Power Supply Design

For the project I'm working on, I need to design a power supply which results in an output voltage of 3.8V. I think I'm going to have the GE864 chip controlled by another micro in order to more easily control the timing required for power (turning the chip on and off requires a 1s pulse, restarting requires a 200ms pulse on a different pin).

Although I haven't selected the microcontroller I'm going to use yet, I have designed the circuit which will interface with its output, which corresponds to the GE864's input (ON or RESET pins). I have designed my power supply circuit with an input of +5V from the micro and an output of +3.8V going to the GE864.

Right now I'm getting power from an old PC power supply. I found the pinout for the main harness and switched the green and black wires in order to turn it on and off. Using my multimeter, I measured different output voltages between pairs of wires. Some were +12V, some were +5V. I decided to start with the +12V coming from one of the floppy connectors.

I ran power from the yellow wire on the floppy harness to the power block on my breadboard, and did the same with ground. To clean up the input some, I used a 100uF electrolytic capacitor. In order to bump down from +12V to +5V, I just used a 7805 +5VDC Voltage Regulator from Radio Shack (way overpriced, I know, but instant gratification). I chose to go this route instead of just using the +5V from the PC's power supply because I wanted a more accurate reading. Coming directly from the supply wouldn't have given me as close to +5V as bumping down with the regulator.

So now, I have a clean +5V to work with. But I need to get to +3.8V. For this, I decided to use the LM317T Adjustable Voltage Regulator, and I'd just design a circuit around it.

I just decided to use a good old-fashioned voltage divider. The LM317 was too tricky. The voltage divider was straightforward and since I was on campus, obtaining the parts was free, as opposed to  unnecessarily expensive.

Designing a circuit with my specifications was not difficult at all (Prof. Dragon is the MAN!). Here are my calculations:

Vin to the voltage divider was my +5V from the voltage regulator. Vout is the +3.8V that I need to power my chip. I selected a random value for R2 = 3k and just calculated R1 from the voltage divider formula.

Here's the schematic representing my circuit:

I used the 1N401 diode to protect against backwards current and the capacitors to filter the input and output. My theoretical output was 3.75. I actually measured 3.73, which should be fine for this

SO glad I got this working now! Next I can work on how I actually want to go about turning the chip on and off. I'm thinking of using a separate micro to send ON/OFF signals to the GE864 instead of doing it manually.

Project Schematic

I'm working on a project using the Telit GE864 chip, and have been having a hard time learning PCB design software so I can make the schematic for my circuit.

I went back and forth between PCB123, ExpressPCB, and CadSoft Eagle. Although PCB123 and ExpressPCB seem like fine products, I eventually settled on Eagle because the others are proprietary. If I designed my schematic and board with their software, I had to buy from them (I think one of them has a free pizza offer when you order a board, which was tempting, but didn't quite seal the deal). And, from what I understand, Eagle is used in industry, so I figured it would be good to get familiar with it anyway.

My trouble was, I had never designed a schematic around a chip before. The only experience I had was designing simple circuits in MicroCap with simple components (resistors, capacitors, diodes, power/ current sources, etc), but nothing with an actual chip.

I recognize "the cloud" as an invaluable resource for engineering/ programming help. I registered for the forums on Sparkfun. Within 30 minutes, I had a reply. Within an hour, I had my solution. Someone pointed me in the direction of the Sparkfun tutorials, which are immensely helpful. This one, in particular, described how to create a breakout board in Eagle. . . . which is EXACTLY what I needed to do.

I found that instead of creating the library for the chip I want to use, I could just download the one the geniuses at Sparkfun had already created. While this simplifies my task greatly, it still leaves a great deal to be learned, which I completely appreciate!

Now I'm researching more on the chip and deciding what type of power supply I want to use. I may just design schematics for all of the suggested supply circuits in the GE864's instruction manual, for more experience with Eagle.

I'm very excited right now!