A PSU or Power Supply Unit aka AC/DC adpater is then means by which a low tension (voltage) appliance receives its power from the 240 AV mains supply, the this is usually accompanied by the nominal current that is drawn from the PSU on load in milliamps. (e.g. 500mAmps = 0.5 Amps) most modern DC adapters are unregulated, where the output voltage is only regulated by the load current, and a smoothing Capacitor is sometimes used to reduce AC ripple on the DC Level.
If the PSU is regulated the DC Voltage that you measure using an AVO even off load will be the same as the rated current.
Also most, not all, are double insulated (indicated by small square inside a larger one) this means that it doesn't need a earth on the plug, the give away is a plastic pin instead of a metal one found in most domestic plugs with a 13A fuse.
Using Ohms Law P= I x V means that we can find out either the Current in Amps Power in Watts or the Voltage of any unit this is also called the VA rating
The Simple test is if the adapter gets really hot as soon as the you apply AC power then this may indicate catastrophic failure OR a dead short and you must turn OFF immediately the plastic may even start to melt, you may also smell electrical burning. This will also happen if you ever use a non-specific adapter on your device that has a different output voltage this will almost certianly cause damage to your device.
The main test if to test the step-down transformer using an AVO (Amps Volts Ohms) this is what is used to reduce the 240V ~AC voltage to something more in line with the required DC supply say 14-16V ~AC, a diode bridge converts this to DC, if it is regualted there will be either and either a zener or resistor drops this to the rated 12V and a capacitor to smooth the level.
You need to make sure this is socket is working correctly by
checking with either a lamp electricians screw driver or a AC test plug,
dont use this socket you have an earth fault or someother problem,
usually two lines at the outside of a AC socket tester means all is
correct.
- First measure the resistance (Ohms) across the two AC pins that is the Live and Neutral this should have VERY low resistance in the range 300 to 2,000 (2K) Ohms. if this is the case then your primary winding is probably OK if you see 1 on all settings this means open circuit (O/C)
- Then connect the power supply to a 240V AC socket, and check the output voltage with multimeter set at volts DC you should see a stable voltage of the rated value for a regulated adapter or slightly higher by about 5% if its unregulated as this is a reading off load, if you were able to test the voltage with the adapter on load you would see the rated DC voltage.
- If you connect an unregulated AC/DC adapter to the device and open up the cover to test it under load and the voltage is low around 5V or less then the adapter will need replacing even if the off load value appears correct in above test ( ** ref the table below)
NOTE: If you have a AC only output then the test done in 1 can also be done, if there is a diode bridge, Zener Dicde OR Capacitor on the secondary this test wont work so you might have to remove the cover by unscrewing the tamper-proof screws and test it before the connection to the PCB, without the power on of course, check out my tip
HERE for help on how to remove the screws.
If you have a faulty power supply its usually the thermal fuse that blows on the secondary winding, this means that you will get considerably less Voltage at the output jack of around 5 VDC or less and falling if this is the case the transformer is faulty and will need to be replaced with one of the same step-down ratio. OR you will need a new AC/DC Adapter, check out this table for examples of the results that you might find.
** The Amstrad Black PSU that reports 24V and 383 Ohms on the primary may still not deliver the rated voltage under load and therefore I have marked this as a false positive and therefore FAULTY.
NOTE: The AC/DC Chargers are constant current devices (the symbol is usually two interlocking circles and even though you can do similar tests on the transformer primary coil its the current that it delivers under load that is important the only way to test this is using a clamp meter on the cable to see the current under load this should match the rated current.
COMPUTER ATX PSU
Most ATX power supplies are known as switched mode can only be tested under load when connected to the PC motherboard this is a bit tricky using a plain old Fluke, AVO or Multimeter the best way is to buy dedicated PSU tester for the type of Power Unit you have, it will check the DC voltages in the range + 12V,-12V,+5V,+5VBs and +3.3V on both the SATA and IDE supply cables these cost about £20 ($35) you can test the unit in as little as 5 mins.
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