Power Supplies
One thing that I started building early during my time as an electronics enthusiast were power supplies. With transistorized radios and cassette recorders it was a pain to be buying batteries all the time. Rechargeable batteries were available but expensive. Also remembered that the GE Nicad Charger had a hinged cover because there was a *chance* of a explosive rupture of the cell during charge, yikes, that cover didn't inspire confidence.
I can remember in high school getting directed by the librarian to a bunch of defunct wall warts in the library garbage can. Then it was just a matter of taking them home, figuring out how to open them up, changing the rectifiers and filter caps and adding 3 terminal regulators. It was fun and I actually learned a lot.
Here is a variable supply I made later out of an old Knight KG-240 tube amp:
Now in the boring 20s of the 21st century power supplies are everywhere. The 60 Hz transformer wall-wart has been replaced by switching supplies. Every compact fluorescent or LED bulb is basically just a power supply with a lighting element. Laptops and computers that are long obsolete usually still have working power supplies. And of course there are USB chargers from phones everywhere.
So why build your own when commercial supplies are cheaper/smaller/more reliable ? I have to admit that I've bought some power supplies recently. First I tried an MFJ switcher which was nice and compact with meters. But eventually the fan noise got me. Next I bought an old Heathit HP-1144 power supply (meant for the SB-104). But the HP-1144 wasn't meant for modern rigs with their high surge currents at powerup - had a lot of trouble with nuisance fuse blows. Finally ended up with an Astron linear supply which other than some hum works fine. However I do intend to build my own 12 volt, 20A supply "real soon now".
Linear power supplies are relatively simple: enclosure, switch, connectors, transformer, rectifier, filter cap and some kind of regulator circuit with appropriate heat sinking.
Here is the power supply I built when I got the Elecraft K2 rig.
This supply is linear, built into a soldered together PCB box with a screw on PCB lid. I purchased the toroidal power transformer from Digi-Key. The circuit is fairly simple with no current limiting or crowbar circuitry. It uses a low-dropout design with a MOSFET as the pass transistor. As I recall bypass caps were added across the rectifier diodes to minimize hum when using the supply with direct conversion receivers.
It worked fairly well except for one thing: it wouldn't start-up under the K2 load. So I'd have to turn on the supply first with the K2 off and then separately turn on K2. This wasn't a huge problem and I actually do the 2 step process with the Astron and Yaesu. But I eventually came up with the idea of bridging the transistor with a relatively high resistance of 4.7k ohms. This apparently gave enough drive at powerup to the voltage divider on the output to allow the supply to turn on. My Dad could do this kind of fix with small engines, just a small adjustment with his thumbnail on some tiny screw on the engine and the roto-tiller would roar to life on the next pull. I wish he was still here.
BTW, you do keep a lab notebook don't you ? Somewhere to write your ideas, your experiences, your purchases. A place to document your projects. I got the idea of writing the date on the project from W7ZOI probably in EMRFD (Experimental Methods in RF Design). Then it's just a matter of consulting the notebook for the schematic and notes. Maybe I'll put up a blog post on notebooks in the future.
Best Regards,
Chuck, WB9KZY
http://wb9kzy.com/ham.htm