Voltage Regulation
PHYS 309
A. Introduction—Zener diodes and voltage regulator chips
Name:
As you know, diodes are essentially one-way “current valves.” Zener diodes are special types of diodes.
How it looks:
Circuit symbol (note the “Z”):
Direction of conventional current flow:
Like all diodes, Zener diodes work via a p-n semiconductor junction. When you try to send current
through regular diodes “backwards” they completely shut down. However, Zener diodes will resist that
current flow only up to a point. Zeners suddenly conduct once the reverse voltage is above a specific
value. This voltage has various names such as “breakdown voltage,” “Zener voltage,” or “avalanche
voltage.” It’s a very specific value, and thus Zener diodes are often used as voltage regulators.
Another way of regulating voltage is via a voltage regulator chip.
One such regulator is the 𝐿𝐿𝐿𝐿7805 chip. It takes an input of up to
35𝑉𝑉 and gives an regulated output of 5𝑉𝑉. It will also put out up to
1.5𝐴𝐴 yielding a power of 7.5𝑊𝑊. Obviously there are some internal
resistors and transistors that drop the voltage from 35𝑉𝑉 to 5𝑉𝑉.
Others are labeled intuitively as 𝐿𝐿𝐿𝐿7808, 𝐿𝐿𝐿𝐿7810, 𝐿𝐿𝐿𝐿7812,
𝐿𝐿𝐿𝐿7815 and 𝐿𝐿𝐿𝐿7824.
The datasheet for the 𝐿𝐿𝐿𝐿7805 shows a typical regulator circuit.
The two capacitors serve to further remove any “wiggle” in the dc
voltages. Voltage regulators may be put in series with each other.
The output of one may become the “0V” of the next one.
B. Assignment
One-week:
Build the circuit shown below. It’s a combination of a voltage source, a circuit with Zener diodes, and the
all-important circuit-protection resistor 𝑅𝑅1. You should have a 𝑑𝑑𝑑𝑑 input of ≈ 15𝑉𝑉. The open wire at the
“top” of the three LED circuits is to be used as a switch.
Use 6 identical LEDs in this circuit.
Use a multimeter to check that the
LEDs all activate at the same voltage.
Record the value of the LED voltage
and your Zener breakdown voltage.
LED voltage:
Zener voltage:
Figure 1
Voltage regulation-1
Voltage Regulation
PHYS 309
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Name:
One-week check: Part of this grade will be neatness and judicious use of hookup wires to make a
clean layout.
o Demonstrate that your multiple-Zener diode circuit works by connecting the free wire (top of
the LED circuits) to D4, then D3, and then D2. The three LEDs should light in sequence,
serving as a primitive voltage indicator.
o Unhook that free wire. Now use a 25Ω variable resistor as a voltage divider—instead of the
Zener diodes—to light the LEDs in sequence as you turn the resistor knob. Hint: You know
the middle connection should connect to the wire at the top of the LEDs—where should the
other two wires connect?
o Design your own circuit using at least 2 𝐿𝐿𝐿𝐿7805 chips, a number of LEDs, and currentlimiting resistors to demonstrate that you can build a “stacked” voltage source (i.e. 5𝑉𝑉 and
10𝑉𝑉). You must have the lab power supplies set at 18𝑉𝑉.
Writeup: Due at the start of the lab time 2.0 weeks from today.
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I will email you the 5Spice .sch file of the Zener/multiple LED circuit for simplicity.
What are the breakdown voltages of your Zeners, and how were they used to make the various
voltages that you could have for driving the LEDs?
o What did putting those Zeners in series accomplish?
o Why did the LEDs light in the order they did as you moved that free wire around? Hint:
discuss this in terms of the LED voltage you recorded above, and how those relate to the
Zener voltages.
How did using the 25Ω variable resistor make it easier to control your primitive voltage indicator?
Hint: Think how the voltage changed on the middle connection of that resistor as you turned the knob.
Describe the purpose of the 220Ω resistor in series with the Zener diodes. Hint: Think how the input
voltage compares to the voltage at the top of the three Zeners. What problem would arise if that 220Ω
were not present?
What is the power dissipated (𝑃𝑃 = 𝐼𝐼𝐼𝐼 = 𝐼𝐼 2 𝑅𝑅 = 𝑉𝑉 2 /𝑅𝑅 ) by the 220Ω resistor assuming the input is
15𝑉𝑉? Remember that your Zeners are present.
I will email you the starting 5Spice .sch file for the two-𝐿𝐿𝐿𝐿7805 circuit. Finish it out with the LEDs
in your test circuit.
o Describe your own voltage regulator circuit using the 𝐿𝐿𝑀𝑀7805 chips works—this would be a
fairly simple discussion of how the output of one becomes the input of the other.
o Be sure to mention the purpose of the 220Ω resistor above the two 𝐿𝐿𝐿𝐿7805 chips.
o Again, quickly describe how your stacked LEDs serve as an indicator of the output of the
voltage regulators (i.e. how many LEDs were fully lit by the 5𝑉𝑉 and how many by the 10𝑉𝑉).
Voltage regulation-2