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How to test your AC/DC Adapter or Computer Power Supply Unit.

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.

  1. 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)
  2. 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.
  3. 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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what is psu?


Power supply unit computer
A power supply unit (PSU) converts main AC to low-voltage regulated DC power for the internal components of a computer. Modern personal computers universally use a switched mode power supply. Some power supplies have a manual selector for input voltage, while others automatically adapt to the supply voltage.

Apr 25, 2014 | HP Pavilion a1253w Notebook

1 Answer

fault finding


Power is going to the motherboard, and you know this by assuming?

An assumption would be you see the Power Supply fan spin, LED lights light up, and maybe computer case fans spin.

Not an assumption, and you would have tested the 3 main voltage power rails, coming out of the Power Supply.

The Power Supply in your computer is an SMPS.
Switched-Mode Power Supply. (Also is known as the short abbreviation - PSU. Power Supply Unit)

http://en.wikipedia.org/wiki/Switched-mode_power_supply

The Power Supply in your computer is a Converter.
It converts HIGH AC voltage into 3 LOW DC voltages.

[Depending on country,
USA = 120 Volts AC. UK = 240 Volts AC. Japan is 100 Volts AC, but may depend on area. Australia = 240 Volts AC. India = 240 Volts AC. HOWEVER, do not write the above in stone. I may have made an error ]

The 3 low main DC Voltages coming out of the Power Supply is;
A) 3.3 Volts DC
B) 5 Volts DC
C) 12 Volts DC

Orange wires carry 3.3 Volts DC
Red wires carry 5 Volts DC
Yellow wires carry 12 Volts DC
ALL Black wires are Ground wires. They can also be called Negative wires.
This is a DC circuit now. There is a Positive, and a Negative.
Orange, Red, and Yellow wires are power wires, and also Positive wires.

The first part of your diagnosis will be to test those 3 main voltage power rails.

[Digressing;
Using an example;
There are many Red wires coming out of the Power Supply.
These are 5 Volt wires. They are Connected TO, the 5 Volt power rail in the Power Supply.
ALL 5 Volt wires end in one place, in the Power Supply.
The 5 Volt power rail.
When you test just ONE red wire, you are testing the entire 5 Volt power rail coming from the Power Supply.

This also goes for the Orange wires, and Yellow wires ]

With the Red 5 Volt wires, and Yellow 12 Volt wires, you could just use a 4-pin Peripheral power cable to check them,

http://www.playtool.com/pages/psuconnectors/connectors.html#peripheral

Multimeter set to DC Voltage, the red (Positive) probe lead of the multimeter; touches the female metal terminal connector, for the Red wire.

http://www.mcmelectronics.com/product/28-9420&utm_medium=Affiliate&ref=cj&utm_campaign=CommissionJunction&utm_source=CommissionJunction?t=2&utm_expid=8634549-14

The black (Negative) probe lead of the multimeter, touches a female metal terminal connector, that goes to a Black wire.

You should be reading 5 Volts DC.

Same thing for the Yellow 12 Volt wire.

With an Orange 3.3 Volt wire, this changes.
A straightened out paperclip is inserted, down into the BACK of the ATX main power cable's connector; AND into a socket hole with an Orange wire in it.

http://www.playtool.com/pages/psuconnectors/connectors.html#atxmain20

http://www.playtool.com/pages/psuconnectors/connectors.html#atxmain24

The straightened out paperclip, slides down into the socket hole, with the Orange wire in it.
Slides down into the socket hole, RIGHT NEXT TO the orange insulation of the wire, and MUST go down far enough; to Touch that female metal terminal connector.

EVERY wire going down into the ATX main power cable's connector, ends in a female metal terminal connector.

Same thing is down with a socket hole that has a Black wire in it.
ALL Black wires are Ground wires. (Negative)
You can choose ANY socket hole that has a Black wire in it.

Now touch the two probe leads of the multimeter, to their respective straightened out paperclips.

Red (Positive) probe lead of multimeter, to straightened out paperclip in Orange wire socket hole.
Black (Negative) probe lead of multimeter, to straightened out paperclip in Back wire socket hole.

You should be reading 3.3 Volts DC.

(Or if your multimeter kit has special probe lead, that would take the place of a straightened out paperclip, of course use it instead)

Know this;
A) If ALL of the LED's ('lights') were on at once, they would use less than 1 Watt of power.

B) EACH fan uses 2 to 3 Watts of power.

C) A typical CPU (Processor) can use 51 to 130 Watts of power.
Just depends on what Processor (CPU) it is.

This is why a Power Supply with a weak voltage power rail, will not have enough power to turn the Processor ON, but will have enough power to light those simpy LED's, and spin fans.

[LED - Light Emitting Diode ]

Regards,
joecoolvette

May 11, 2013 | Dell Computers & Internet

2 Answers

wht kind of current comes out?


The computer's switch mode power supply produces DC (direct current) electricity.

Nov 05, 2012 | Computers & Internet

1 Answer

What voltage is applied to the lamp inverter PCB. ? MP7650


Most lamp invertor boards receive rectified and filtered mains dc voltage as the input power supply.
So if you have 240 V AC mains you would typically get around 339V DC. ie 240 times root 2. the peak AC voltage.

Jun 13, 2011 | 3M MP7650 Projector

1 Answer

the green light is blinking on the supply unit but it not coming on or booting up , i need to know if the supply unit is bad


Close examination of your power supply may reveal if the psu is bad.

When the power supply is plug in ac socket the green indicator led inside the motherboard will lit steady green. See if by turning on the cpu you will see some sparks in the power supply connector attached to the motherboard. This is a clear indication of a bad or shorted psu.

You can also smell if there is a burning inside the psu.

See if the fan inside the psu is working if not then its a clear indication of a bad psu.

Using a multimeter if you have one measure the dc voltage coming from the supply connector. Insert the black probe to one of the black wires in the connector. And the red probe to the red wire in the connector. the reading should be +12VDC. Put the red probe to the red wire and it should read +5VDC. Any readings below this, means bad psu.

Measure also the output voltage when there is a load in the power supply. Connect a CD-ROM drive or hardisk and measure again the voltage. Dropping of voltage when put some loads is a clear indication of a bad power supply unit (psu).

I hope this will help you.
Thank you for using Fixya!

Jan 11, 2011 | HP ATX-300-12Z (ATX30012ZBD) 300-Watt...

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