DC power supply specifications

- the specifications and points to note when buying a DC power supply (PSU) for an electronics laboratory whether it is a 12 V DC power supply, a 5 V DC power supply or any other voltage.

A lab DC power supply is an essential requirement for any electronics test or development laboratory. The DC power supply will be used to power circuits or boards that do not have their own power supplies. In this way it is possible to run up, test and develop a variety of circuits under the required conditions.

DC power supplies are used in a variety of applications part from their use within the laboratory. Any circuit that is not powered by a battery will need a power supply to enable it to be powered from the mains. These power supplies normally have a fixed voltage and their requirements can be well specified for the particular applications. Often values such as a 12 V DC power supply or a 5 V DC power supply may be used to supply power rails on equipment.

For a power supply to be used in a development or test work, the specifications needs to be such that it can provide power for a variety of circuits and under different conditions. The power supply may need to have a variable output voltage, variable current limit, and the noise and ripple limits will need to meet the requirements.


Choosing power supplies

Power supply design and specification may appear to be straightforward at first sight, but this is not necessarily so. The choice may not be obvious because there are many manufacturers offering a variety of different DC power supply products. Choosing the right supply for any given application can be difficult. But fortunately there are some basic rules that can help in the choice of a supply.


Basic specifications

When choosing any DC power supply for lab or bench use it is necessary to carefully consider the requirements for the current application, and any future ones that can be foreseen. The requirements can then be compared to the various DC power supplies that are available. A careful check should be made to ensure that all the crucial requirements have been met. Output voltage and current are obviously the most important. Often the overall power that a range of power supplies may be quoted in watts. A 10 amp 30 volt supply has an output power of 300 watts.

It is necessary to look at other specifications like the output ripple, as well as the line and load regulation. Ripple is important especially for switching regulators as described later. Load and line regulation figures are also important. Typically figures for these should be a few millivolts or less than 0.1% of the maximum output voltage.

In some instances it may also be necessary to take account of input specifications. Many DC power supplies are auto-ranging and can accommodate US / UK mains voltages. However, larger supplies may not be auto-ranging, and could require manual switching. A few may be able to accommodate only limited input ranges. Accordingly it is wise to check the supply is suitable for the area where it will be used.

Remember to read the small print in the specifications. There have been examples where some small open frame DC power supplies do not like leads of only a few metres or capacitive loads, some even breaking into oscillation. Look carefully at the whole specification because limitations like these are normally hidden away in the small print.


Allow sufficient capacity

One of the major problems that can occur is allowing insufficient capacity in the power supply. Often additional requirements and facilities may be required and these quickly consume power. Sometimes the current consumption for a board or circuit can rise as much as 50%. In view of this and the fact that the power supply will be used for other projects, it is wise to allow an ample margin for increased current consumption.


Fixed or variable?

The answer to this point depends to a very large degree on the application. For applications such as powering a large logic board it is often best not to have a variable DC power supply. If the output control were knocked or moved then it could have disastrous effects on the board. Even comparatively small changes can destroy chips, especially the low voltage logic chips that are widely used today.

For applications where supplies may need to be used on a variety of rails, or where the supply voltage does need varying, then a variable voltage DC power supply is obviously needed.


Is remote sensing required?

In some applications it may be necessary to have long wire runs. If the current drawn is reasonably high, then it is possible for the voltage drops along the lines from the power supply to the item being powered to be significant. For example, when drawing a current of five amps, a resistance of only 0.1 ohms will cause a voltage drop of 1/2 volt. This level of voltage drop can be crucial for an item such as a logic board. Even when smaller voltage drops are present they can still cause problems because an accurately controlled voltage is usually required at the unit under test. These voltage drops could easily result in false failures being detected.

To prevent this happening, power supplies with remote sensing capabilities should always be used at the outset. These power supplies use four wires running to the load. Two to supply the power and the remaining two act as the sense lines so that the DC power supply regulates the voltage at the point where the power is being delivered. Thus voltage drops along the line need not be taken into account.

Fortunately most lab or bench DC power supplies have an option whereby remote sensing can be used. Typically the supply and sense connections are lined at the power supply itself, but there is always the option to break the link and sense the voltage at the point where it is delivered.


Beware switching regulators for some applications

Switching power supplies are ideal for many applications but some may have high levels of voltage spikes on the output arising from the switching action in the supply. By their nature the spikes are short lived and their energy content is small. Accordingly, when ripple measurements in root mean square values (rms) are given they may not appear large. However, the peak-to-peak values of ripple give a more relevant picture. Like any form of ripple, once the spikes enter the item being powered, false levels or AC measurements may be detected and spurious failures may be reported. It is also possible these spikes may cause the unit under test to malfunction if they are large.

Fortunately, many switching supplies perform very well. Manufacturers use techniques such as zero-transition switching that reduce the levels of spikes to single figures of millivolts. These levels are more than adequate for most applications. However, where very low levels of ripple are essential a linear DC power supply is often best, although it will be larger, consume more power and dissipate more heat.


Summary

DC power supplies are a standard piece of equipment used in any electronics development or repair laboratory. Often they are taken for granted, and sometimes little thought may be placed into their purchase. However a analysing the requirements enables the right DC power supply to be bought for the given application. Additionally, good supplies, well matched to their requirements and from reputable suppliers will provide many years of trouble free service.

By Ian Poole

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