For charging different laptops I use an universal laptop charger ( 230V AC -> 12-22V DC ). It features a USB port which I have no use for. What I could really use is a current meter, permanently connected to the charger, for laptop diagnostic purposes. One could really benefit from knowing laptop’s current consumption during different stages of startup. Connecting the amper or current meter to the power supply is easy, the issue is the current meter operates on a 9V battery which drains pretty quickly if you use it a lot. Solution would be to power the current meter from the same power supply instead of the battery. It could be powered from the unused USB port, but since the port outputs 5V, output needs to be converted to 9v. Of course that output could only be used for powering devices with low current consumption. By calculating the maximum power delivered by a typical USB step-down converter ( 0.5A * 5V = 2.5W ) we can then, by inverting the power formula, conclude the highest output current would be 2.5W / 9V = 0.28A. 0.28A is sufficient current to power a simple current meter without over-stressing the buck converter.
Tools and materials:
- Phillips screwdriver size #1
- Sharp ESD safe tweezers
- Soldering iron with a chisel tip
- Scrap board for harvesting the suitable SMD resistors from it
- Datasheet for a particular step-down / buck converter
The whole process would have been far too easy if a schematic diagram was available for the mentioned laptop charger. We need to locate the step-down converter without a “road-map” by visually examining the board. It should be near the USB port, have some small resistors around it and an electrolytic capacitor in the vicinity. When the mentioned step-down converter is located, its datasheet needs to be acquired. Datasheet gives invaluable information about the chip such as pin-out, maximum ratings, formula for calculating required resistance… It’s usually obtainable by googling the numbers and letters written on the chip, together with the word datasheet. Datasheet for MC34063A is available here.
I have gathered all the needed information for the particular step-down converter (MC34063A), and presented it in the picture below.
After locating the comparator pin, in my case it is pin number 5, we need to find the two resistors (marked R1 and R2) connected to this pin. This could prove difficult without the schematics diagram for the particular board, but with the help of a multimeter in a continuity check mode we can probe the resistors nearest to the comparator pin. Resistors with the lowest resistance relative to the comparator pin are the resistors we are looking for, and are connected directly to the mentioned pin.
Note the codes written on each of the two resistors and use this handy resistor value calculator. When we obtain the resistors’ values from the calculator, they can be used in the formula for adjusting the output voltage (written in the datasheet and in the picture above) to confirm they are the ones we are looking for. In my case the R2 has a value of 3.3K ohm and R1 is a 1K ohm. If I take the formula for adjusting the output voltage ( Vout = 1.25 * (1+ R2/R1) ), for this particular step-down converter, and put in the values from the two resistors, I get 5.375 = 1.25 * (1+ 3.3K/1K). I can conclude the buck converter is configured to output around 5.3V to the USB port. With the use of the multimeter I can confirm the voltage present at the VCC pin on the USB port matches the voltage I got from the equation, which means I’m targeting the right resistors. How do I make the step-down converter to produce 9 instead of the 5V? By inverting the formula and then replacing the existing resistor with the one carrying the value I get from the formula. I could replace any one of the two mentioned resistors to get 9V output, but for the sake of simplicity, I only replaced the R2 resistor since R1 has a really nice round value of 1K ohm. A simple equation inversion for calculating the needed resistor value is described in the picture below.
As per my calculations, I needed to find a 6.2K Ohm resistor. In my case a small surface mount (SMD) resistor would be suitable. I used one of my scrap boards to harvest the needed resistor from. Best tool for the job is the soldering iron with a chisel tip or a hot air wand.
Then I replaced the existing R2 3.3K Ohm resistor with the harvested 6.2K Ohm resistor. I could use any SMD resistor with the resistance value close to 6.2K Ohm but it’s advisable to do a re-calculation, when deciding on a resistor not exactly matching the needed resistance value, to see how it would affect the output voltage.
As a finishing step, I soldered a positive wire from the current meter to the USB slot’s VCC line and ground wire to ground line. For a safety measure, I filled the USB slot with hot glue. This is to prevent myself accidentally connecting an USB powered device to it in the future, since 9V would surely damage the device connected.
What about the current meter, is it working?
No, not really. The current meter I tried to do this with, can’t work as a current measuring device for the same circuit it is powered from, it would need to be isolated to work. Therefore, I went back to the 9V battery. I have ordered a current meter capable of measuring the same power supply, but since it can be powered from as low as 4.5V, it really defeats the whole purpose of this 5 – 9V conversion.
Word of caution
Electronics repair is not difficult but it requires a lot of patience, calmness, focus, a steady hand and some common sense. When in doubt or when you do not fully understand something, educate yourself first by googling, before following the step in question. If you do not feel competent enough to follow a given procedure it is best to seek help from an expert technician in your area. I cannot be held responsible for any damage caused to yourself, your equipment or to the device you are repairing as a result of following this tutorial.
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