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.

Feb 282013
 

led lights

Switched-mode power supply units are power supply devices of a modern type, they are highly efficient, small sized and have low weight.

switching_power_supply_psin10012Switched-mode power supplies rectify and stabilize the voltage needed to supply power to an appliance operated by DC, a LED lamp, LED rope light, CCTV cameras, promotional items or other devices. They rectify and correct the current pulse, clearing the interference in it. In technics they are also called key stabilizers whose main operating principle is to maintain the value of the output voltage constant.

Switched-mode units serve to convert the current with mains frequency (50/60Hz) into current with high frequency (50 kHz), which provides much greater power and efficiency. This feature is combined with small and compact size, and with far less weight than standard transformer solutions, which makes switched-mode power supplies very popular in everyday life.

original_36233

 Waterproof LED Driver

The size of a switched-mode power supply is determined by its power, and the higher the power is, the more massive the power supply is. These power supplies are produced with different primary and secondary voltages designed for powers from a few watts up to 1,000 W. It is sometimes more practical if a switched-mode power supply has more than one secondary voltage (usually 12 VDC and 24 VDC)

The efficiency of switched-mode power supplies in contemporary models can reach 93%. This in turn leads to reduction of losses in the form of heat dissipation in the environment. The more powerful and less precise switched-mode power supplies still produce heat, so they need a fan that provides them with the necessary cooling. Most modern switched-mode power supplies have overvoltage, overload and short circuit protection, which makes them a reliable and durable energy source.

original_31928 Waterproof LED power supply

One of the drawbacks of switched-mode power supply devices is that they emit a high-frequency signal, which may lead to electromagnetic interference in computer systems and precise devices (such as a pacemaker). Therefore, switched-mode power supply units have a metal casing, which absorbs the majority of these emissions, and the larger and more powerful ones also have an EMI filter.
According to the environment in which they operate, there are waterproof and non-waterproof switched-mode power supplies. Waterproof power supply units can be used to supply high power LEDs, LED rope lights and LED tapes, they have a waterproof casing and IP65degree of protection, which makes them suitable for outdoor installation. Non-waterproof power supply units are designed for indoor use and have perforated casing for better cooling. According to their implementation they can be mounted on a DIN rail or in a standard way.

original_26038

DIN rail switched-mode power supply

The massive influx of switched-mode power supplies in our everyday lives enhances the requirements towards their designers to reduce the size, weight and interference of these devices. Nowadays switched-mode power supplies are becoming more and more functional and ergonomic.

 

Jun 102012
 

One of the main problems that need to be resolved in the design and construction of systems for video-surveillance is unconditionally the provision of power supply and connection lines. This broad topic directly affects not only the source of voltage, together with the power wires, but also the different lightning-protection modules and grounding elements. Practice shows that the correct design of the power module of video -surveillance systems can help avoiding a lot of trouble in the process of operation of these systems.
The main and most common mistake made when designing and installing video-surveillance systems is trying to implement the project in the cheapest possible way on account of using inappropriate power supply units and connection lines. This sometimes happens in spite of the bitter experience from mistakes already made by constructors of these systems, connected with reducing the reliability and the protection against accidental events impacting negatively the equipment as a whole.

The dalliance of construction electricians regarding power supply requirements creates prerequisites for errors, leading to malfunctions and failures of the operation of different components of video-surveillance systems, but also to serious damage of the whole equipment. Very often power supply failure leads to damages of expensive cameras (as the one, shown in the picture) and other electronics.

kamera za videonabludenie

In fact, in 99.9% of the cases of circuit- technical decisions, in the terminal step of the cameras there isn’t any contemplated galvanic, optical or other separation of the power supply line from the line of the video signal.

kamera

This means that these errors in such circuits provoke damages in the video signal of recording and control systems.
As a result, a cheap power supply unit unconformable with the technical assignment leads to failure of the whole video-surveillance system. At present three types of circuits for implementation of the video-surveillance power supply source are most common: linear, pulse and standby.
Considering the first constructive solution we need to specify that its practical realization has to follow a precise proof test of the whole equipment. Such caution is justified by the fact that linear stabilizers are most sensitive to inductive interference and generally to interference in reactive forming elements and the various fluctuations in the power supply. The principle of operation of the linear stabilizer is based on reducing and straightening the input voltage. If we have 220 VAC at the input, which is converted to 12 VDC at the output and we have alteration of the input voltage to 235-240 VAC, then this can instantly damage the video-cameras and the recorder, and the presence of a fuse in the circuit would save the system only from a fire and it is quite wrong to over-estimate its security functions. We should also mention one more considerable drawback of the linear stabilizer – its little efficiency.

All disadvantages of linear stabilizers mentioned so far are completely excluded in pulse converters. The inductivity of the power supply unit, its cable length, the external HF interferences, the arcing contacts and the difference in potentials of various locations in the system have little impact on the power supply unit. By using pulsed power supply units we can build a system not only with high reliability but also with high price/quality ratio.

A standby power supply source is most often used in specialized sites where the video-surveillance system is powered directly by a standby battery or by other sources of alternative energy in order to prevent failure in the operation of cameras in case of mains voltage drop, often intentionally switched off by thieves or criminals. In this case the problems in the mains line are minimized and the only condition for the absence of trouble is the fuse in the electrical circuit.

 

Connecting line
In order to build connecting lines in video-surveillance systems, depending on the installation conditions, there are three main types of cables: two-wire cable, twisted pair cable and combined cable. The most common of these is the two-wire cable.

fror
This cable is suitable for powering video-surveillance systems because of its temperature parameters and its protection from various influences, although its parallel conductors can accumulate difference in the potential if the cable is used for especially long distances. When building a power line it is necessary to take into account the presence or absence of other nearby power conductors. We need to pay special attention to the inacceptable parallel position of power and low-voltage lines, otherwise quite strong interference may occur in the line connecting the cameras in case of switching the power lines. You should know that such two-wire cables in most cases are unshielded and unprotected from HR interference. For this reason, very long cables begin to function as an antenna and receive all external interference.
The twisted pair cable, in turn, is not intended for use as a power cable, but despite that it is often used by various assembly groups.

ftp

The main mistake is the use of the twisted pair cable both as power supply cable for the cameras and as signal transmitting cable, no matter if the signal is analog or digital. This leads to two main problems: First, the proximity of the wires, twisted around each other in pairs, has a very strong negative effect on the quality of the image and second – the small cross section of the conductor significantly limits the length of the line. This type of cable must be used very carefully in video-surveillance systems, because while designing the track of the system we must take into account the voltage drop depending on the length of the line. The use of a twisted pair cable is inappropriate when connecting a video camera with a coaxial cable designed for a load of 75 Ohm.

kabel za videonablyudenie

When choosing a cable we must consider the complexity of the site, the proper evaluation of the interference from external sources and the safety requirements. According to high standards for picture quality and safety, the video signal cable must be mandatory shielded, the power cable must have a section calculated with 30% reserve and must be made of quality copper.

 

Commutation
As practice has shown the most reliable connections are made through a quality solder or a tight connector. The conductors must be properly cleaned and tinned because over time the copper base of the cable gets oxidized which causes interruptions during the operation of video-surveillance systems. The higher the oxidation level is, the greater the voltage drop is at this point of connection and at high currents this may lead to arcing, heating of the wire, short circuit, and consequently – a fire. Many manufacturers provide an M-F terminal clamp, which is suitable for indoor installation, but is completely inappropriate when installed outdoors. Finally, we will add that despite the wide variety of systems for video-surveillance on the market, we must be extremely careful because very often behind the beautiful packaging there are conductors with very low content of copper in the wire and the beautiful golden contacts get rapidly oxidized by the moisture in the air. You should trust only established manufacturers and retailers who have responsible attitude towards customers and a good reputation, as is the case with Vikiwat Ltd.