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Scrounging for Power Transistors

BoardAndPartsWhile 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.

LampModule

PartsThe 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!

Posted in Scrounging, Scrounging Parts.


3 Responses

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  1. dfowler says

    Joseph,

    You need to ask the guys that make the gizmo. I could not give you that even if I had it.

  2. Joseph says

    May i have a shematic diagram of “semiconductor checking gizmo”, I realy want to buy a kit but a big note says “Sales to U.S. and Canada Only” and i am in Philippines
    so Please send me a copy in my e-mail adress

  3. Berni says

    I like to keep old circuit boards too, you never know when you will need some part off it.

    Oh and that semiconductor testing thingy looks nice, makes me want to make one.Hmmm or even beter! Implant one in the digital scope im makeing