Power Harnesses

The power harness for each vehicle that will be tracked by the Fremont area hams should have a power harness that safely provides sufficient current to the radios for proper operation, even at high power settings, without causing voltage lags for the GPS and TNC equipment.

We use red & black twin lead for the power leads, as a safety feature, so that anyone connecting equipment can be sure that they have the polarity correct before they connect the equipment. (Some groups use the same size polarized connector, but use a reversed polarity to the specification we use here.)

For simplicity, we will rely on a single point of contact to the power source (item #1 below). We prefer using large spade lugs for side-post vehicle battery terminals, but we have found a wide variety of terminal sizes and access among the rental vans we have equipped, as well as the local vehicles we use to test our trackers before the event. The main power harness (item #2 below) provides power to the voice radio, as well as to the APRS tracking package. The final power splitter (item #3) is contained within the tracker package itself, providing power to all the devices in the package.

The tracker package gets the heavier (#12 AWG wire) because some trackers will be equipped with two radios, a single high-power radio, and additional equipment.

Connecting to the Power Source (item #1)

The harness is divided into three components, so that you can connect the harness to power by a variety of methods. For example, you could use large spring-clamps (with insulated sleeves, of course), or large crimp-on ring-lugs. You could even use a high-current cigarette lighter plug, although a lighter connection typically isn't meant to deliver a large amount of current. (However, you could use a high-current rectifier diode and a gel-cell battery connected to the power line could supply the transmit current you need.)

Make sure that your power leads are fused on both the positive and negative leads. That is because it is possible for damage to com in through the antenna, seeking ground, rather than through the positive lead. By fusing both leads, you are taking extra precautions to protect your loaner equipment.

While this section of the harness does not need to be very long (less than 2 feet), the fuses should be as close to the power source as possible. This will help reduce the chance of shorting the power source without fuses.

Make sure you include some spare fuses in your grab-and-go kit, to replace the fuses in your power harness, as well as any fuses in your equipment.

For Baker to Vegas 1997, we are planning to use ring-lugs, anticipating that most of the vehicles will have side-post battery terminals. However, we'll also have a few heavy spring-clamps as well, for the vehicles that will have top-post battery terminals. We're planning to connect to the vehicle battery, to ensure that the harness has sufficient current for all the equipment.

The Power Harness (item #2)

The length of 17 feet (shown in the illustration) is enough to get from the battery into the vehicle. If you are preparing a harness for a specific vehicle, you should measure twice before you cut!

The 12 feet from the power source (item #1) should get us well into the passenger compartment. There is still five feet to get to the voice radio, and five feet to get to the APRS package.

Soldering needs to be done well on the junctions, to ensure good current flow. Insulation will be electrical tape on each lead (because you cannot get heatshrink around individual leads), and then heat shrink over BOTH of the taped junctions.

Start with a 17' (minimum) length of 12-gauge red and black 'zip cord', and a 5' length of 18-gauge read and black 'zip cord'. Take each cord, and separate the red and black leads for 1.5" on each end. (This sort of forms a "Y" on each end of the cable.)

1) On the 12-gauge cord, strip 1/4" of insulation from both leads on one end of the cord
2) solder a female molex pin to red and black leads on the leads you stripped in step 1
3) strip 1/4" of insulation from the other end of the #12 cord, and solder male pins there
4) Mark the 12-gauge cord, approx. 5' from the female molex pins installed in step 1
5) cut off 1/4" of insulation from each lead, approx. 5' from the pins installed in step 1
6) at this mark, separate the red and black leads, for a length of 5 inches (see illustration)

7) As in the illustration, remove 1/2" of insulation from the red and black leads (not adjacent!)
8) bare 1/2" of wire from both leads on one end of each #18 cord
9) lay the #18 red wire left-to-right, with stripped end to your right.
10) lay the #12 wire, left-to-right, next to the #18 lead, with the female pins to the left
11) match the exposed wire on both red leads, side by side, and twist the wires as shown below.

12) solder the wires together. Get a good solder flow, but don't burn the insulation
13) wrap electrical tape around the solder joint, using at least three layers on the wrap
14) fold back the 18-gauge black lead to match the exposed 18-gauge black lead

15) solder the wires together. Get a good solder flow, but don't burn the insulation
16) wrap electrical tape around the solder joint, using at least three layers on the wrap
17) slide a 4-inch piece of heat shrink up the 18-gauge cord, from the end with the male pins.
18) cover the electrical tape and junctions with the heat shrink, and shrink it down
19) add an extra layer of heat shrink on top of the first, is you think it is needed...
20) solder female molex pins to the remaining ends of the 18-gauge cord
21) install the male molex pin pairs into a FEMALE molex connector (red lead in the 'pointy' end).
22) install the female molex pin pairs into MALE molex connectors (red lead in the 'pointy' end).

23) Finally, admire your handywork! Nicely done!

The Power Splitter (item #3)

Each vehicle had a GPS Package. Not every vehicle had a voice operator. As a result, we created a power harness that ran power from the vehicles battery, using gauge red/black zip-cord. This cable terminated in the vehicle in a "Y", with two of the molex jacks. One of these jacks was for the GPS Package, and the other was for a voice radio (if installed).

Inside each package, we then needed to split the power from this single jack on the power harness to provide power for a number of devices in the package. This was done with a Splitter Harness, to provide three power connectors within the package, sourced by a single power connection coming into the box.

The use of the 12-gauge wire on the splitter provides enough current for a high power transmitter, if needed. The two 18-gauge leads provide ample current for the TNC and GPS devices.(If you use pass-through power taps, as we have, be sure that they are connected to one of the 18-gauge leads. If you connect those taps between the radio and the harness, you may limit the current available to your transceiver, and the transmitter may cause a voltage drop for the TNC or GPS unit, which may then cause erratic operation!)

The power connectors used are Molex(tm) "standard series, 0.093".
The MOLEX part numbers are;
03-09-2021 (Plug) and 03-09-1027 (Receptacle)
02-09-1119 female terminal (18-24 gauge) and 02-09-2118 male terminal (18-24 gauge)
02-09-1104 female terminal (14-20 gauge) and 02-09-2103 male terminal (14-20 gauge)
1545PRT is a package set, with parts for (3) complete sets of connectors (18-24 gauge).

We have found it cheaper to buy the individual components in quantity (10- or 25-packs) of the plug and receptacle, and bulk bags of the terminals. Our local source is Minute Man Electronics in Fremont, but they are available at many other outlets and mail-order houses as well.

Because of the currents needed for the transmitters, it's very important that you use good assembly techniques. That means;

  • soldering all connections (good solder joints, please!)
  • avoid exposing bare leads on the connectors and splitters
  • using at least one layer of heatshrink on the junctions, and preferably two, for strength.

Building the Power Splitter

I offer these step-by-step instructions for how the power junctions between the gauge and 18-gauge leads should be made. The male connection will be "on the right" in the instructions below. Junctions will be made "to the left", to allow us to use heatshrink on all the junctions. As a result, we will be building "from the left". We will insert the pins into the molex connector shells at the very end of the process. The harness should look something like this.

Start with a 9-inch section of 12-gauge red and black 'zip cord', and two 4-inch sections of 18-gauge red and black 'zip cord'. Separate the red and black wires, so you have six individual wires.

1) strip 1/4" of insulation from one end of each #12 lead and solder each lead to a female molex pin
2) cut off 1/4" of insulation from each lead, approx. 2" from the pins installed in step 1
3) bare 1/2" of wire from one end of each #18 red and black lead
4) lay one #18 red wire left-to-right, with stripped end to your right.
5) lay the #12 wire, left-to-right, next to the #18 lead, with the soldered pin to the left
6) match the exposed wire on both leads, side by side, and twist the wires as shown below.

7) solder the wires together. Get a good solder flow, but don't burn the insulation
8) slide on one layer of heatshrink from the right, covering the exposed joint, and shrink it
9) add an additional layer of heatshrink, if you like...
10) follow steps 4-9 again, for the black lead...
11) cut off 1/4" of insulation from each lead, approx. 3" from the pins installed in step 1
12) follow steps 4-10, to add the second red and black leads, remember to put on heatshrink!
13) strip 1/4" from the remaining ends of the #18 and #12 wires
14) solder female pins to the ends of each of the #18 leads
15) solder MALE pins to the remaining ends of the #12 wire
16) wait for the pins to cool from the soldering
17) insert the FEMALE PIN pairs into a MALE JACK shell (red wire on the 'pointy' side).
18) insert the MALE PIN pairs into a FEMALE JACK shell (red wire on the 'pointy' side).

19) admire your handywork! Nicely done!


Copyright 1996-2002, David K. Z. Harris, N6UOW
Questions? Comments? Additions? Email b2v at baker2vegas.org

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