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Bench Power Supply Part 2

Jan
12

Now that I've got my specs figured out, it's time to start some high level design. This will allow me to get the layout of the power supply set before diving into the small details. Hopefully this will make the design process more efficient.

One of the biggest things that will affect this high-level design is one particular design specification. That is, the call for a switching knock-down stage. The reason I chose to include this is efficiency. Many lab power supplies I've seen out there have one thing in common: Many of them use linear regulators like the LM7805 or LM317. These are good devices, but they all have very low efficiency, especially when the dropout voltage is high. Enter switching regulators. Switching regulators can have very high efficiency (upwards of 95%) which allows for higher current handling, and less heat dissipation. However, they have a drawback. Switching regulators typically have more noise on their outputs. They may be OK for some circuitry, but this inherent noise will not do for the lab power supply I intend to build.

To get the best of both worlds, I plan to use both types of regulators in my design. The switching regulator will take care of most of the voltage dropout first, while leaving about 2-3 volts for the non-switching (a.k.a. linear) portion to drop second. This will reduce power dissipated in the non-switching section of the power supply, which has numerous advantages, including (hopefully) eliminating the need for a noisy fan, as I'd like to make this thing as small, quiet, and cool as possible. This would definitely not be possible without the switching section in front.

Now, it's time to make some initial part choices:
Parts List:

  • Linear Output transistor: P-Channel MOSFET IRF9540
  • Switching Regulator: LM2679-ADJ
  • Switching Regulator Inductor: Digikey# 553-1121-ND
  • Switching Regulator Capacitor: Digikey# P15372CT-ND
  • Current Sensor: ACS712

This should help lay the groundwork of the power supply. Next we'll look at putting in some control circuitry, including op-amps and so on...

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Bench Power Supply Part 1

Jan
11

Graduation is on the horizon, and I've spend too many years using wall-warts as my primary bench power supplies. I'm ready to finally build something I can be proud to have on my bench. So, I'm setting out to build a really high quality bench power supply.
Every good project starts with a list of goals and in this case that means setting the specifications for my power supply. I think it's good design practice to decide what you seek to accomplish before you spend too much time designing. So, without further delay, here are the initial specs that I'll be designing to.

Design Specifications:

  • Dual Floating Outputs
  • Adjustable Voltage, 0-30V, Steps of 10mV
  • Adjustable Constant Current 0-5A, Steps of 1mA
  • Soft output On/Off Switches (Default: OFF)
  • Output On/Off Indicator LEDs
  • OLED Text Display
  • Voltages/Currents set with single rotary encoder
  • Serial Computer Interface (Read/Set Voltage/Current)
  • High power efficiency, switching knock-down stage, regulated final stage
  • ICSP Header for firmware updates
  • Made from low cost parts

Now that the specs have been written down, I'll begin designing the circuitry. Stay tuned for part 2.

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Weather Station

May
29

I've been spending some of my spare time tinkering with the famous Arduino again. This time I've managed to build a network enabled weather station platform. It's pretty simple so far. It is currently only measuring temperature using an LM34 temperature sensor. It's just strung out my window, and is programmed to take a reading every 15 minutes.
The communication is done through an Ethernet Shield. This device sits on the Arduino and allows it to make basic HTML requests and pushes data to a PHP script on my website using the URL and the PHP _GET function. The data is then stored in a CSV file and displayed via a Google Graphing API. Here is a peek at that graph. BTW, this is live data you're seeing and the time is local time in Spokane, WA.

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Getting students on the air

Jan
05

Most hams will remember the first time they spoke on the air, weather it was on an HF radio of an elmer down the street, or a VHF handheld. I was in high school and got interested in the school radio station, KGHP, during my freshmen year. Leland Smith, a teacher, introduced me to the radio station after school one day, and the memory of fading two songs together for the first time was pretty cool.

Peninsula High School is one of very few high schools with a radio station these days. The cost of keeping these stations on the air isn't too attractive to most school districts, but the students, the school district, and members of the community have done a fantastic job at getting some support and funds to keep the station on the air. Spencer Abersold has been at the front of this effort to keep the lights burning at KGHP. Spencer and some of the students involved with the station have managed to get enough money recently to do a station re-model. It makes me happy to see a local radio station going so strong.

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More features for the repeater database

Jan
04

I've been spending a bit more time on the repeater database. It's evolved from a simple html table with data from various sources, to a full-blown database-driven system that supports user editing and has more features than you can shake a stick at. You might ask, "Are all these features necessary?" and the answer is no. I didn't do this to try and compete with some ham radio repeater websites. I just did it to learn about databases, and to have some fun, while getting a useful list of repeaters in the area. The database now has a google map for every repeater, as well as websites, and other information in every entry. It's probably overkill, but like I said, it was a learning experience.

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