CHAPTER :2 MEASURING VOLTAGES AT DIFFERENT LEVELS What is a multimeter? A meter is a measuring instrument. An ammeter measures current, a voltmeter measures the potential difference (voltage) between two points, and an ohmmeter measures resistance. A multimeter combines these functions, and possibly some additional ones as well, into a single instrument. A multimeter can serve many purposes including checking for voltage signals at different points in a system, testing the output of the power supply and checking for continuity in a circuit or cable. Digital multimeter (DMM)? One simple test that you can perform on a power supply is to check the output voltages. This shows whether a power supply is operation correctly and whether the output voltages are within the correct tolerance range.You must measure all voltages with the power supplies connected to a proper load. Which usually means testing while the power supply is installed in the system. Measuring voltage: To measure voltage on the system that is operating you must use a technique called back probing on the connectors (These are the red,yellow, orange, black and white wires coming out of power supply). You must measure the voltages when system is powered on. Nearly all the connectors you need to probe have openings in the back where the wires enter the connector.The meter leads are narrow enough to fit into the connector beside the wire and make contact with the metal terminal inside. The technique is called back probing because you are probing the connector from the back. You must use this back probing technique to perform all of the following measurements.To test a power supply for the proper output, check the voltage at the Power_Good pin for +3v to +6v of power. If the measurement is not within this range, the system never sees the Power_Good signal and therefore does not start or run properly. In most cases, the power supply is bad and must be replaced.Continue by measuring the voltage ranges of the pins on the motherboard and Hard Disk power connectors. If you are measuring voltages for testing purposes any reading within 10 % of the specified voltage is considered acceptable, although most manufactures of high quality power supplies specifies a tighter 5% tolerance. For ATX power supplies, the specification requires that voltages must be within 5% of the rating , except for the 3.3v current which must be within 4%. Desired Voltage Loose tolearance Min (-10%) Max(+8%) Tight tolerance Min (-5%) Max( +5%) +3.3v +/- 5.0v +/- 12.0v 2.97v 4.5v 10.8v 3.63v 5.4v 12.9v 3.135v 4.75v 11.4v 3.465v 5.25v 12.6v Power_Good signal has tolerances that are different from the other signals, although it is nominally a +5v signal in most systems. The trigger point for Power_Good is about +2.5v, but most systems require the signal voltage to be within the tolerances listed. Signal Minimum Maximum Power_good +5v 3.0v 6.0v Replace the power supply if the voltages are measured out of these ranges.Again, it is worth noting that any and all power supply tests and measurements must be made with the power supply properly loaded, which usually means that it must be installed in a system and the system must be running.
Posted on: Tue, 09 Jul 2013 05:34:06 +0000
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