PC Repair II Miami
Ray Merry, Adjunct Professor
Notes on Current, Power, and Power Supplies
Flow of charged particles across or through something.
Usually electrons (negative charges) from negative terminal to ground or
Letter I stands for the current.
Measured in Amperes.
Force creating the current, abbreviated by the letter V,
measured in Volts
Force impeding the current. Depends on material and cross section (larger area less
current). Letter R stands for
resistance. Measured in Ohms
Ohms Law: I =
12 Volts across/through 6 ohms = 12/6 or 2 amps current.
12 Volts across 1 ohm = 12 amps
12 Volts across .1 ohm = 120 amps!!!
This would burn up most things.
Beware of short circuits. 0 Resistance = infinite current!
AC vs DC Alternating vs Direct Current
What is a ground!
Danger with High
110 vs 220
varies with country and here with use.
Even 110 VAC is
potentially deadly, depending on your resistance.. i.e. are you grounded?
Usually 12 Volts no
problem for you, but it is for shorted parts!
Power, P = I2R
A power supply can supply only so much power!
Too many devices NG!
Max power 250 watts or so.
Power = Heat, more power, more heat
Lower Voltage usually means less power and less heat.
Voltage Furnished from Power Supply
Voltage of power supplies
+12 used for motors, e.g. FD drive motor
+5, +3.3, used for cards, some fans, CPU’s
Lightning and Other Surges
Surges are sudden
increases or decreases in Voltage and Current.
Large motors going on
through wiring & shorts
Uninterruptable Power Supplies…UPS
Power up, power down a problem
Limits to useful voltage of about +-10V for most PS
Power down or off, PC off!
UPS supplies power from a battery which is converted to 110
VAC, then back down!
Bigger battery (amp-hours) more time
Less Power more time.
PowerGood signal if not on, PS shuts off.
PS_On signal, tells Power Supply to turn off the power!
Power management APC
Measuring Voltage, Current & Power
Positive lead is red, negative is black: Digital meters
read negatives, not analogue ones Voltmeters measure voltage, don’t mix AC
with DC ranges! Always set on the
largest range first.
Ohmmeters measure resistance, power must be off!!!
Wattmeters measure power in watts, power must be off!
Ammeters measure current in amps, power must be on
Multi-Meters meters which measure all or most of the above.
Recording Voltmeters.. make a recording of the voltage over
time, great for long term fluctuations.