Published:2011/8/8 22:34:00 Author:Li xiao na From:SeekIC
The faithful companion of every electronics fan is his laboratory power supply. It’s always ready when you need it, it can provide the right voltage for every project, and it’s just about indestructible. The fact that a lab supply is part of the standard equipment for every workstation is demonstrated by the enormous number of products available. In this article, we examine a selection of lab supplies, describe the various options and features, and point out what you should look for when buying a power supply.
The following pages present brief descriptions of no less than 31 laboratory power supplies. What all of them have in common is that they convert ac mains voltage into a clean, well-regulated dc voltage. At least that’s the intention. It won’t come as any surprise that not all of them can do this equally well, as you can see from the descriptions.
However, capacity is not the only thing that matters in choosing a suitable power supply, although it naturally forms the starting point.
Power
Nowadays, besides conventional (linear) power supplies, there are also various units available that use switched-mode conversion or a combination of these two conversion methods. Switched-mode power supplies have the advantage that they can provide higher efficiency at lower weight for the same amount of output power, since the necessary transformer is much smaller. Particularly with relatively high-power supplies, this can represent a considerable advantage.
However, before buying o power supply you should determine how much capacity the supply you’re looking for should have, in terms of both voltage and current. If you only need to occasionally power relatively simple circuits, a voltage range of 0-12 V is often more than adequate, and a current rating of 1 A may cover all your needs. However, the required current rating can be quite different if you’re working with (fast) digital circuits. Although such circuits often operate at fairly low voltages, the amount of current you need can quickly rise to impressive values. Naturally, the same is true for people who need a power supply specifically for working with car equipment. On the other hand, if you use the power supply for developing audio amplifiers, you will probably need high voltages as well as high currents. It’s a good idea to consider this question carefully before you buy, since regardless of whether it’s a linear supply or a switched-mode supply, you naturally have do reach deeper in your pocket to get more power.
Bells and whistles
Perhaps you would rather spend the extra money on additional features, such as a second output to provide a negative voltage. That can be quite handy for powering circuits from a balanced supply (such as op-amp designs). In this case it’s convenient if the two channels can be coupled (tracking mode), so both voltages can be set using a single knob.
There are also a few other important considerations and things to look for. Fine-adjustment controls for voltage and current, for example, are not a matter of course. If it’s necessary to be able to precisely adjust these parameters for your applications, we recommend selecting a power supply with digital controls for these adjustments. Digital knobs are more convenient for making fine adjustments, since there’s no mechanical limit on their range of adjustment. With an analogue potentiometer, at some point you simply run out of range, and then you have to resort to the coarse-adjustment knob. Of course, conventional potentiometers have the advantage of providing continuous adjustment, instead of using discrete increments like their digital counterparts. Incidentally, a ten-turn potentiometer is also a reasonable alternative, although it has the drawback that if a relatively large adjustment is necessary, you literally have to twiddle your fingers for o while before you get the voltage or current you want.
For adjusting the settings, you’ll naturally need a voltmeter and an ammeter. All the supplies we examined here have this measurement capability on board. Most of them even have separate meters for voltage and current. The test team’s preference is for LED displays, since, they are generally easier to read than LC displays and can be read more quickly than moving-coil meters.
Remote control
It’s also possible to make settings and read values remotely. The simplest method is to use an analogue volt¬age (or resistance) to control the output voltage. Other possible interfaces include RS232, USB, and of course the professional General Purpose Interface Bus (GPIB).
Test engineers: Arjon Floris Marcel Kcer.ders
Project editor: David Daamen
Project support: Susette de Leeuw
Some of the power supplies also perform the actual regulation at a distance. Such units have separate ’sense’ terminals that are used to measure the voltage to be regulated at the load instead of at the supply output.
Reprinted Url Of This Article: http://www.seekic.com/blog/project_solutions/2011/08/08/Programming_with_word__write_Visual_Basic_programs_using_MS_Word_(2).html
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