Introduction: Fixing a Laptop Adapter
The modern laptop power supply is a marvel of modern engineering. Switching power supplies have been around since decent power transistors began to appear in the late 1950's but comparing those with what we have today is like comparing a Model T to a modern car. This power supply is actually a DC to DC converter, where the 120-volt AC line voltage is rectified to 170 volts DC, it is then chopped at hundreds of kilohertz and passed through a high-frequency transformer, where it is again transformed to a lower voltage AC and then again rectified to DC. This is done at over 90 percent efficiency. The major breakthrough that allowed such high efficiencies to be achieved was the development of solid state devices with incredibly high switching speed. The waste heat and hence inefficiency of a switching circuit is mostly produced during the time that the solid-state device is switching from on to off and vice-versa. As the transition time of the device is lowered, the waste heat goes down also. This is a long way from a linear power supply where 30 percent efficiency is considered to be good efficiency.
You will need the following to do this instructable:
1) Soldering gun, solder and solder sucker
2) Long-nose pliers and flat blade screwdriver
3) Multimeter and oscilloscope
4) Glue that will glue plastic parts together.
5) Two 680 microfarad, 25 volt electrolytic capacitors. Preferably the type that has the leads coming out of one end.
( Values of capacitance down to 470 microfarad would work OK, but keep the voltage rating.)
Step 1: Check the Output Waveform and Schematic Diagram
Looking at the output waveform as shown here, it can be seen that there are only approximately 10 volts of DC with spikes every 22 milliseconds riding on top. The output is supposed to be at least 19 volts with millivolts of AC component riding on the DC. Experience tells me that the solid state components are very robust in these circuits, the weakest link is the electrolytic capacitors which can go at any time.
Step 2: Dismantle the Adapter
Wait a few minutes for the input capacitor to discharge and take the adapter apart and gently pry up the circuit board from the inside of the bottom case. Make note of where all the main parts are as shown in the above diagram.
Step 3: Remove the Two Output Capacitors and Solder in the New Ones
The two output electrolytic capacitors are on the far right of the board as shown in the picture, unsolder them from the board and use the solder sucker to remove the old solder. Make note of the polarity of the old ones before you take them out and put the new ones in the same way. The negative side goes toward the heat sink as viewed from the top. Take the new capacitors and solder them in with the soldering gun taking care not to hold the soldering gun tip on the board for any more time than is necessary (about 10 seconds).
Step 4: Reassembly and Testing
Before you put everything together, check and recheck your work. Double check the input wires from the line cord where they connect to the board. Check the output wires where they connect to the board. Once you are satisfied that everything is OK, plug in the charger and look at the output. It should look like the above picture, with a smooth DC level at about 19 volts with no noticeable AC component. Note: I have taken both the before and after oscilloscope readings in "AC mode". After you are satisfied with your work, reassemble the board in the box and re-glue everything together.
Step 5: Conclusion
Once I had taken the output electrolytic capacitors out of the circuit, I could see that one of the capacitors had physically burst at the bottom. This is caused by the buildup of gasses within the electrolytic eventually causing it to burst rather than exploding. Kind of like a safety valve.
I tested the value of both capacitors and the values were down to around 10 percent of their rated value. This would keep them from properly filtering out the switching transients (spikes) produced by the chopping action of the switching transistor and the subsequent collapsing of the magnetic field of the high-frequency transformer. There are other factors that come into play when an electrolytic capacitor ages, such as ESR (equivalent series resistance) but I will not get into that here.