Download Electrical Sensors - Red Hook Central School Dst

Potential Divider Circuits
• Sometimes elements on circuit need less than the
total voltage potential.
• Circuit that can be adjusted to divide the p.d.
(voltage) called potential divide.
• Which type of circuit divides voltage?
• Series circuits divide total input voltage between
resistors.
A simple series circuit divides the voltage
among resistors.
Sketch the circuit below.
Derivation of the potential divider equation.
V out
Basic pd divide • I = Vtot
equation.
– Req
• I = Vtot
(R1 + R2) V2 =
Vtot
R2
(R1 + R2)
Around R2 I = V2/R2 Sub in & rearrange
To find partial voltage:
V2 = (Vtot) R2
R 1 + R 2.
The equation:
V2 = (Vtot) R2
R1 + R2.
Describes the output voltage of a particular
resistor as a fraction of the total current based
on the fraction of resistance offered by a
particular resistor.
Ex 1: 2 resistors are in series to 10V. Use the
equation to find V1& V2.around R1 & R2.
3W
12 W
10 V
V2 = (Vtot) R2
R1 + R2.
V1 = 10 V x (3 W) = 2 V
15 W
V2 = V – V1 = 10 – 2 – 8V.
Ex 2: Two resistors are connected to 8V as
below. Find V1& V2.
5 kW
V1
• V1 = 2.4 V
• V2 = 5.6 V
8V
12 kW
V2
Ex 3: Given 3, 5 kW resistors as below, find the
Vab, Vbc.
Compose an
equivalent
circuit
a
a
5 kW
8V
b
8V
b
2.5 kW
c
c
Apply the equation
• Vab = 8 x (5 x 7.5) = 5.3 V.
• Vbc = 8 - 5.3 = 2.7 V.
Proportionality Equation for series circuit.
V1
V2
R1
R2
I
• Since I = V1/R1 = V2/R2.
• V1/R1 = V2/R2.
• V1/V2 = R1/R2.
Ex 4: Use proportionality to find V1.
4W
V1
• 2.5 V
8W
5V
Ex 5: Choose an equation to solve for V on the
10W resistor.
7W
10 W
100 V
• Using potential divide equation
• 50 V
3W
Potentiometers
variable voltage
Kerr 145 potentiometer
• Demo, explanation, problem.
• Nov 04 pg 15
• Kerr pg 150 #32.
Use your data booklets to
sketch the circuit using the IB
symbols.
It’s as if wire is many resistors in series. The
voltmeter can read the pd around some of the
resistance by touching specific points.
6.0 V
V
Simplest potentiometer has slider contact.
2V
• At top V = 2 V
• Halfway V = 1 V
• At bottom V = 0V.
2V
• This is equivalent.
• Why?
Ex: If there are 30cm above & 70cm below the
contact, what is the voltmeter reading?
30cm
2V
70cm
• The reading
around70cm.
• Partial R =
• 70 cm
• Total R =
• 100 cm
• V = 2V x (70/100)
• 1.4 V
Ex: If there are 40cm above & 60cm below the
contact, what is the voltmeter reading?
40cm
2V
60cm
• The reading around
60cm.
• Partial R =
• 60 cm
• Total R =
• 100 cm
• V = 2V x (60/100)
• 1.2 V
I can add another resistor to the
circuit and determine the
reading on the voltmeter.
The maximum resistance of this variable resistor is
100 W. When the sliding contact, S, is at contact A,
what will the voltmeter read?
• It will read 6V (the resistor is bypassed): the maximum
reading of voltage in this circuit.
• What is the reading of the voltmeter when the sliding
contact is moved to B?
We have, in effect, the following situation.
Therefore, the voltmeter will read 3V.
It is not possible to make the reading of the
voltmeter vary from zero up to the full voltage of
the supply using this circuit.
When all 3 connections of the 100 W variable
resistor are used as below, what will the
voltmeter reading be? X is a wire, the contact is
at B.
• Zero. The contact bypasses the bulb like a
short circuit. The wire x is in parallel with the
bulb.
The variable resistor is 100 W. What resistance
does it supply on this circuit?
• 50 W, since resistance is directly
proportional to length.
Uses of potentiometers
• To control volume.
• To control lights (dimmer switch).
Electrical Sensors
• Devices whose resistance
changes with changing physical
conditions.
Strain Gauge
When stretched
R increases b/c
it gets longer &
skinnier.
Find symbol put
in notes.
Light Sensor
LDR – light dependent resistor
Resistance
decreases
when light
shines on it.
See symbol.
Thermistor – resistance varies with
temperature. Usually the R decreases
w/increasing temperature (counter to our
wires).
Symbol.
Using Potential Divide with Sensors
LDR - When light stops shining, its resistance increases
and Vout increases. If it Vout gets high enough, the switch
will activate perhaps putting on lights. The p.d. divide is
needed to create a p.d. to activate switch.
12V
switch
Use of Sensors
Ex: The cell has an emf of 12-V and no internal resistance.
The p.d. required to activate the switch is 5-V. Find the
value of R1 that will cause the switch to activate when the
resistance of the LDR is 200 kW.
switch
Rearrange:
Vout =
(Vin)
R2
R1 + R2.
solve for R1.
R1 =
(R2)
Vin - Vout
V out.
Solve for R1.
280 k W.
Which would be a good sensor to
use with a fire alarm?
What would be a good use for a
strain gauge?
Hwk. Read Hamper.
115 – 123
do pg 123 #21
Nov 04 pg 15