## Posts Tagged ‘limiting resistor’

### Transistors – Voltage Regulation Part XVI

Monday, November 5th, 2012### Transistors – Voltage Regulation Part XIII

Monday, October 15th, 2012
Last time we learned how the Zener diode, an excellent negotiator of current, is involved in a constant trade off, exchanging current for voltage so as to maintain a constant voltage. It draws as much current through it as is required to maintain a consistent voltage value across its leads, essentially acting as voltage regulator in order to protect sensitive electronic components from power fluctuations. Now let’s revisit our example power supply circuit and see how Ohm’s Law is used to determine the amount of electric current, ## Figure 1
If you’ll recall, Ohm’s Law states that current flowing through a resistor is equal to the voltage across the resistor divided by its electrical resistance. In our example that would be R. In fact, the voltage across _{Limiting}R is the difference between the voltages at each of its ends._{Limiting} Applying this knowledge to our circuit, the voltage on one end is V. According to Ohm’s Law the equation which allows us to solve for _{Zener}I is written as:_{PS}
V – _{Unregulated}V) ÷ _{Zener}R_{Limiting} And if we have a situation where V, such as when the voltage of an unregulated power supply like a battery equals the Zener voltage of a Zener diode, then the equation becomes:_{Zener }( V) = 0_{Zener }And if this is true, then the following is also true:
R= 0_{Limiting} In other words, this equation tells us that if V, then the current _{Zener}I will cease to flow from_{PS} the unregulated portion of the circuit towards the Zener diode and the external supply circuit. Put another way, in order for I to flow and the circuit to work, _{PS}V must be greater than _{Unregulated}V._{Zener} Next week we’ll continue our discussion and see why the resistor ____________________________________________ |

### Transistors – Voltage Regulation Part XII

Sunday, October 7th, 2012
Let’s continue our discussion with regard to the example circuit discussed last time and see how the Zener diode works in tandem with the limiting resistor to control current flow and hold the output voltage at a constant level. ## Figure 1
To recap our discussion from last week, the unregulated power supply portion of the circuit in Figure 1 generates an unregulated voltage, V and converts it into a steady output voltage, _{Unregulated}V. Because these output terminals are connected to the ends of the Zener diode, _{Output}V is equal to the voltage put out by it, denoted as _{Output}V._{Zener} The Zener diode, an excellent negotiator of current, is essentially involved in a constant trade off, substituting electric current that originates in the unregulated power supply portion of the circuit for voltage, I, through it as it needs, its objective being to keep _{Z}V at a constant level, and it will continue to provide this constant output, despite the fact that V_{Output} varies considerably._{Unregulated} So, where does the current I, that is, the current flowing from the unregulated power supply area, as shown in Figure 1. _{PS}
I splits off from _{Z}I and continues on to the Zener diode, while current _{PS}I splits off from I on its way to the total internal resistance, _{PS}R, in the external supply circuit. _{Total} What this means is that when you add I together, you get I. Mathematically speaking this is represented as:_{PS}
= I_{Z} + I Why solve for I, _{PS}V, _{Unregulated}V, and _{Zener}R relate to each other with regard to the Zener diode. _{Limiting}____________________________________________ |

### Transistors – Voltage Regulation Part XI

Monday, October 1st, 2012
Without limits on our roadways things would get quickly out of hand. Imagine speeding down an unfamiliar highway and suddenly coming upon a sharp curve. With no speed limit sign to warn you to reduce speed, you could lose control of your car. Limits are useful in many situations, including within electronic circuits to keep them from getting damaged, as we’ll see in a moment.
Last time we introduced the Zener diode and the fact that it performs as a voltage regulator, enabling devices connected to it to have smooth, uninterrupted operation at a constant voltage. Let’s see how it works. ## Figure 1
In Figure 1 we have an unregulated power supply circuit introduced in a previous article in this series. We learned that this power supply’s major shortcoming is that its output voltage, V._{DC} It also varies with changes in, R changes when components are turned on and off by microprocessor and digital logic chips. When _{Total}V is not constant, those chips can malfunction, causing the device to operate erratically or not at all._{Output}But we can easily address this problem by adding a Zener diode voltage regulator between the unregulated power supply and the external supply circuit. See the green portion of Figure 2. ## Figure 2
Our power supply now consists of a Zener diode and a limiting resistor, I, flowing through the Zener diode. Without this limiting resistor, _{Z}I could get high enough to damage the diode, resulting in system failure._{Z}Next time we’ll see how the Zener diode works in tandem with the limiting resistor to control current flow and hold the output voltage at a constant level. ____________________________________________ |