Vehicular Charging Systems Part Ill By Frank Oropeza | Editor’s Note: Although manufacturing abilities and components have evolved since this series was first published in 1984, electrical parameters of alternators have remained virtually unchanged. Even the 10SI remains popular because of its economical retrofit possibilities. Diodes Diodes are of two types, rectifying and zener. The rectifying diode is a switch that conducts current from cathode to anode when voltage polarity is as shown (Figure 1). BATTERY/ BATTERY Li o/14v ro * R7 > BATTERY] | °?”> puttin Figure 4 — 1968949/ Lbs LN79000 (Note: CR1 clamps the signal to ground) Circuit ground, hence, we now rectify almost all Zener Diodes Rectifier Distribution of the stator signal except for the amount we use for the ground clamp (0.7 volt). The rectifier configuration of CR1 and CR2 is recognized in the electronics industry as a voltage doubler. The other 2 phase signal function to result in a con- stant averaging relationship as illustrated —py} + a Alec = Figure 1 = es c A ee gee _— a The forward drop or voltage to turn on a silicon diode is 0.7 volts. Once turned on, the diode resistance is very low. Conversely, a diode in a non-conducting state appears as a high resistance path, just like an open switch. Diodes are tech- nically considered diodes up to 3 amps and beyond that we refer to them as rec- tifiers. Such as, the rectifiers utilized in a rectifier assembly, wherein 6 rectifiers function to rectify the alternator output to a DC voltage and the battery functions as a large capacitor to store the generated energy. Let us look at what the 6 rectifiers do with the 3 phase alternator signals seen in Figure 2. Rectifier CR1 clamps the signal to Zener diodes perform like diodes when biased in the forward direction. They exhibit special characteristics when biased in the reverse direction and are ideally suited for sensing voltage levels. Zener diodes are available in rat- ings from 2.7V to several hundred volts. in Figure 3. Additionally, zeners are rated as a func- OPEN While on the subject of tion of power dissipation to accommo- diodes/rectifiers, let us consider date various circuit applications. As the CLOSED | 4 typical problem in the industry. voltage is increased across a zener Twenty-five amp rectifiers are diode, it remains in a non-conducting state until the zener thresh- -~—A—-% -f- – -y @vots old is reached. At this point sigh the diode conducts and acts AVERAGE _| like a low resistance or Figure 3— 3 Rectifier Response closed switch. Zener diodes also exhibit a temperature characteristic that increases or decreases as a function of temperature and in regula- available from suppliers as a function or price. Rectifiers or any semiconductor devices are classified by a sorting process by manufacturers and they use a leakage voltage criteria to determine the selection process. Rectifier leakage current effects are a function of applied voltage, hence, at a higher voltage, a higher leakage cur- rent results. tor circuits this effect can be used to an advantage to simplify circuit design and cost. Zener diodes play an important role in voltage regulators. Their primary func- tion is to sense when the battery voltage is out of range and as such, the zener diode is the decision making element of a voltage regulator. Based on the distribution, the 50 volt rectifier is a bargain because it is the bot- tom of the barrel. In turn, the rectifier with a higher voltage rating costs consid- erably more money. How They Combine to Comprise a Regulator The basic elements required in a volt- age regulator circuit have been discussed and now we will combine them to evolve a regulator circuit. We will use an A cir- cuit configuration since it is very popular and we will also relate the difference between the Delco 10SI, 25SI, and the Leece Neville regulators (Figure 4). Resistor R1, R2 and R3 form a voltage divider network that features a variable voltage range of 1 volt centered at 14.0 volts. Capacitor C1 is a tantalum capacitor that functions to filter excess noise and ripple voltage while providing a low impedance to prevent regeneration. Zener diode CR1 is the circuit component that decides when to switch transistor Q1 “on” or “off? depending on the bias voltage set by potentiometer R2. When zener diode CR1 is off, transistor Q1 if cut-off, indicat- ing that battery voltage is low. Hence, the voltage from trio at R6 is clamped to 1.4 volts at the collector of QI.