Those Little Black Chips
ME456: Mechatronics Systems Design
Chapter 9:
Electronic Building Blocks
• Integrated Circuits
– Expand possible tasks for a microcontroller
– Substitute hardware for software
– This chapter includes
• The Transistor (NPN)
• The Digital Potentiometer
Prof. Clark J. Radcliffe
Mechanical Engineering
Michigan State University
http://www.egr.msu.edu/classes/me456/radcliff
The Transistor (NPN)
• A current amplifier …
– Makes
small
currents BIGGER
Important 2N3904 Specifications
Note high maximum ratings
• The 2N3904 Transistor
β = 30-400
C= Collector
B = Base
E = Emitter
BUT 200mA maximum current
2N3904 Datasheet
Maximum Thermal Dissipation
• Posted on the ME456 website
Depends on package BUT is always < 1 watt
Your package, max dissipation P = 0.65 W
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Electrical Characteristics
When the transistor is off
Electrical Characteristics
When the transistor is on
•Transistor will tolerate supply voltage on Collector
Δv > 40-60 volts
•Expect current leakage through collector
ileakage < 50 nA
Gain: icollector = β*ibase
Text says β = 416 but that is a maximum…
Min Current Gain: 30 < β <100
Max Collector-Emitter Saturation Voltage: 0.3 volt
Note: Power dissipated, P=VI < 0.3volt*0.2A = 0.06 W
Still… A powerful driver
100 mA load driven by 1 mA
AND load powered by unregulated supply
Æ Here, 10 mA is driven by 0.1 mA
Requiv =
R1R2
RR
R
=
= = 50 kΩ
R1 + R2 R + R 2
Max(ibase ) =
Vmax
Requiv
=
5 volt
50 kΩ
= 10 − 4 Amp = 0.1 mA
β = 100 is more reasonable
Potentiometer
produces voltages
0 < V < 5volt
Still… A powerful driver
100 mA load driven by 1 mA
AND load powered by unregulated supply
Æ Here, 10 mA is driven by 0.1 mA
Max(icollector ) = Max(β ⋅ ibase )
Max(ibase ) = 0.1 mA
= 100 ⋅ 0.1 mA
= 10 mA
Saving BS2 power…
• With Transistor drive, you can
use the 9v battery instead of
BS2 supply
– Saves your limited 50 mA BS2
supply
• Move Vdd connection to Vin
NOTE: This IS NOT dependent on collector supply voltage
– use your 9v battery instead of BS2 supply!
– Now you are driving a 10 MA
load with 0.1 mA!!!
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More Important I/O
Move LED connection from Vdd
-> to Vin
– Save I/O pin power
Circuit will work the same
Now you are supercharged!
BE CAREFUL
This can’t be done with all circuits
(Careful analysis required)
The Digital Potentiometer
The Digital Potentiometer:
Acts the same as a standard potentiometer
Adjusts the wiper to change the resistance
to adjust the voltage at the wiper.
The Digital Potentiometer
The Digital Potentiometer
acts the same as a standard
potentiometer but controlled
digitally.
The chip has
(3) potentiometer connections
(2) power connections &
(3) control connections
How it works
Each element = 78.125 ohms
128 *78.125 =10K ohm total.
With any single tap closed
10K ohm resistance is split & varying “wiper” voltage
The tap is digitally controlled
opens and closes 1 of 128 possible switches
(really transistors).
AD5220 Pin Summary
PIN
Purpose
1. CLK
Receives clock pulses to move the wiper terminal.
2. U
U/D
/D
A high signal moves the wiper (W1) towards A1, and a low
signal moves the wiper (W1) towards B1.
3. A1
The potentiometer’s A terminal
4. GND
The ground connection. Vss on the HomeWork Board.
5. W1
The potentiometer’s wiper (W) terminal.
6. B1
The potentiometer’s B terminal.
7. CS
CS
The chip select pin. A low signal to this pin enables the chip.
8. Vdd
The Digital Potentiometer Circuit
Which Vdd could be Vin??? Why?
Connect to +5 V ( Vdd)
What do the bars over the symbols mean???
3
Digital Potentiometer
Programming
The wiper moved by setting direction and sending pulses
Set Direction
Low for Down to B1
Pulse clock to
move tap 128 times
Reverse direction and repeat
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