The last years I have been focusing on Arduino projects where a USB wall plug with 5V output is almost the only power supply that I needed. When I started a project a few months ago to generate audio waves with an Arduino I needed a dual power supply to build the analog amplifier step and had none.
So I searched the web and found some good deals on new and used lab power supplies. But they would have costed me quite some money, and I although a lab power supply is always useful, I felt it was overkill to buy a lab grade power supply. Maybe you are lucky and you can find a good deal on a used dual adjustable power supply, but that wasn't my case.
So, looking for a cheaper option, I realized that I had a bunch of used laptop chargers lying in a box ( I never throw used chargers away!). Surely I could use two of them to improvise a dual power supply, but I needed an adjustable one to experiment with different supply voltages. Since I wasn't in a hurry I thought about adding an analog regulator as the final step but looking around in the web I found some interesting switching adjustable step down converters that fitted the bill. The step down converters I found have an LCD display for tension and current can handle 4A and they only cost about $5 each, including postage!
And, both power supplies are adjustable completely independent. You can connect them to deliver +V/0/-V for an opamp setup or use them in completely separate circuits.
The limitation with the selected low-cost stepdown converters is that you cannot set an output current limit. If you need an adjustable current limit on your power supply, then you should find a different stepdown converter and can still use the laptop chargers as a first step to save space and money.
This project involves dangerous mains AC voltages. Only proceed if you are familiar with this type of wiring!
Parts you will need:
- 2 used laptop chargers with at least 15VDC output
- 2 step down converters (see #1 below)
- Suitable housing (see #2 below)
- On-Off switch
- Nylon screws and bolts
- Cable, connectors
#1 - I used are these stepdown converters.
#2 - The box I used was a Hammond Electronics RM2055M made of ABS and 190 x 140 x 50 mm (roughly 7.5" x 5.5" x 2"), check your local supplier for something similar. You can also use a metal case. Forced ventilation is not required.
- Assorted tools: metal hand saw, pliers, screwdrivers...
- Soldering iron
- It took me 2 nights to assemble the whole unit once I had received all the parts.
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Step 1: Preparing the Laptop Chargers
The two used laptop chargers I used were not too different and both had a 19VDC output rated at 4.5A. So even factoring in the voltage drop accross the converter I can easily get 15VDC out of them, and the stepdown converters can deliver up to 3A, more than enough for my needs (Photo 1 above)
To prepare the chargers, crefully strip them of their plastic housing. I held them in a bench screw and carefully used a metal hand saw to cut the housing in two. I didn't use a power tool in order not to cut too deep and damage the elecronics inside. Be careful not to damage the charger electronics!
The naked chargers look like photo 2 above.
Carefully unsolder the AC plug.
Strip the cable that goes to the laptop, being careful not to damage the insulated wires inside. Leave at least some 150mm (6") of wires exposed. You will need these wires to connect to the converters that will be situated at the front of the new housing.
My chargers have 3 output wires:
- Black: GND
- White: +19VDC
- Blue: Not needed. This 3rd cable is called "ID" and is used by the laptop to identify the type of charger so the manufacturer makes sure you are using the right one
Cut off the blue wire (or the corresponding one in your case) close to the PCB, taking care not letting the open end touch any other electronic parts.
Step 2: Prepare the New Housing
I bought a new housing from a local supplier, the housing itself is made by Hammond and is 190 x 140 x 50 mm (roughly 7.5" x 5.5" x 2") which I had previously estimated would be the minimum size that matched my components (Photos 1 & 2)
I suggest that before you buy a new housing, you strip your chargers, lay them on the table and estimate the size of housing to suit your needs.
The next step depends on the housing that you buy and your tooling and materials. In my case I replaced the front plate of the new housing with a transparent piece of plexiglas cut to the same size as the original cover. (Photo 3).
I marked and drilled the necessary holes. Again, this could vary in your situation.
The original chargers were sealed and did not have ventilation holes or slits. Nevertheless I drilled some holes for ventilation in the bottom and back cover of the housing - just in case. I would have preferred to cut some slits instead of making holes but didn't have the tools to do so.
I thought about applying paint to the inside of the transparent front panel, using a mask so only the LCD's would be visible, but I liked the tech look of a transparent front.
Step 3: Final Assembly
Place the two naked chargers in the new housing. In order to simplify the wiring place the chargers so that their AC and DC ends are on the same side (that is: AC ends together on one side, DC ends together on the other side).
Use hot glue to fix the chargers to the bottom of the housing, or use some brackets to hold them in place.
Please note that these laptop chargers were designed to work inside an insulated and sealed case. There is no guarantee that the heat sinks around the charges will be potential free, so make sure you follow the following
- Do not let the heat sinks touch each other. This might create a short when you power up the chargers or when you connect them to your circuit. The same holds if you use a metal housing: don't let the metal of the chargers contact each other through your metal housing; insulate them appropriately.
- Do not drill holes into the chargers to fix them with screws. You might damage some components or short the electronics inside the chargers.
- Do not deform the chargers (same reason)
To complete the electrical wiring do the following:
- Connect the output of charger #1 to the input of stepdown converter #1. Be careful to respect the polarities of the wires. Use a multimeter to check the polarity of the output wires.
- Repeat for charger and converter #2
- Fix the banana posts on the front cover.
- Connect the outputs of the two stepdown converters to the respective banana posts.
- Fix the stepdown converters using nylon screws and nuts. If you choose to use metal screws, be sure to use insulation washers so you don't short anything on the PCB's of the converters.
- Connect the AC wiring so that the AC switch can turn the whole unit on or off.
Close the box and begin to play :-)