YourITronics

The major disadvantages of usual linear power supplies are high power dissipation, the size and the appropriated weight. When looking for an alternative solution, I decided to use a switch mode power supply (SMPS). The efficiency of such power supplies is around 70 % to 90 % at a power density of 0.2 W / cm³. Because homebrewing was out of the question due to lack of time, I tried the modification of a PC switch mode power supply. The later are mass-produced goods and available for less than 50 DM.

Depending on the PC model, these are rated anywhere between 150 and 240 W. For supplying socket 7 main boards they have four different output voltages of +5 V, +12 V, -12 V and -5 V. They are mainly primary switching power supplies with power switches arranged in a half-bridge configuration. The outputs can drive the usual 20 A (+5 V), 8 A (+12 V) and 0,5 A (-12 V, -5 V). At approx. 205 W output power and a typical efficiency of 75 % this means a dissipation of only 68 W. I had acquired an unbranded PC power supply, measuring 140 x 100 x 50 mm (W, D, H) and weighing 350 g. Most power supply units are designed according to the same principle (half-bridge configuration) and hence the following described modification should be applicable also to power supplies from other producers.

Several protection circuits are included in the original power supply. Excessive primary current due to a very high secondary current leads to a high alternating voltage at the T3 output. If this voltage is above a fixed threshold the TL494 stops immediately generating cyclically pulses and changes to the intermitted mode (on / off). The circuit and the load are protected likewise against over-voltage at the +5 V output or short-circuit at the -12 V and -5 V outputs. Switching off is executed via H-signal to the IC1 protection input (pin 4) too.
If you see a KA7500 or IR3MO2 PWM regulator IC on the board, each one is a pin compatible second source to the TL494CN. IC3 is a dual comparator from LM339 type. Some power supplies are not equipped with this IC, but with a two transistor discrete monitoring circuit, offering the same functionality.

Get 13.8 V / 15 A out off a PC Power Supply 

[VIA]

This is a successor of the PIC16C71 4-digit LED f-counter & V-meter. Some hard to find parts used in the previous version, which are out of production for some time, has been omitted. A rather early PIC16C71 has also been replaced by 28-pin device PIC16F876. The later is capable of driving 4 digit LED display in multiplexed mode while measuring frequency, power supply voltage as well as handle two analog inputs to display SWR/PWR signal strength in a bargraph manner. There is no need for external LED display driver chip as well as external data EEPROM since it is already implemented in PIC16F876. Reduction in the number of used chips also results in smaller dimensions of the counter compared to its predecessor.

In operating mode a push-button allows the user to choose between the frequency, bargraph or supply voltage to be displayed. The frequency display mode can also be changed with longer (>1s) push-button pressing. For example if the frequency to be displayed is 14.065.9 MHz the user will see on the four digit display either “065.9″, “4.065″ or “14.06″. The default display mode after power-up can be changed in the set-up menu. The set-up menu is entered at power-up while holding the push-button pressed.

The production of Siemens’s (now Infineon’s) miniature 7 seg. LED HDN1077 displays in low current version (suffix O) was abandoned recently. Therefore I’ve designed another display PCBoard which suits the same f-counter base board but uses newer Agilent’s HDSP-U103 miniature 7 seg. LED displays. Their current consumption is even smaller. They need no more than 0,5mA per segment for acceptable brightness. Assuming that during normal operation in average only one half of the segments lights, the average current consumption of the counter is about 20mA. In power-save mode the consumption reduces to less than 10mA which is important in case of battery powered equipment.

The latest software version of the frequency counter is for radio-amateur and non-commercial use downloadable from this page for free. The PIC controller is in-circuit programmable if the MC34064 reset circuit is not soldered to the PCB. The instalation of this IC is strongly recommended after the frequency counter has been built and tested.
50 MHz frequency counter, voltage meter & SWR/PWR indicator
[VIA] - [Circuit Schematic]

Andrew writes: This is a bench power supply with regulated DC outputs -12V, -15V, +5V, +12V, +15V, and variable and one +35VDC unregulated output. I based this bench supply almost exclusively upon Andrew Kilpatrick’s design. I did add a few things, but it’s essentially the same and I am not writing here to claim any shred of originality. I spent somewhere around 80$ US on the project (including the GIANT transformer), which isn’t bad compared with commercial supplies boasting similar features. Please note that I DID find a few minor errors in Kilpatrick’s schematic, which have been fixed here.

The first part of the circuit is the rectifying/filtering board. The idea here is to rectify and smooth the positive and negative sides of the AC current from a large transformer (must be center-tapped) to produce +35V and -35V DC with a common ground. These two lines are then fed to regulators which are connected to the jacks on the front panel.

Since I had a bunch of small prototyping PCBs (cheap from Hong Kong with factory defects), I used two and split the circuit into two parts: the rectifying/filtering board and a regulating board. In the end, however, the regulators needed to dissipate more heat than I thought, so I threw out the second pcb and mounted all the regulators on a hefty Pentium II heatsink I had laying around.

The bases among the TO-220 regulators used in this project are connected to different sources, so I had to find a way to isolate them. In the end, I mounted the 78xx regulators directly to the heatsink with sheet metal screws and sandwiched the LM317 and the two 79xx’s with a metal bar (which I also mounted with screws) to the heatsink, isolating them with some heatsink pads I found in the power supply of a 1989 IBM PS/2 Model 70.

[Link] 

The U232 module provides conversion between rs-232 and usb interface. Its very versatile and can be added to existing projects. For example you can see in the pictures that the board previously had an RS-232 connector and after the U232 module was soldered the board has USB connectivity features. The USB port on the host device supplies all power for the module. Several models are available for both DB-9 and DB-25 connectors with long and hort footprints.
[LINK]

Member Spikenzie etched his own 8×8 LED matrix PCB. He’s also posted helpful instructions and artwork used in his process -

The 8×8 LED matrix is a building block. There is no processor or circuitry other then then LEDs and the copper traces. It is simply an eight by eight 64-LED matrix on a PCB. The 8×8 has 16 pins on one edge, 8 connect to the rows and the other 8 to the columns. This allows the maker to use their preference of controlling circuitry.

The way a matrix works can be a bit mysterious at first. Of course building one yourself is the best way to learn

[VIA] - [LINK]