Download Ground, power and resistors

Ground, power and resistors
TechVents Ham Radio Module 2
What is ground?
Expressions:
• You need a good grounding
• Finding common ground
• Not grounded in reality
4 reasons for ground
1. Reference point
2. Where power and current want to go
(pushed by V)
3. What you and equipment need for safety
– Surges, failure in equipment should not go
through you
– Static in your body can destroy equipment
4. Active part of an antenna and transmission!
Ground as safety
Ground as reference point
• This is will be the main use.
• In a circuit drawing, ground can look like:
– 0 Volt reference
– “minus” terminal on the battery
– Explicit ground
But what is the critical difference between the figure on the left and the other two?
Difference: the meat sack!
• Imagine you’re holding one of those circuits
on the top line – and something shorts out
• Your hand to ground is now the ground
connections
– Power will take easiest route to ground
– Without direct ground connection, that route
might be through you, the meat sack
• So: ground required for higher power devices!
– Small battery operated won’t make a difference
Real-world example
• Radio or appliance has a metal case
• Internal component failure cause short that
makes a live internal wire touch the case
• Is the case connected to ground?
– YES: power flows to ground. Probably blows fuse.
– NO:
• Nothing happens – nowhere for power to go.
• Then you walk up and touch the case
• Mmm… fried meat sack!
Back to our basic circuit
• Voltage: pump creating pressure (potential)
difference
• Current: how much is flowing through
Better resistance analogies
Not bad, but still useless
Now we’re getting
somewhere. Useful
work that looks like a
resistance to the
circuit.
Back to our basic circuit
• Voltage: pump creating pressure (potential)
difference
• Current: how much is flowing through
• POWER = VOLTAGE X CURRENT
– Static electricity shock: 10,000 V but little current
– Slow moving river, lots of current, but little
potential change
• But get to a waterfall and that “potential” difference
comes in.
Lab 1: which has most resistance?
• 7W bulb?
• 40 W bulb?
• 100 W bulb?
• THESE ARE ALL RATED FOR 120 Volts!!!
• Which is using most power? Test with meter.
• Why is 100W bigger bulb than 7W? More
power means is has to throw off more heat!
Could be same resistance value…
Pretend the stripes are all the
same. Because I didn’t want to
redraw this.
The bottom one can handle *way* more power
What is a resistor?
• Something to make silly circuit diagrams
– Unless it’s a representation of a full circuit for
calculation purposes
• A useful component for controlling how much
current and power will flow through!
• Usually made of carbon
• How much current flows through?
– V=IxR
– I=V/R
-> so it depends on the voltage (pressure)
applied!
Resistor for current control
• Presenting: the diode
– Diode:“one-way voltage control”: like a valve
– Without separate resistance, it’s effectively a short
circuit. That’s bad.
– LED:
• Light emitting
– ALWAYS ADD RESISTOR
• To control/limit current!
LAB: Battery polarity tester
Connect battery one way, and the RED LED turns on
Connect it the other way, and the GREEN LED turns on.
If both turn on, one is wrong
- Log lead / round side to + on power source
- Short lead/flat side to -
Combine the equations
• 10 Volt battery, 5 ohm resistor. Power?
–P=VxI
– V = I x R -> I = V / R
• Substitute in equation 1
– P = V x (V / R)
– P = V2 / R
– P = 102 / 5
– P = 20 Watts
Now take practice test 2
- Have a calculator handy
- Find it on
http://techvents.wordpress.com/docs