at the emitter of Q4 and anode of D5
drives the voltage tripler circuitry via
diodes D6, D7, D8 and D9 and the
series of 1μF capacitors. ZD2 clamps
the output voltage at 33V.
Microcontroller details
Pin 18 of IC1 is the MCLR reset
input. It’s pulled high via a 10kW resistor and ensures that IC1 is reset on
power up. The MCLR input, the clock
(pin 16), the data line (pin 17) and the
5V and ground supply connect to an
in-circuit serial programming header
(ICSP) to allow IC1 to be programmed.
The header isn’t required if the IC is
already preprogrammed, such as the
one included in our short-form kit.
Link setting
When installed, jumper JP1 ties IC1’s
RC7 (pin 1) input low. This selects a
test mode for checking that the sensor
impedance is correct (300W).
In this mode, the narrowband output produces a value corresponding
to the sensor cell’s impedance. Since
this impedance depends on the sensor
temperature, it’s a good way to check
whether that part of the control circuit
is working and verify that the sensor
is not being overheated by exhaust gas
when installed in a vehicle.
As mentioned earlier, when jumper
JP2 is shorted, the WFMD produces
fixed outputs at lambda = 1 for calibration.
Next month, we will describe the
construction procedure, how to set up
and calibrate the WFMD and install the
sensor in a vehicle’s exhaust system,
as well as how to install and use the
SC
Bluetooth app.
We replaced the narrowband sensor used in a 2000 VW
Caravelle with the Bosch LSU4.9 wideband sensor and
connected the narrowband ‘S’ curve output of the WFMD
to the vehicle’s ECU to simulate a narrowband sensor
signal. The yellow trace is the wideband output and cyan
the narrowband output. It cycles between rich and lean
about once every two seconds because the ECU is adjusting
the fuel injector duty cycle based on the narrowband
output. The wideband signal doesn’t visibly vary much
because it’s only ranging over 0.98 to 1.02 lambda, as
shown in the video at siliconchip.au/Videos/WFMD (taken
from a computer using the Bluetooth interface).
siliconchip.com.au
Australia's electronics magazine
May 2023 79
