Power supplies for communications servicing
The choice of a power supply for servicing communication equipment is important_particularly the dc power supplies that power mobile and portable equipment on the test bench. The power supply must provide a clean, well-regulated dc voltage, and it must be immune to strong RF fields. Otherwise, RF transmitting equipment can seriously impair the operation of the power supply. A good bench supply must meet some important parameters.
Regulation Two kinds of regulation apply to power supplies. One is the load regulation, which refers to the power supply’s ability to maintain a constant output voltage under widely varying load current demands. The other is the ability of the power supply to maintain a constant output voltage under varying ac power line or source voltage.
* Load regulation _ A well-regulated power supply is a must. The current demand from transceivers will vary with the mode of operation. In the receive standby mode, with the receiver squelched, the unit may draw a few milliamperes. In the receive mode, with the receiver receiving an on-channel signal and producing full audio output power, the unit may require a few hundred to several hundred milliamperes, depending on the audio output power rating of the receiver and the efficiency of the receiver design.
When the transmitter is keyed, the current requirement could be 10A-20A or more depending on the transmitter power and the efficiency of the transmitter design. In spite of the widely varying load demands, the power supply voltage should remain constant. The degree of regulation is usually described as a percentage. The formula for defining the percentage of regulation of a power supply is:
% Regulation = (Vnl – Vfl/Vfl) x 100
where Vnl is the no-load voltage Vfl is the full-load voltage. For example, if a power supply is set to an output voltage of 13.6V under no-load conditions and then drops to 13.1V under full-load conditions, the percentage of regulation of the power supply is:
(13.6 -13.1/13.1) x 100 = (0.5/13.1) x 100 = 0.0382 x 100 = 3/82%
Typical load regulation should be no greater than 1% on a good-quality power supply. Much lower values are possible with higher-quality dc supplies.
* Line regulation _ Suppose that a power supply is rated at 1% regulation over an ac input line voltage of 110V-120V. This should be measured at some fixed value of load current, say 50% of the rated load current. If the power supply voltage at 50% load current is 13.5V, then this voltage should not vary more than 0.135V as the line voltage changes from 110V to 120V. Typical equipment ratings for power line voltages are 105Vac-125Vac.
Ripple Ripple on the dc output voltage should be kept to a low value. Otherwise, the ripple can adversely affect the equipment you are testing and lead you to believe the problem is in the equipment when it is actually in the power supply. Any hum or extraneous noise introduced by the power supply must be negligible in comparison with the hum or noise present in the equipment under test. Ripple from the dc supply can be measured using an oscilloscope. Ripple levels of 5mV-10mV should cause no harm in troubleshooting communications equipment. Don’t accept ripple levels much greater than this. Much lower ripple levels are common with higher-quality equipment. Lower-current supplies usually are accompanied by lower ripple levels. Ripple specification is usually stated at the continuous load rating of the power supply.
Voltage adjustment A voltage adjustment allows the power supply to be operated over a range of voltage to meet the requirements of different equipment. Generally, it is desirable to operate mobile communications equipment requiring 6A-16A at 13.6V. This is spelled out in EIA standards. However, when testing a transceiver, it is often desirable to run the voltage above and below the standard test voltage to flush out voltage-related problems within the equipment. Generally, equipment can be operated at a voltage 15% above or below the standard test voltage without causing harm to the equipment. Be careful not to exceed the maximum dc supply rating of the equipment. You may cause more trouble than you had at the beginning. Most supplies designed for servicing 12V equipment are adjustable up to 15V-16V, so it is not possible to exceed the safe dc supply rating of the equipment.
Metering It is highly desirable to have separate metering for both the voltage level and the current level of the power supply. Although analog meters are less expensive, the digital meters are much more accurate, probably 10 times more accurate than the analog type. For example, the specification for an analog meter might be 65% while the specification for the digital meter might be 60.5%. However, analog current meters are sometimes more advantageous. If you are using a power supply with the analog metering, it is a simple matter to use a more accurate digital voltmeter to set the output voltage to the proper point and then mark the scale of the analog meter at the proper reference point for the standard test voltage.
Safety features Safety features such as overvoltage protection and current limiting are important in order to protect the equipment being serviced or tested. Imagine what would happen if the power supply voltage suddenly increased beyond the acceptable input level of the equipment under test. The equipment would be likely to suffer serious damage from the over-voltage condition before the power could be removed. Over-voltage conditions are often handled by a crowbar circuit that blows the power supply fuse when the output voltage exceeds a preset threshold. It happens in the blink of an eye, preferably before any damage to the device under test can occur.
Current limiting is another important feature of the power supply. Generally, the current limiting control is adjusted to supply the needed amount of current to the device under test, but not much more. If the current requirement of the DUT exceeds this level, then the power supply goes into a foldback condition and limits the current to the preset level.
RF immunity Power supplies that are used in servicing communication equipment such as transmitters must be able to operate in an RF field without adversely affecting the operation of the power supply. This is one of the most important factors when choosing a power supply suitable for testing communication equipment. Here, the specifications are rather arbitrary. For example, a specification might read “RFI resistant” or “No deterioration of performance when power supply is subjected to an RF field”. Exact field intensity levels, such as microvolts per meter, would allow better comparison of power supplies for the RF immunity factor.
Duty cycle There are two specifications concerning duty cycle_continuous duty and intermittent duty. Continuous duty is easy to understand. It is virtually all the time. For example, if a power supply is rated at 20A continuous duty it can be operated at a load current of 20A 24 hours per day, seven days per week.
On the other hand, if a power supply is rated at 30A intermittent duty, what does “intermittent” mean? This must be spelled out in the manufacturer’s specifications. For example, one manufacturer states “transmit duty 20 minutes, 20% duty.” This indicates that the supply can deliver at the intermittent level for four minutes out of a 20-minute period.
There is also the ICS duty. This stands for intermittent communications service, where the intermittent service is on a 50% duty cycle or five minutes on and five minutes off. For example, a power supply may be rated for 15A continuous duty or 20A ICS.
Summary Sometimes, not as much care is put into the choice of the bench power supply as the other test equipment on the bench. The power supply should be one of the most important considerations of all the bench test equipment. It is used in virtually every test procedure requiring dc power on the bench. If you are working from a poor-quality dc bench supply, you are asking for trouble. Shortcomings with the power supply can lead you on a wild-goose chase. The trouble might appear to be in the equipment when the power supply is the real culprit. Good dc power supplies are not terribly expensive, considering that they are the workhorses of the test bench. Carefully compare manufacturers’ specifications and get the best power supply your budget will allow.
Until next time*stay tuned!
Kinley, a certified electronics technician, is regional communications manager, South Carolina Forestry Commission, Spartanburg, SC. He is a member of the Radio Club of America. He is the author of Standard Radio Communications Manual: With Instrumentation and Testing Techniques, which is available for direct purchase. Write to 204 Tanglewylde Drive, Spartanburg, SC 29301. Kinley’s email address is [email protected]