I had a serial GPS mouse lying around (Thanks Alex) and my boss was kind enough to give me a retired HP Ipaq 2210 pocket PC from work. The 2 combined and I had a pretty reasonable touch screen car navigation system. Only problem was a power supply.
I didn’t want to use a cigarette lighter adaptor because I would have wires going everywhere; I wanted it hard-wired so the the GPS mouse sits on the dash with the cable disappearing down behind and a single thin cable coming out from the centre console for the Ipaq. I also wanted the GPS mouse running full time so there was no delay when it was searching for the satellites but I only wanted the IPAQ to be powered when the ignition was on.
I made 3 attempts before I was successful. First was 2 LM7805 5volt regulators. These got way too hot even with a heatsink; I would have needed a huge heatsink. Second attempt was the contents of 2 cigarette lighter to USB adaptors supposedly able to deliver 1AMP; yeah right! These things just about burst in to flames when I turned on the IPAQ! The third and successful attempt uses a LT1074 switching regulator and is detailed below.
The LT1074 was provided as a sample from Linear Technology, which is great since they cost about $NZ40 to order from RS!
I couldn’t find exact matches for all the components in the reference schematic.
Here’s a list of parts I used
C1: Electrolytic 470uF (25v)
C2: Green Cap 0.01uF (This was a guess! All I really new is that it wasn’t an electrolytic because the schematic shows no polarity symbols!)
C3: Low ESR electrolytic 220uF (25v) (The application noste AN35 said to use low ESR and place it very close to the the LT1074).
MBR6745: This a SCHOTTKY-BARRIER RECTIFIER DIODE the recommended MBR745 is rated at 7.5Amps I used an ERC81-004 rated at 3Amps. Robbed from an old dot matrix printer PSU.
R1:I couldn’t find 2.8K @1% so I used 2x 5.6K @1% in parallel; both 1/2watt.
R2:2.2K @1%; 1/2 watt
R3:2.7k @5%; 1/2 watt
L1: This is of unknown value; robbed from an old dot matrix printer PSU. The application notes AN35 describe a rather humerus “alternate” method of selecting and inductor: (Click to read)
I haven’t done any PCB etching since high school so I thought I’d give it a go. I used Eagle CAD PCB design software which allows boards 100mmx80mm to be designed using their freeware version. Eagle is a bit clunky and counter-intuitive but once you get the hang of it, it’s really very good.
I followed the laser printer method for my etching as described in this instructables article.
That’s it! A bit of shame I don’t really understand how it works! It does the job though and with very little heat.