Jul 232013
 

Power supply sources are among the electronic devices that are most used in laboratories for testing various household and industrial appliances and equipment. A power supply unit can be defined as a source which supplies energy and power. Unlike industrial energy sources, such as hydroelectric power plants, heating or renewable energy, we are going to focus only on power supplies that are used for testing and evaluating the parameters of an appliance or an electronic circuit, and we are going to make a research of the possible development of the investigated object.
The initial division of power units can be done according to the type and specifications of their input and output signal:
AC input – DC output (adapters, desk power supplies for laboratories, chargers for batteries);
AC input – AC output (separating transformers, AC laboratory desk power supplies with amendable secondary voltage – automatic transformers, frequency inverters);
AC input – a combination of AC-DC outputs.
DC input – DC output (DC-DC converter);
DC input – AC output (inverter, generator).
We would like to clarify that the word „desk“ means not only easily transportable equipment, as several devices discussed below have very high weight, but also shows the purpose of use, i.e. the devices are used on desks in laboratory and engineering test configurations.

Adjustable AC-AC power supply unit

When testing electrical equipment powered by alternating voltage, it is often necessary to assess how the equipment will react when subjected to higher than the nominal voltage, and also when subjected to lower than the nominal voltage. Standard voltage variations are +/- 10%, but the deviation during actual operation can be greater if we connect a very powerful consumer to the same line, so it is important to test the equipment with voltages beyond its standard fluctuations. Sometimes an engineer-constructor in the process of designing a new device needs to test it with stress (peak) values ​​in order to eliminate the weaknesses occurring during the test, and thus to make it resistant to more extreme loads, thereby considerably extending its life.

AC-AC+DC sinhro

Laboratory power supplies with synchronous AC-DC output and AC output

For this purpose we need an AC power supply device with adjustable output voltage. Another possible use of the adjustable power supply unit is to see what happens with the electronics, when the test device is connected for a long time through a very long power cord (or extension), which has some voltage drop or the electrical circuit cannot maintain standard nominal values ​​of current and voltage. The latter is characteristic for consumers which are remote from the power substation, usually in holiday areas or suburbs. The electronic device is reached by voltage whose values ​​are lower than those which were set when the equipment was designed and this continues for a long time. An AC-AC power supply unit can operate similarly to test various devices before they are put on the conveyor.

The change of the output voltage in the power supply unit is accomplished by switching individual coils or through the use of an autotransformer. The typical front panel of a power supply unit usually has an ammeter, voltmeter and / or power meter, a commutator of different voltages or a rotary potentiometer for gradual voltage alternation. There are also sockets or terminals for several different channels or even some type of contact, a commutator between the channels and a power button for switching the unit. If necessary, the adjustable AC-AC power supply unit or one of its channels can be used for switching and gradually adjusting a soldering iron.

 Adjustable AC-DC power supply

An AC-DC power supply is the widespread adapter, also called a battery eliminator. This name describes their main function – to replace the batteries with the huge advantage that the power supply can change the value of the output DC voltage and is typically much more powerful than a universal adapter, for example. On its front panel there is usually a commutator of the main DC voltages, a switching button and sockets for connecting the DC consumer (e.g. for a banana plug). A special case of this power supply is a charger for a car battery, but it can also be used to power radios, car stereos, etc.

DC supply

Laboratory linear single-channel DC power supply with LED indication and gradual adjustment of current and voltage

One variation of this type of power supply unit is a power supply device which maintains constant (stabilized) voltage, irrespective of the magnitude and the alteration of the load (constant voltage supply). These devices are also adjustable, so their front panel is equipped not only with a voltage regulator but also with a voltmeter, and occasionally an ammeter, by which the voltage is set. They can have one or more single outputs, and an option to connect the device to a more precise measuring system. This unit can also work as a replacement of batteries (battery eliminator), if necessary.
The next version of AC-DC adjustable power supply is a power supply unit, the output of which is fed with constant voltage and constant current (constant voltage / constant current supply). These devices are very common and are present in all electrical laboratories, research centers, university laboratories, telecommunications and automation. They are made of electronic components and modules, selected at very high standards for quality and precision. The output provides stabilized output voltage, and stabilized output constant current and they can be adjusted gradually and independently of each other.

2chanel DC supply

Laboratory two-channel linear DC power supply

The source of  stabilized constant voltage and constant current is made of a step-down transformer, a rectifier and a filtering unit, a source of comparison voltage, a circuit for comparison of stabilized voltage and respectively stabilized current, an amplifier, a stabilizer, a circuit for DC measuring, etc. When the output voltage changes due to changes of the supply voltage or the current through the load, the alternating signal is compared with the comparison voltage by means of the scheme for measuring stabilized voltage, then the error signal is amplified by the amplifier for comparison and is fed to the amplifier as a control signal (positive feedback) in order to change the output voltage to a specified value. The amplifier for comparison consists of an operational amplifier with high amplification, so that it is able to make the stabilization even at small voltage changes. Devices of this type can be connected both in series and in parallel so that there are higher voltages or currents.

A variation of this laboratory linear DC power supply is when the device is equipped with two, three or more outputs (multiple output supply). This laboratory linear DC power supply can be programmable, as we will mention further in the text, its two channels can be connected in parallel and in series, depending on the purpose of measurement. Contemporary power supplies are equipped with LED screens, which display the parameters of current and voltage of both channels, buttons for gradual regulation of voltage and current of the front panel, protections against overloads and current rush, cooling system, etc. The more expensive power supply units also memorize the settings made in previous measurements, which is very useful during repetitive tests.

Thus we gradually come to the next category of power supplies – programmable linear DC power supply units.

These are DC power supplies, equipped with their own software, participating in a computer controlled system for testing or production. These laboratory devices work with different types of interfaces, the most common of which are: IEEE-488, also known as GPIB. Another popular interface is RS-232, and in the various units are used some network interfaces, e.g. Ethernet, USB interfaces, etc. These devices have detailed instructions regarding the interface, with which they work for the user’s facilitation. The computer connection is also useful because it gives the opportunity to download volt-ampere characteristics of the studied device easily and quickly.

In the next part of the article we will focus on how you can choose your required power supply, pulse power supply units, which are indispensable, brief summarized instructions on how to work with the most common power supplies, control of the technical characteristics of EFT, avoiding pulsation and extraneous noise, achieving the highest precision of a device, selecting operation mode, etc. We will look three-phase power supplies with high power and DC-DC power.

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