Background Application: 1995 Ford Ranger Manual Control Blend Door Actuator
When the user changes the control potentiometer position the blend door actuator moves a baffle (blend door) to blend air from hot and cold sources in the heating and cooling system.
In my blend door actuator pin 8 ground also connects inside to pin 4. Pin 7 at 12 volts is input to Q1 shown below. Pin 6 is what I marked as Vreg output, however, based on feedback, I think this output is not regulated, but rather, Q1 provides either current limiting or crude thermal shutdown function.
Problem: Q1 NPN Darlington Malfunction and Inability to Understand Design
Reverse engineering effort produces this circuit (note 18 ohm "dummy load" is used to eliminate complexity of the actual op amp servo motor control circuit):
simulate this circuit – Schematic created using CircuitLab
The "Darlington" component Q1 is marked ST Mal KSD1692. I am unable to find an ST part by this indication. The Fairchild KSD1692 datasheet shows the pinout that I am using.
Based on diode drop tests, the Darlington pass transistor Q1, as shown in the circuit, failed inside the blend door actuator. Zener diode D1 is rated 9.1 volts.
Because the pass transistor is bad, and I don't have a replacement blend door yet, I do not know the design value for regulator output Vreg. Does this bias circuit makes sense? If so, then how does the bias circuit work to set the value for output voltage Vreg? Edit: The new question is now how does the Zener and BJT provide an effective or not so effective thermal shutdown function?
PS - The parts in the circuit seem to be 12-14 volt tolerant, so I removed the parts D1, Q1, and Q2 from the malfunctioning blend door and applied 12 volts to the node indicated Vreg. This makes the blend door actuator work as expected in a bench test.
New Material - Edit
The "dummy load" should be 120 ohms for measured motor resistance instead of the 18 ohm load shown above. A stalled motor would give the armature resistance at zero rotations per minute and the current limit, if provided, should protect the motor during stall conditions.
This is a single sided board with components on the face and traces on the bottom. There are 12 resistors on the circuit board all large values 3.4k or more. I don't see a low value current sense resistor that would otherwise connect to the base of Q2.
In the additional diagram below (which relates to a new question) there is a possible sense path indicated at the bottom right. The short trace shown to ground below is actually 2-3 inches of black trace on the face of the board. This trace goes under the pin header so I can't follow it back to the base of Q2. Maybe it provides some sort of control function keeping Q2 off or in the active region until some fault condition occurs. I can't make sense of it yet.
In the new diagram the user manual control, the "Hot/Cold" knob is shown at the top right and the top and bottom wipers, etched on the face of the circuit board in black trace material, are shown at the bottom right. A conductive "comb" device attaches to the final output gear which moves the blend door. The comb fingers make contact with both wipers so the comb voltage picked up on the top wiper transfers to the bottom wiper.
The input voltage Vs = Vreg (above) is the output from the circuit above, but note there must be some feedback path that keeps the base of Q2 either at zero volts or biased in the active region during normal operations otherwise Vs would not be in the expected operational range.
If one disconnects the "Hot/Cold" knob the L2722 control chip forces node A to half the source voltage shown as node A = 0.5Vs. The L2722 forces the motor to move in the direction that moves the comb to the center of the two wipers. So with no sense input from the "Hot/Cold" knob the L2722 design drives the motor until the comb moves to the center and node A settles at 0.5Vs.
If the operational "Hot/Cold" knob is set exactly to the middle position then there is 0.5Vs on both sides of the 10k and 93k resistors and the wipers are in the middle position too.
If the user sets the "Hot/Cold" knob to some other pick-up voltage, but not in the center of the pot, then the L2722 drives the motor, which drives the comb position, until the voltage at node A is restored to 0.5Vs, and then the motor stops. Node A only connects to a high impedance op amp input so current flowing through the 10k resistor should also flow through the 93k resistor.
I think the Top and Bottom wiper can be replaced in SPICE model as follows. The Top wiper would be a voltage controlled voltage source. The Bottom wiper would be a variable resistor. But so far I am stumped by how this servo feedback circuit works.
