Oz DIY-WB Schematics (20 Dec. 2001)

Here are notes to understand how the Oz DIY-WB works. It also servers as a reference for when you are constructing the PCB. In general you should install the components in the order they are described so that tests can be performed along the way.

Power Connector & Low Current Power Supply

The power connector CON2 has pins numbered 2, 1 from left to right when viewed with the connector to the bottom of the PCB. The left pin (2) is ground (GND) for the whole circuit. The right pin (1) is the battery (VBatt). Vbatt goes to the transient suppressor D7 and is filtered by C12 (25 uF) and then regulated to 8.0 Volts by U4 (78L08). Capacitor C13 (10 uF) ensures the regulator is stable. Also shown is the voltage divider R28/R29 which is an input to U2B described later.

Note, U4's silk screened outline is rotated 180° to its correct orientation. When these components are installed and power and ground connected to CON2, the Vcc voltage of 8.0 Volts should be measured at pin 14 of the U3 (designated as U3p14) socket (this is beside the R7 label).

Virtual Ground

U2B, a section of the LM324 quad op-amp is configured as a buffered voltage divider to produce a voltage of 4.0 above GND (using half Vcc set by R28/R29). If we think of the 4.0 Volts as a virtual ground, then the real ground is at -4.0 Volts, and Vcc is at +4.0 Volts around the virtual ground potential. R33 limits the VGnd current if there is a short anywhere, and C14 filters the regulated voltage. The Virtual ground (VGnd) is connected to the sensor's Vs/Ip common point and to a few other circuits points to control the Ip feedback loop and to switch off Ip during heater warm-up. Measurement at U2p7 should be 4.0 Volts.

Heater Drive & Temperature Sense

Although more complex, this circuit can be broken down to a number of functional blocks.

The voltage regulator (either an LM317, or a low drop-out 1086) must be used with a large enough heat sink that the device does not shut down through thermal overload. The 1 ohm 5W resistor may also be mounted on a heat sink for better thermal stability. The optional shunt resistor R40 may be selected to provide faster heatup (max heater current is 1.45 Amps)

The 1 Ohm resistor determines the current limit through the regulator. The LM317 maintains a 1.25 Volt potential between the Vout and Adj terminals and a 1 Ohm resistor develops 1.25 Volts at 1.25 Amps. By padding R41 with a larger value resistor (say 5.6 to 10 ohms), the NTK's current limit of 1.45 Amp can ge precisely set (6.8 ohm in parallel with 10 ohms gives a calculated current limit of 1.43 Amps). R35, the 1 k resistor in conjunction with the othe circuit components can further limit the heater current.

The LM431 limits the votage across the heater to about 10.4 Volts and also enables the opto-isolator U5 (and thus the Ip drive circuitry). The LM431 is an "adjustable" zener and turns on sharply when its Ref input exceeds 2.495 Volts. R38 and R39 (4.7/1.5 k) set this voltage at 10.31 (= 2.5 x(4.7 + 1.5)/1.5). When this voltage is reached, the LM431 start to conduct and pulls the junction of D5/D6, which was sitting at Vbatt (biased by R37), to ground. This action turns ON Q3, lights the LED, and activates the opto-isolator U5 which in turn activate the Ip drive circuitry.

Ip (Pump Current) Drive & and Ip Off

The sensor's pump cell requires a small (less than 10 mA) positive or negative current to provide the oxygen source or sink capability that the sensor's operation relies upon. The Ip drive circuit is also switched off until the heater circuit determines it is up to operating temperature.

Transistors Q1 and Q2 provide increased drive current from the IpDrv signal (from the Vs/Ip Feedback circuit). This is a fairly standard configuration and is in some respects similar to each of the output stages in the LM324 itself. The IpOn signal actually controls the right most 3 bilateral switches of the CD4066B. When IpOn is

Ip Sense, Stoic Reference & Vout

U2C works as a difference amplifier see page 3 of National's AN-20.pdf (198 kb) with Vout = R14(v2-v1)/R12

Vs Feedback and Ip generator

Optional ATtiny and +5 Volt regulator

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