Simple, resistor based Dummy Load

Another afternoon project. Some time ago I was working on a 80 watts 12-to-36 Volts DC-DC boost converter. Not one of my most successful projects but anyway.

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So I needed some kind of load but my home made constant current dummy load can only handle 20-something volts. A few 100 ohms 15 watt resistors were just what I needed. So I took 6 of them and made a simple, single-sided PCB that holds the resistors as well as 6 switches.

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The resistors are upright and have a bit of an air gap between them. That’s important, they get incredibly hot with 36 volts accross them. I know, I had to learn the hard way 😉

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Together with the aluminium front it has a nice weight for its size and with it’s silicone feet it sits firmly on the bench.

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Variable Voltage Power Supply using a LM317

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A classic afternoon project. I was in need of a variable voltage and didn’t have a proper lab power supply available. But I did have a solid 12 volts from an old computer PSU. So I built myself this little thing.

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It’s really nothing more than a LM317 in a TO220 package with a pot, two capacitors and banana jacks. It measures about 65x55mm and has rubber feet so it sits nicely on the bench. All the parts I found laying around here. The little heat sink can handle 5 to 10 watts without getting overly hot.

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Over a short period of time you can burn quite a bit more so you can draw up to 1.5 Amps (the LM317’s internal current limit) from anywhere between 1.25 and 10 volts when connecting it to a 12V supply.

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Xilinx Prototyping Board

Some time ago I wanted to try out programming CPLDs in VHDL. I was entirely new to both of those topics and I didn’t have a real need for a CPLD at the time. So I built myself a nice little prototyping board for the Xilinx XC9500XL complete with some push buttons, LEDs a 7-segment display and a PIC16F688 used mainly as a clock source.

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As you can see, I’ve used a socketed PLCC44 package. The CPLD comes in two versions: XC9536XL with 36 macro cells and XC9572XL macro cells. 36 cells won’t allow you to do very much I had to soon find out and even 72 can be quite limiting depending on what you are planning to do. Prices don’t differ that much so I recommend you get the 9572 straight away.

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Notice that I’m using the 3.3V version in this project. There is a 5V version of these chips as well and many of the DIY projects you find on the web use them. But they are increasingly hard to find and might cost several times more. When I was doing this project, Farnell was selling them around $15 in small quantities compared to maybe $4 for the 3.3V version.

The board is powered from anywhere in the range of 4 – 16V so you can easily power it from USB. All the CPLD pins are accessible from the 100mil headers you on the top and the bottom of the board. On the right you can see the programming headers for the PIC and the CPLD, respectively. I’ve also included a few 3.3V ‘outlets’ that I can use to power other boards connected to it.

Here’s a list of related projects:

http://bitcycle.org/electronics/1st_CPLD_project/

http://hackaday.com/2008/12/11/how-to-programmable-logic-devices-cpld/

http://startingelectronics.com/projects/xilinx-CPLD-board/

http://dangerousprototypes.com/docs/XC9500XL_CPLD_breakout_board