Power Supply Notes

Home Up Objectives Sat AM Assignments Sound & Multi-Media Hard Drive Info Hard Dr Lab #1 Slides on Current Power Supply Notes Lab 10 Power S Lab11 Diags PC R II Sat Grades PC Repair II Labs Lab1 CDROM Lab 2 FDISK & P M Lab3 Upgrade HD Lab 4, Drive Image. Lab 5 Floppy Dr Lab 6 Memory Lab 7 Sound Card Lab 8 PC Prices Lab 9 Multimeter

PC Repair II  Miami Dade CC
Ray Merry, Adjunct Professor 

Notes on Current, Power, and Power Supplies
 

Current:

Flow of charged particles across or through something.  Usually electrons (negative charges) from negative terminal to ground or positive. 

Letter I stands for the current. 

Measured in Amperes.

Voltage:

Force creating the current, abbreviated by the letter V, measured in Volts

Resistance:

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 = V/R

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 Voltages….

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

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

 -5, -12 scarcely used

 

Lightning and Other Surges

Surges are sudden increases or decreases in Voltage and Current.

Causes:

Large motors going on

Lighting

Transformer/Generator/Power Problems

Interference through wiring & shorts

Solution: Surge Suppressor

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.

Power Supplies

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.

 

Home Objectives Sat AM Assignments Sound & Multi-Media Hard Drive Info Hard Dr Lab #1 Slides on Current Power Supply Notes Lab 10 Power S Lab11 Diags PC R II Sat Grades PC Repair II Labs Lab1 CDROM Lab 2 FDISK & P M Lab3 Upgrade HD Lab 4, Drive Image. Lab 5 Floppy Dr Lab 6 Memory Lab 7 Sound Card Lab 8 PC Prices Lab 9 Multimeter