An 8.5 Volt 1.5 Amp External Power Supply

This is a power supply circuit that I have recently wired out to obtain a regulated power supply voltage of 8.5 volts and 1.5 amps. The purpose of the power supply unit is to power up my VHF hand-held, which used to be run on built-in batteries. Now that I wanted to use my hand-held as a base device and also to run it for an extended period of time without the use of the default battery. So I have decided to use an external power supply for powering the rig. I have utilized a 12 volt 5 amp two-wheeler battery as the powering source for the circuit. The voltage requirement for the rig is 8.5 volts and 1.5 amperes current.

Because the current requirement is about 1.5 amps, and that it cannot be delivered by a 78 series IC voltage regulator, I have decided to use a 2N3055 NPN transister in series mode to deliver the power.. The voltage is taken out from the leads of the 12 volt DC battery. As it is DC voltage, no rectification is needed. The DC voltage is directly connected to a forward biased high power diode such as 1N4005 diodes in series to prevent any polarity reverse. The voltage out from the diode is filtered using a small capacitor of about 0.1 Mfd for eliminating any RF component in the circuit. Now the voltage is given to the collector of the 2N3055. The base of the 2N3055 transistor is regulated by the use of a 9 volt 7809 IC voltage regulator. The base lead is connected to a 100 Mfd capacitor to give a smooth base voltage to the transistor, which ensures the emitter current to be steady and smooth. The emitter voltage is further smoothed by the use of a 100 Mfd capacitor to maintain an adequate voltage when PTT is pushed on. A 0.1 Mfd capacitor is connected across the emitter and ground to bypass any RF signals that may cross into the power supply circuit. Now the output voltage is taken out from the emitter of the 2N3055 transistor, which gives an approximate voltage of 8.5 volts and 1.5 amps. An LED circuit is also given in the circuit which acts as a power indicator and also it facilitates to drain any charge that is present in the capacitors.

During operation of the rig, the current consumption will be higher and the 2N3055 transistor will get easily heated up. So it is important to give an adequate heat sink for the transistor. An extended coverage heat sink is given to dissipate the heat generated by the transistor. Output leads can be taken out from the output of the circuit to measure the voltage reading and also an ammeter is connected in series to give the current consumption of the rig. I have also included an external cooling fan to cool down the rig during long operations.

Please see the diagram and also the working circuit that I have constructed.

A Passive Field Strength Meter

Field strength meter has always been one of my favorite circuits that I loved to construct. This one is a passive field strength meter I had made recently. Field strength meter (FSM) is also called as the signal strength meter. Field strength meter when used in a radio receiver it is called as the S-meter. This instrument is used to measure the electromagnetic field around your place. The frond-end of this circuit can be modified to receive selectively whatever field that we are needed to measure. It can be tuned to the radio frequency spectrum to receive radio frequency signals that are around your place. It is a simple circuit to construct and also an useful one. The unit of measure of field strength is volts per meter.

The circuit consists of a tank circuit, a diode detector, and a digital readout meter. The input radio signal is collected by the use of a small piece of wire. A small coupling capacitor is used to sample a part of this energy to be fed to a tank circuit or a tuned circuit. The tank circuit consist of an LC, which is an inductor and a capacitor in parallel. The tank circuit can be tuned to any frequency range by changing the capacitor value or inductor value using variable inductor or variable capacitor. A combination of LC inductor and capacitor connected can also be used to select the different frequency ranges. The hot end of the the LC is connected to the detector diode at the anode end which is placed in forward direction. The diode acts as a detector to detect the incoming radio signals which is an  alternating current (AC) into direct current(DC). A second diode is also used in the reverse direction from the negative side for isolation and polarity.


The output direct current from the diode is filtered using a filtering capacitor of 0.01 microfarad. The RF component in the signal is further filtered using a radio frequency (RF) choke connected to the output of the diode (cathode side). This output of the radio frequency choke may be devoid of any high RF signals and further filtering may be done if necessary to relieve off any stray RF signals using a second bypass capacitor of 0.01 microfarad connected at the other end of the RF choke. The signal here is pure DC, which can be attenuated using a suitable resistor combination or a variable resistor to be fed to a microvoltmeter or to a digital multimeter. The readout of the signal strength is got from the digital multimeter with the selector switched to the microvolt range.
 
The FSM is mostly used for the purpose of detecting any powerful radio signals around the place and to determine the signal strength of the transmitter. It can also used for testing purposes in measuring the output of the oscillator, tune the IF stages of the transmitter, testing the output of the transmitter stages, and to tune the output of the transmitter power amplifier. Sometimes it can also be used to detect the mobile phone radiation from your hand-phone.