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Three Power Supplies for the Garmin GPS-20 Sensor Board

TAPR has had great success in providing the Garmin GPS-20 Sensor Board to the Amateur community via group purchases helping keep costs low for experimenters.

There are various uses for the GPS-20, most notably for APRS and for the MIC-E and Totally Accurate Clock (TAC) projects. I am sure there will be more projects in the near future.

The GPS-20 requires +5VDC, plus or minus 5%. This equates to a range of +4.8VDC to +5.2VDC. Over several cycles of GPS-20 group purchases, some amateurs had problems providing an adaquate power supply resulting many times in the unit being destroyed. The following three methods are provided to help amateurs contruct power supplies for their units. These are:

Have fun with your GPS-20 and let us know what new and exciting projects you have constructed.

Note: Measure the output voltage of any power supply you build BEFORE attaching it to your GPS-20 and make sure it falls into this range. These power supplies are easy to construct and there's no rush. Double check your work and go slow.


Since the GPS-20 is an OEM (Original Equipment Manufacturer) product it does not come ready to use out of the box. There is no display, only a 12-pin JST connector that provides two RS-232 compatible serial ports, a One-Pulse-Per-Second (1PPS) output, and power. There is also a MCX style connector for the antenna connection. Included with the TAPR GPS-20 purchase is:

  • a JST 12-pin connector shell, pins, and wire to make the computer/power connection,
  • an RF pig tail (RG-174 coax) with a MCX connector on one end and non-connected on the other, and
  • documentation.

You supply the enclosure, antenna, and power supply. This article describes three candidate power supplies you can build for your GPS-20. The GPS-20 requires a well regulated +5VDC plus or minus 5% at 170 mA (typical). The GPS-20 documentation warns that any voltage transients, over voltages, or reverse voltages will damage the circuitry. Each power supply varies in complexity and purpose. Choose the one that fits your situation.

I am assuming that the voltage input for each of the power supplies is +12VDC such as from a power-cube type AC-to-DC adapter. Make sure the adapter can supply the necessary amount of current, usually 300 mA will do. The third design described below uses a Power Trends switching regular and is capable of using battery power from 4 AA batteries.

Take care in constructing these power supplies. Double check your work. Measure the output voltage before applying power to your GPS-20. And in all cases, observe polarity!

7805 Power Supply

The 7805 is a +5VDC three-terminal fixed-voltage regulator chip. It is a popular low-cost voltage regulator used in many projects and it is relatively easy to build a power supply with. The disadvantage of the 7805 is that it is a "lossy" regulator and therefore not very efficient. Thus it will not work well using AA batteries, for example.

The 7805 is commonly packaged in a TO-220 case and is rated at 1.0 amps, plenty for the GPS-20. However, in order for the 7805 to handle its designed rating requires a heatsink be installed on the 7805. Bare this in mind when you build this version of the power supply. If the 7805 will be in the open air it may be okay without a heatsink, but if you install it inside an enclosure, it will require one. If the enclosure is aluminum, simply attach the 7805 to the enclosure, it will act as a heatsink.

Figure 1 shows the schematic diagram for the voltage regulator. Construction is simple, you can use a project PCB board such as Radio Shack's Cat. No. 276-148 or construct it dead bug style. Keep the electrolytic capacitors close to the voltage regulator to avoid causing the 7805 to oscillate.

Figure 1 - 7805 Power Supply

The 7805 is available at most electronic parts suppliers. The Radio Shack part numbers for the 7805 power supply are:

  • 7805 Voltage Regulator: 276-1770
  • TO-220 Heat Sink: 276-1363
  • 10 uF, 35V Radial-lead Electrolytic Capacitor: 272-1025

LM317T Power Supply

The LM317T is an adjustable voltage regulator. It behaves and performs very much like the 7805 fixed-voltage regulator with the added benefit that it is adjustable. The LM317T is also a "lossy" regulator and therefore not very efficient.

This design is helpful if you get a finicky GPS-20 that wants just a little bit more than +5.0VDC to start up. With the LM317T you can adjust the regulator to output +5.1VDC. But be careful, you do not want to exceed the upper limit of +5.2VDC on the GPS-20.

Figure 2 shows the schematic for the LM317T power supply. The design is similar to the 7805 power supply. It has a few extra parts, most notably, the variable resistor R2. With R2 you can adjust the voltage output of the power supply.

Figure 2 - LM317T Power Supply

The LM317T is available at most electronic parts suppliers. The Radio Shack part numbers for the LM317 power supply are:

  • LM317T Voltage Regulator: 276-1778
  • TO-220 Heat Sink: 276-1363
  • 0.1 uF Monolithic Capacitor: 272-1069
  • 1.0 uF, 35V Tantalum Capacitor: 272-1434
  • 270 ohm, ¼ watt resistor: 271-1314
  • 4.7K ohm trimmer resistor: 271-281

Power Trends PT5100 Power Supply

The Power Trends PT5100 is a switching regulator. It is pin-for-pin compatible with the 7805 fixed-voltage regulator. However, its physical size is much larger than the 7805 and it costs quite a bit more. The advantage of the PT5100 is that it is more efficient voltage regulator, thus it is a good candidate for any application that requires batteries. I have built this power supply for my GPS-20 and it runs on four AA batteries. The above two power supplies were unable to operate on AA batteries.

Figure 3 shows the schematic for the PT5100 power supply. It is very similar to the 7805 power supply except that the voltage output capacitor is 100 uF vice 10 uF.

Figure 3 - Power Trends PT5100 Power Supply

The PT5100 is available from Digi Key (1-800-DIGIKEY) and the part number is PT5101A-ND. The Power Trends home page is located at http://www.powertrends.com/.

The Radio Shack part numbers for the remaining parts of the Power Trends PT5100 power supply are:

  • 10 uF, 35V Radial-lead Electrolytic Capacitor: 272-1025
  • 100 uF, 35V Radial-lead Electrolytic Capacitor: 272-1028


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