Transcript Ohms Law & Watts to Horsepower

Applying Math to Wire Sound
Using formulas and
conversions to wire up sound
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Problem
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We have a classic Camaro and we will be
installing a 1200 watt amp and a head unit
that is rated at 100 watts.
What size alternator will be needed to ensure
proper operation of the charging system?
How much circuit protection will be needed to
ensure proper operation?
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Seen this in math?
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Convert 4500cc to in3. 1 in3 = 16.387cc
4500 cc
1
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
1in
3
16 . 387 cc

4500 in
3
 274 . 608 in
3
16 . 387
16 = 5y, Solve for y.
y
16
 3 .2
5
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Math Terms
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Ohms Law: E = I x R
E = Voltage
I = Amps
R = Ohms
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Watts Law: W = E x I
W = Watts
E = Volts
I = Amps
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Horse Power
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1 Hp = 746 Watts
One Hp can raise
550 lbs 1 foot in one
second
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Applying Ohms law & Watts law
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Ohm’s Law : E = I x R
•
1998 Ford F-150 needs 180 starting amps to crank
the engine. What is the resistance if the voltage is
12V?
• 12 = 180 x R
12
 R  0 . 07 ohms
180
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Applying Ohms law & Watts law
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Watts Law: W = E x I
If a 500 watt speaker
need 10 amps to
operate, what is the
voltage requirement?
•
500 = E x 10
500
 E
10
50  E
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Applying Unit Conversions
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A10,000 watt lighting
system is being
installed on and off
road Chevy Silverado.
How much horsepower
is being used to power
these lights if the
available voltage is 12
volts?
•
10000 watts
1

1hp
746 watts

10000 hp
 13 . 4 hp
746
1 horsepower = 746
watts
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Let’s look at the original
problem again!
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We have a classic Camaro and will be
installing a 1200 watt amp and a head
unit that is rated at 100 watts.
What size alternator will be needed to
ensure proper operation of the charging
system?
How much circuit protection will be
needed to ensure proper operation?
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Calculations
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Given values
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12 volt system
80 amp alternator
We will be adding
1300 watts in sound
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Using Watts law to find the total
amperage needed for the sound
system
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Recall, Watt’s Law:
W=E x I
Watts = Voltage X
Amperage
Total wattage added =
1300
Voltage = 12
1300 Watts Divide by
12 Volts = 108 Amps
The system will require
108 amps working at
max output
1300
 I  108 . 33
12
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Calculating Total System
Amperage
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We need to find the reserve amperage of
our camaro.
We know the alternator is rated at 80
amps and by researching our stock
vehicle option package our peak draw is
60 amps so our reserve would be 20
amps.
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Putting it All Together
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System voltage = 12 volts
Total Amp draw of our stock car = 60 amps
Our reserve = 20 Amps
Our new system will require 108 amps max
for a total of 1300 Watts
By adding 108 amps to our car the total peak
demand would be 168 amps.
Add in our reserve and our Camaro will need
a 188 amp Alternator to operate under peak
load.
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What will this cost us in
performance?
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1300 watts divided by 746 = 1.74
horsepower
1300 watts
1

1hp
746 watts

1300 hp
 1 . 74 hp
746
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Try This Problem….
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A vehicle with a 12 volt
system and a 100 amp
alternator has the
following circuits: 30 amp
a/c heater
30 amp power window/seat
15 amp exterior lighting
10 amp radio
7.5 amp interior lighting
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Name: ___________________
1) Find the total resistance of the entire electrical system based on the car’s
system described above.
2) Find the unused amperage if all of the above circuits are active.
3) If a 1000 watt amplifier is added to the audio system, how much unused
amperage (negative or positive) will be left?
4) Will a larger amp alternator be required to operate the circuits? If yes, calculate
the minimum amperage needed?
5) If a larger alternator is needed and given alternators come in 100 amp intervals,
what size alternator do you need to fulfill the requirements of the charging
system?
6) You have successfully installed your amplifier. Should you be concerned about
a reduction in horsepower? To answer this question calculate the following:
–
Based on the original alternator size, how much horsepower is being used by the
alternator?
7) By adding the new alternator, how much horsepower will the engine lose if the
total engine horsepower equals 385 hp?
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Now it is your turn…..
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Think of a problem similar to the car stereo
problems we have solved today or even one you
have experienced yourself.
Write it up and make your recommendations for
how to solve the problem, be sure to show your
work as we have in this lesson.
Your problem analysis is due next Friday!
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Have a great Weekend!
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We are done!
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