1985/01/21 Caterpillar

All Caterpillar Equipment With 24-Volt Electrical Systems

Converter Assembly

Most Caterpillar products (except D3, 910, and 931) use electrical systems of a nominal 24-Volt rating. Users sometimes wish to operate nominal 12-Volt devices, such as business-band radios, CB radios, tape players, etc. There are several ways to do this, including:

1. Center-tap the battery.

2. Using a dropping resistor.

3. Using a separate 12V battery, and recharge it at night.

4. Using an electronic 24V to 12V converter.

Of the above, 1, 2 and 3 are unsatisfactory, for the following reasons:

1. Center-tapping causes early battery failure and other potential problems; see Service Magazine, November 8, 1982, Page 7, "Do Not Center-Tap Batteries In 24-Volt System To Operate 12-Volt Accessories".

2. A dropping resistor would only work if the voltage and load were constant. The 12V devices mentioned above are not constant loads.

3. A separate battery is hard to mount and protect, and requires charging with an external battery charger at frequent intervals. Battery life will be short unless a special deep-cycling battery is used.

The 9G6981 Converter Assembly was designed to provide power to a 12V device. The converter also prevents damage to the vehicle's electrical system and to the 12V device.

9G6981 Converter Assembly.

Applications

The 9G6981 Converter Assembly can be used to power any nominal 12V device from a supply voltage between 18 and 32VDC, as long as the device requires less than 4 Amperes (up to 5A momentarily). If more current is required, two or more converters can be connected in parallel.

Design Features

The 9G6981 Converter Assembly is entirely solid state, with no moving parts. It is a switching regulator, which is more efficient (generates less heat) than the more common series-pass regulator. It contains circuitry which prevents damage caused by input voltage that is too high, reverse polarity, short-circuited output, high temperature, RF interference, etc. The circuitry is sealed into a black anodized heat sink, and connects to the machine through Sure-Seal connectors.

If an overload (more than 4 to 5A) is drawn from the converter assembly, it will shut off until the overload is removed protecting both itself and the load that is being powered. Likewise, if the converter assembly overheats, for example, by having a coat laid over it, it will shut off to protect itself.

Installation

The converter assembly should be mounted in a cool, dry place, preferably inside the cab. It should not be mounted in the engine compartment or where it will get wet.

The preferred mounting location is where there is free movement of air across the converter assembly, and where it is unlikely that a coat, hat or other obstruction will be hung or laid on the converter assembly.

Special Instruction Form SMHS7749 gives information concerning installation and connection of the converter assembly. The reference to a 10A fuse in this Instruction has misled some readers. The fuse is only to protect the wire between the fuse and the converter assembly. The converter assembly itself does not require a fuse. It is not true, as the Special Instruction states, that using a larger fuse will damage the converter assembly. The recommendation of a 10A fuse does not mean that the converter assembly will produce 10 Amps.

There are two cables coming from the converter assembly. One has a two-pin Sure-Seal Connector, and the other has a three-pin Sure Seal Connector. The two-pin connector is the 24V input, and the three-pin connector is the 12V output (one pin is not used). Use the 6V3000 Sure-Seal Repair kit to make mating connectors as required.

Two-Way Radio Applications

There are three general types of two-way radio equipment that might be used with the 9G6981 Converter Assembly:

1. CB Transceivers.

2. Low-power business-band tranceivers (up to 20 Watts RF output).

3. High-power business-band transceivers (over 20 Watts RF output).

Any legal CB transceiver can be powered with the 9G6981 Converter Assembly.

Low-power business-band transceivers can also be powered by the 9G6981 Converter Assembly, provided the input current requirement in the transmit mode is less than 4 to 5 Amps. Check the radio manual, or measure the current while actually transmitting. To measure the current accurately, use a 6V3030 or 6V7070 Meter on the 10A Scale, or the 6V4910 Clamp-on Ammeter. Connect the transceiver to a power supply and transmit into a 50-Ohm dummy load or into a correct antenna located at least six feet away from the meter. If a battery is used as the power supply, be sure the voltage stays above 12V during the test; if the supply voltage is slightly low, the current draw will be affected. Never transmit without an antenna or dummy load connected to the transceiver.

When checking the transmitter current draw specs, do not confuse the input current requirement given with the RF output power (given in watts). Be sure to use the input current requirement for the transmit mode (sometimes abreviated 'XMIT" in the radio literature), because the current requirement in the receive mode (RCVE) will be much less. In general, the input current requirement of a solid-state transceiver can be estimated by:

This gives an approximation only. Some transceivers are more or less efficient and may vary considerably from the formula given.

High-powered 12V business-band transceivers, in general, cannot be powered by one 9G6981 Converter Assembly unless they are detuned. Detuning involves intentional misadjustment of the coupling between the driver and final amplifier stages of the transmitter. This results in less RF power out as well as less input current.

NOTE: Internal adjustments to business-band transceivers should only be made by a qualified communications technic.

Detuning results in less power being transmitted, and in some applications, may result in unsatisfactory signal strength when the mobile unit is in a fringe area. Detuning does not affect the receiver of the mobile transceiver, and will not reduce the ability of the transceiver to receive weak signals. In most applications, mild detuning will not interfere with normal communications.

Reducing RF power output does not result in a proportionally-reduced signal strength. For example, cutting RF power in half reduces signal strength by 3.01 db, only slightly more than the minimum perceptible change which is about 2 db. Cutting RF power output by 20% reduces signal strength by 0.97 db, a change which cannot be disconcerned by the receiving station.

IF RF power output is reduced either by detuning or by using a smaller transmitter, the communications capability may be restored or even increased by the antenna and coaxial cable. Most business-band or governmental radio equipment will be in the VHF (148-174 MHz) or UHF 440-470 MHz) bands. Many antennas are available for mobile transceivers, but most of them are either 1/4 wave or 5/8 wave. They can be visually distinguished by their lengths.

Where possible, a 5/8 wave antenna is preferred because it amplifies the signals (both transmitted and received) approximately 3.2 db, compared to the 1/4 wave antenna. This is equivalent to increasing the RF power output of both stations by about 109%.

There are many manufacturers of good antennas. Larsen is one of them. The Antenna should be as high as possible. On top of the cab is preferred. Either the 1/4-wave or 5/8-wave antenna should be mounted on a large horizontal metal surface, because the surface actually becomes part of the antenna. If this is not possible (for example, on a fiberglass boat) special center-fed antennas are available. Vehicles with fiberglass roofs can use the regular mobile antenna if a large piece of sheet metal is glued to the underside of the roof to form a ground plane.

Coaxial cable should be of the best possible quality. The large RG8/U type is best but is seldom used in mobile applications because it is large in diameter and not very flexible. The cheap CB-type RG58/U is not recommended, especially if it has crimped (not soldered) fittings. A good choice is RG8/X. It is about the size of RG58/U but has qualities similar to RG8/U. Ten feet of poor coax can easily absorb 1.5 db of the signal. This results in a 29% reduction in RF output power.

Correct antenna choice and installation can more than compensate for as much as a 50% reduction in output power. They will also improve the receiver's performance. This may enable the use of a 9G6981 Converter Assembly where it would otherwise be too small.

In some cases, a 24V-input transceiver may be available. This, of course, eliminates the need for any converter at all. It reduces the user's flexibility, because the radio cannot be switched to a 12V vehicle.

Competitive Voltage Converters

Some dealers have observed that converters are available at a lower price than 9G6981.

Some competitive converters work on a different principle. They are series-pass regulators. These converters can have any or all of the following shortcomings:

1. Failure of the pass transistor(s) can result in the full 24V system voltage being applied to the 12V load, probably destroying it.

2. The converter is susceptible to failure caused by high input voltage, voltage spikes, reverse polarity, etc.

3. The converter is not sealed and is susceptible to failure due to dirt or water.

4. The converter will not withstand vibration.

5. A strong RF field, such as that produced by the transmitter, may cause the converter to turn on and destroy the transceiver with high voltage.

6. The converter may emit RF noise and interfere with radio reception.

Caterpillar has not found a competitive converter that has a favorable price/value relationship.

Reliability

The field reliability of the 9G6981 Converter Assembly has been excellent. Only a few isolated failures have been confirmed. Most of the converters returned by dealers actually have nothing wrong with them. If a 9G6981 Converter Assembly appears not to work, check the following:

1. Make sure 24V is being applied to the input of the converter while the load is connected to the output.

2. Make sure the converter is connected correctly.

3. Make sure the input current to the load does not exceed the rating of the converter. If in doubt, measure it with an ammeter while powering it with another 12V source, such as a battery.

4. If the converter is very hot to the touch, make sure that air is free to circulate around the converter, and that it is not in a hot location (such as the engine compartment or in front of the heater outlet).