This and That

Now that I had a basic design to prototype, I needed to collect all of the parts to test out the basic power supply circuit.

First iteration - variable power supply

Austin does not have many options when it comes to electronics parts stores.  Up north you have Frys Electronics.  They have a pretty decent selection of components ( mostly NTE ), project boxes, wire, switches, supplies, tools and what not.  I’m not sure how long they will continue stocking this gear, though, as their real bread and butter seems to be consumer electronics – computers, stereos, tv’s, etc.  None the less, I found most of what I needed to complete a simple breadboard test of the supply.  Here’s a short list of what I used to put everything together.  The prices are from memory, so the accuracy is sure to be lacking. 

• NTE 956 – $2 – adjustable positive voltage regulator, 1.5A, 1.2-37 VDC
• NTE 5312 – $4 – 8A single phase bridge rectifier, up to 70VAC input
• 1000 μF electrolytic capacitor, 50VDC – $4.  ( Always remember to derate your caps.  50% is the rule of thumb, so technically I shouldn’t run this power supply over 25V.  If you want to run all the way up to 37V, get 75V or better caps. )
• 0.1 μF electrolytic capacitor, 50VDC – $0.50
• 1 μF electrolytic capacitor, 50VDC- $0.50
• 10 turn, 5k Ω potentiometer – $12 ( This was a high quality, audio grade pot ).  I went with the 10 turn for precision voltage selection.  Any 5k pot will work, just remember that the shorter the wiper travel, the harder it will be to set your voltage to exactly what you need.
• A knob to fit the pot – $2

I had a breadboard and all the resistors I could ever want in the lab at home, so I didn’t need to buy them.  I probably also could have avoided buying the smaller caps, but I wanted to make sure I had the exact sizes.

With loot on hand, I carefully put the circuit down and double-checked every connection.  You might be wondering about some of the other parts listed in the schematic that I didn’t buy.  Since I was just testing at this point, I didn’t invest in a switch or a fuse, figuring that I would simply connect or disconnect a fused power source to verify the design.

Speaking of power sources, I looked for a decent transformer but couldn’t find one that would give me the voltage/amperage combination I sought.  Optimally, a transformer wound for a 120 VAC primary with a 36 VAC secondary rated for at least 60 VA ( or watts ) would allow me to run the circuit at full capacity – up to the maximum voltage and amperage the regulator can handle.  Check out an online giant like Mouser to find the perfect transformer for the job.

Since I couldn’t find one that matched those specs, I dug through my wall wart collection and found an old printer supply that output 28 VDC and could push 1A.  VDC?!  The schematic calls for a VAC input!  No big deal.  You can connect VDC or VAC to a bridge rectifier and it will correctly provide the positive and negative potentials at it’s outputs regardless of how you hook up the input.  Just check the schematic if you don’t believe me.    Having the rectifier in place gives my power supply the ability to handle direct or alternating current input.

With everything checked on the breadboard, I hooked up the wall wart and did the first smoke test.  Nothing smoked, popped or fried!  Good sign!  Hooking my Fluke to the output, I verified a roughly 1.2VDC signal on the output.  Dialing up the pot, I watched the voltage climb up and up, eventually topping out at about 30 VDC.  Success!

Next up, conversion to dual variable output, dissipation consideration and the beginning plans for a permanent power supply system.