Alternator Regulator Study
Another personal project inspired by electric motors. My friend gave me an alternator he pulled from a mid 2000s GM car in a junk yard (not a subtle jab) here in metro Detroit.
My vision was to turn this into a wound field electric motor. I planned on designing my own wireless power transfer system and making 3d printed parts to test my design. This is obviously not any type of groundbreaking innovation but rather was meant to be a learning project. It would force me to practice CAD, review power electronics, and test out some control theory. I began by taking apart the brush system and control board, then did DMM testing to make sure the windings were still conductive. I then studied how the control board worked, which was a really good fundamentals of electrical engineering review.
Shortly after this point I had the idea for my PCB eMotor and pivoted my time towards that project. I believe the PCB eMotor has some truly innovative aspects while incorporating many of the same learnings I would have gotten from this project. Please follow the PCB eMotor project in the dedicated respective posts.
I will explain my learnings from the control board study below, as there was valuable information in that for me and it became the bulk of my takeaway from looking into alternators.
Exxotest Guide Used
The main source of content I used for looking into alternator controls and operation was this guide I found online from Exxotest. Here is the link
I had to begin with learning about BJT operating principles. There was very little exposure to this in my coursework as most of the focus was on newer switch types. In short, a BJT (Bipolar Junction Transistor) is a semiconductor that controls current flow. When the threshold voltage drop is met across the base and emitter terminals (typically around 0.7V), a small current flows, which modulates the larger current flow between the collector and emitter. This functionality is commonly used as a switch or amplifier and can be doped as an NPN or PNP transistor.
BJT Symbols
Conducting State:
In the conducting state, the rotor is excited and the alternator generates power to charge the battery.
Conducting Mode Sketch
Key aspects of conducting operation:
When rotor is excited, transistor T2 is conducting.
The potential across the Zener diode remains below its breakdown threshold, thus preventing the activation of transistor T1 and allowing T2 to conduct.
With T1 open, the base-emitter potential on PNP transistor T2 reaches a sufficient magnitude to initiate conduction.
Though not specified, my assumption is that the base current of T2, and consequently its conduction, is regulated by the ground resistor (Rg).
The voltage divider formed by resistors Ra and Rp establishes the reference voltage applied to the Zener diode.
The output diode acts as a unidirectional conductor, preventing the rotor excitation current from flowing to ground.
Non-Conducting State:
The non-conducting state prevents current flow to the rotor, preventing power generation and battery charging.
Non-Conducting Mode Sketch
Key aspects of non-conducting operation:
The fully charged battery voltage provides sufficient potential to overcome both T1's Vbe drop and the Zener breakdown voltage, facilitating base current flow through the Zener diode.
When T1 saturates, its low collector-emitter voltage (Vce) reduces the base-emitter voltage (Vbe) of T2 below its activation threshold. This is because a saturated BJT exhibits a very low Vce, typically 0.1V to 0.2V, which is less than the typical 0.7 Vbe activation threshold.
With T1 active, current diverts to ground via the ground resistor (Rg).
This design opens T2 while minimizing T1 current draw, preventing rotor excitation in a high-charge state.
LT Spice Simulations:
Diode Rectification Circuit
Diode Rectification Circuit - Voltage Plot
I am very much a visual learner. I created this LT Spice model so that I could visualize the inputs and outputs as well as exploratory analysis of circuit behavior. I believe the voltage drop between the output and input waveforms is due to the diode. If you take a look at the difference between the cursors, the maximum difference is about 0.9V. This is close to 0.7V, which is typical for a PN junction. The low-value resistor creates a high load, which likely influences the diode's voltage drop and output voltage ripple. I tried adding a 1mF capacitor across the output and this removed all of the ripple in the output voltage.
Zener Diode Test Circuit
Zener Diode Test Circuit - Voltage Plot
I also created this circuit for visualization purposes. The breakdown voltage of this particular Zener diode is 8.2V. You can see that the voltage rolls off at this point and flattens out as current begins to flow through the reverse biased diode.
Conclusions:
This project served as an effective review of circuit fundamentals and offered a practical opportunity to study BJT switches. It was really neat to see an example of a control strategy using only analog components. In the future, I'm interested in looking into the complexities of the Rp resistor in the voltage divider circuit.
