While working on a variable current load for some power supply testing I discovered that I had no medium size (T0-220) power transistors. I needed one to act as the pass element controlling load current. I did a quick look in my box of old electronic gadgets for scrounging and found several candidates which would surely have a power transistor or two. I had some old dead UPS units that surly had power transistors but just as I went for my screwdriver I noticed the inverter circuit from the desk lamp I modified into a camera swing arm in a previous project. I had saved the driver board for the florescent lamp, probably because it had two power transistors. I also found other interesting parts which prove that being a pack rat pays off.
In past scrounging articles I used a heat gun to remove parts, in this case I just used my normal soldering iron and a solder sucker. There were several parts of interest including a power resistor, several 5% resistors and two large caps. The primary goal was the power transistors but I removed everything of interest and tossed the rest in the trash.
The power transistors were a Motorola brand with a strange part number but I did not even have to look them up. I used my semiconductor checking gizmo to find the pin outs, type, and even the beta (current gain) of the two transistors. There were NPN, B C E, with a crappy beta of about 30. A good transistor would have a beta of over 100 but these would do. Also, they were not really in a T0-220 package but I just needed that could take a bit more current then a typical signal transistor so this was fine.
The two 22uF 200V electrolytic caps are a nice find, mostly due to the high voltage rating.
The power resistor was a great find as well. It’s a 1 ohm wire wound and is great as a current measuring shunt. I could have used a small 1 ohm resistor as the low resistance means little dissipation at my operating currents but the high wattage 1 ohm resistor will be handy in cases where I want to measure high currents.
The nice thing about a 1 ohm resistor for current measurements is that you get 1 volt drop per amp of current. It’s Ohm’s law, E=I/R with R = 1 then E = I. In my testing I just kept a DVM connected to this shunt resistor to read the current directly. Of course I could have put the DVM into current mode but then I would have to move the lead connections back and forth when I wanted to read voltages. Using this resistor, I could leave the meter in the safe voltage mode and move the measurement point around without concern for the fact that the meter is a short when it’s in current mode.
The moral of this story is keep that junk. I thought I was being a bit nuts to keep that PCB from the desk lamp. Now I can say for all us pack rats in the world. Go ahead and keep that junk, it will come in handy some day!