Elenco XP-720K

I recently built the Elenco XP-720K, a bench variable DC power supply that comes in kit form. This is my mini-review of that kit and the process.

My first impression of the kit was of the quality. For a relatively small device it had a lot of weight. The metal case for the power supply is from thick metal, not cheap and flimsy. The painting on the case is very nice, and all the components for the inside are of good quality as well. The instructions are clear and easy to follow. All and all it was a fun kit to build.

As I built the kit I didn't photographically document every step of the way, but if you want to check out pictures of someone else building the kit go here.

I did make several changes or "mods" to the design.

1. I replaced the provided potentiometers with some Bourns 10-turn pots (3590S-2-202L). With more turns I have greater control over accuracy. I can easily dial the output power to within 2 thousandths of a volt. I could have added some "fine-tuning" pots in series with these pots for greater accuracy, but I don't need that level of accuracy.
   
2. In addition to the new pots, I used metal knurled knobs from Kilo (OEJL-63-4-5).
   
3. I covered the connections with the voltage regulators in heat shrink tubing to prevent shorts.
   
4. Instead of the provided capacitors, I ordered new high-quality low-ESR Panasonic brand capacitors rated for power supply use. With these high-quality caps the power supply should run for a long time.
   

Here are some pictures of the changes I made to the original design.






If I had to do this over I think there is only one change I would make. The new knobs I used were 5/8" in diameter. I think I would use larger 3/4" or even 1" diameter.

Even though I'm very pleased with this kit, I can think of some improvements I would suggest to Elenco.

• The traces and the pads on the circuit board are too large. I suspect this is to make it easier for a beginner to solder, but it actually does the opposite. Because the there is so much copper on the board, it takes longer to heat the copper to get the solder to flow properly. So this actually makes it harder to solder.
  
• Soldering the 12 wires onto the voltage regulators was not the easiest. If there were some sort of clip to slide onto the regulators, or if the regulators were soldered onto a second board with a ribbon cable connector between them, it would make assembly easier would help in reliability.
  
• Use bridge rectifiers instead of 4 individual diodes. Fewer solder connections and would be quicker and easier.
  
• Offer a model that includes a built-in display for voltage. Always having to use a multimeter to dial-in the voltage will get old fast.
  
• Offer a model that is based around the switch-mode power supply design versus the linear design of the XP-720K. Sure it would cost more, but it would be more efficient and would be even more fun to build.
  

The Elenco XP-720K is a great kit and a useful product once complete. If you find yourself in need of a variable DC power supply, consider this product (or the smaller XP-15K).