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Low -cost Battery Pack Eliminator for GP88, GP68 and GP2000

by: djbida@90


Due to the development of 8ID UHF-Linked Repeater System, the capability of HH radios that has been the most effective way of short distance communication has improved and can now be used in communicating at greater distance. This innovation has situated our HH radios to be the most commonly used communication equipment between operating troops and higher Headquarters. In order to maximize the utilization of this improvement, a new test and experiment has been initiated. This newest innovation is focused on the power supply of our HH radios, and its aim is to strengthen/prolong the lifespan of our HH radios on its on-use and standby mode.

In all the electronic circuits you'll encounter, there will always be a dc power source, +3V, +6V, +9V, +12 V, etc. Without any power source, an electronic circuit is completely useless.

Abnormal low voltage, no power output of battery, and not charging are prevalent owing to bad weather condition, shortage of generated power, improper use of radios, improper charging, worn out, etc

Mobile radio communications equipment especially our Handheld radios such as Motorola GP68, GP88, and GP 2000, are powered by its own battery pack that is rechargeable. But should they be used for a longer period of time, let's say, during one week operations or longer in areas where there are no access to commercial power source for charging purposes, a car battery may also be used as a supply source along with a convenient circuitry that will reduce 12VDC to a lower voltage level safe enough to power on our handheld radios for proper operation.

The electronic circuit featured in this article "the low-cost battery pack eliminator" is the one that will do the task of converting any direct current voltage ranging from 9-24 VDC to a 7.4 VDC, a voltage level that is approximately equal to the voltage that can be measured from the terminal of a newly full-charged battery pack of our handheld radios. It has been designed to be entirely trouble free and maintenance free, as it is equipped with both reversed-polarity and short circuit protection; furthermore, specifications of its components are carefully investigated, tested and chosen for this purpose. So this innovation is indeed a dedicated one.


To have some understanding on the circuit operation, refer to Figure 1. The four diodes arranged in bridge configuration serve as a reversed-polarity protection. With this diode arrangement, it is immune to reversed polarity trouble and therefore will work even input DC lines are connected interchangeably. The LM7808 is a three terminal positive 8-volt regulator with 1 Ampere maximum current capacity. This regulator employs internal current limiting, thermal shutdown, and safe area compensation and is, as a consequence, essentially blow-out proof.

The current drain of Motorola HH radios is approximately 1.6 Amp when operating on high power. The LM7808 alone therefore cannot withstand the current requirement of these HH radios. In order to boost the current capacity of the circuit, transistor Q1 is connected to IC1 output terminal and serves as a series-pass transistor. The transistor Q1 used in the prototype circuit is C4242, an NPN transistor with maximum collector current of 8 Amp and a maximum collector power dissipation of 80 watts. These specifications of the transistor Q1 is therefore more than enough to support the current requirement of our HH radios and will operate properly without any danger of damaging it by excessive current.

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