1, instrument size
That is the size of the meter, which is a very basic problem. Digital display meter should be installed in the cabinet body. So, it is necessary to consider the overall coordination, big may not fit, too small not clear digital display. In addition, the large volume of instruments function expansion is strong, the same function price may will be more expensive, the small size of the instrument may expand the function of the poor. At present, the international standard size of digital display panels are mainly the following: 48 * 24mm; 48 * 48mm; 48 * 96mm; 72 * 72mm; 96 * 96mm; 96 * 48mm.
2, the display number
Which is directly related to the digital display meter measuring accuracy, generally speaking, display number is high, accurate measurement, prices are also more expensive, mainly the following: two (99, special); three (999, rarely); three semi (1999, ordinary digital display meter majority); four (9999, intelligent digital display table mainstream); four and a half (19999); three out of four (3999); five or more than five (common on the counter, the cumulative table and high-end instruments), the user can according to the requirements of the measurement accuracy to select several digital display table.
3, input signal
Refers to the direct input instrument of measuring signal, some industrial signal is directly connected to the measuring instrument, some signal is transformed after access instrument, must make clear the nature of signal measurement, or to buy the instrument can not be used, even damage instrument and the original equipment. To find out the type of signal: current or voltage, AC or DC, pulse signal or linear signal, etc., but also to clarify the size of the signal. Instrument and the name of the input signal is not the same concept, cite a few examples: the input signal is table (name of the current table 0-75mVdc current, input signal is a voltage signal, because the current through the shunt voltage signal). The input signal is 0-10Vdc tachometer (the name was a tachometer, input signal is the voltage meter, because the inverter will speed signal is converted into a voltage signal).
4, working power
All digital display tables are required to work, the main power supply of the digital display table: 220Vac; 110/220Vac; 85-265VAC/DC switching power supply, 24Vdc (general to order), 5Vdc (small panel table).
5, instrument function
Spectral selection of RS232 or RS485 output or price of instrument function is modular and can be selected, the instrument will function with different and somewhat difference, digital display meter mainly has the following optional function: alarm function and alarm output group number (i.e., relay output), feed the power output and the output voltage and power transmission output and transmission output type (4-20mA 0-10V), communication and communication mode and protocol is MODBUS or other protocol), for regulating and controlling instrument, optional features more specific to according to the manufacturer to choose a standard models, and manufacturers to communicate and confirm the correct to order.
6, a few important parameters should be concerned about: measurement accuracy values (smaller more accurate), response rate (value smaller faster response), working environment, temperature coefficient (value more small temperature influence smaller), overload capacity
7, special requirements
If users have special requirements should be put forward, to allow manufacturers to confirm whether meet the requirements, do not take it for granted, such as: IP rating, high temperature working conditions, the situation of strong interference, signal of special occasions, special operating mode, and so on.
The basic method of cooling and its calculation method
There are three kinds of heat transfer: heat transfer, heat exchange and heat radiation.
For transmission or direct heat conduction by physical contact with the hair of heat transfer heat transfer. The machine is not the same temperature of body or the body temperature of each part, divided into sub mutual transfer. Heat conduction and electrical flow concept is very similar, the heat is always from the temperature of high temperature and low heat conduction process in the thermal resistance of the as current flow process in electrical resistance. The thermal flow in RT is the thermal resistance and tau temperature difference. And heat resistance and Delta for conductor thickness, lambda is the thermal conductivity, a conductor cross-sectional area. This kind of, in switching power supply design, to the heating source of dissipation power rate, temperature rise of tau = Phi RT. by in actual application, the heat flow from the heat source out to of the heat radiator to to the several different material of the heat conductor, the deposit in the same thermal resistance on, in the calculation, total heat resistance for a plurality of heat resistance and.
2) of converter heat quantity to and it close in a fluid layer through the heat conduction and the fluid by heat, volume expansion, density becomes smaller, on the flow to the, surrounding a high density of fluid over-current to fill, fill fluid filled to the ceiling thermal expansion to the on flow, such as the cycle, from heat element is a surface take away heat quantity, the process called on flow and heat transfer. 1 – product flow, and a 1 – flow on heat transfer calculation generally uses Newton proposed the formula: Phi alpha = a (theta theta 2) [w], [the fluid contact surface area [m2] alpha heat line number, theta 1 for wall temperature [k] and theta 2 fluid average temperature. The visible and thermal flow diameter and the flow heat transfer coefficient alpha, Section A and solid surface and flow of the temperature difference (theta theta 2) multiplication positive ratio. The flow heat exchanger is a kind of complex heat transfer process, it not only decided to heat process, and decided to gas dynamic process. Simply speaking, influence on the flow and heat transfer factors have two aspects: (1) the fluid physical properties, such as density, viscosity, expansion coefficient, thermal conductivity, specific heat; (2) the fluid flow condition, is natural the flow is forced to flow, flow is turbulent flow is also layer. Because the layer flow, heat transfer and to rely on each other to do the flow between the heat conducting layer; and the turbulent flow, the wall close to the bottom layer of the outer layer flow, fluid production vortex intensified heat transfer. In general, it is the same one in the circumstances, the turbulent flow heat transfer coefficient than the layer flow heat transfer coefficient is several times, even more.
3) radiation heat shot caused by difference in temperature of electric magnetic wave propagation for radiation heat radiation. Its process than the thermal conduction and the convection complex much. It is the body part of the heat energy is converted into electric magnetic wave energy, can transfer the electric magnetic wave medium such as air, really cool one’s heels, to pass broadcast to, when you encounter the object, a divided by again transformed into heat energy, the remaining is anti shot back. On the surface of the table, at B) (sigma, namely the object scattered hair to the red line is the thermal radiation of a kind. In the real space or air, radiation to the radiation ability of phi, decided to matter, surface condition (such as color, rough degree), surface area and surface temperature. Phi epsilon = BA T14- T24 and the sigma is wave Er hereby Mann constant, the value of 5.67 x 10 8, a radiation surface deposition [m2] t two body surface of absolute temperature [k] and E as a surface black degree. Body color is deep, the more rough, radiation force is strong.
The three elements of electromagnetic interference of switching power supply are: disturbance source, transmission route and disturbed equipment. Therefore, the suppression of electromagnetic interference should also from these three aspects of the manpower, take appropriate measures. Should first restraining the disturbance source, the interference is eliminated; followed by eliminating interference source and coupling interference between devices and radiation, cut off the route of transmission of electromagnetic interference; the third is improving the anti-jamming ability of disturbance of equipment, reduce its sensitivity to noise. The current suppression of interference is basically the coupling between the electromagnetic disturbance and the disturbed device.. Common methods are shielded, grounded and filtered.
1) using the shielding technology can effectively inhibit the electromagnetic radiation interference of the switching power supply, that is, with good electrical conductivity of the electric field shielding, with high permeability of the magnetic field shielding. Shield has two purposes, one is to limit the internal radiation of electromagnetic energy leak. The second is to prevent external radiation interference in the inner region. The principle is the reflection, absorption and guidance of the electromagnetic energy by the shielding body.. In order to suppress the radiation produced by the switching power supply, electromagnetic disturbance influence on other electronic equipment, fully in accordance with the method of magnetic shield to process a shielding cover, and the shielding cover is connected with the casing of the system and as one can to the electromagnetic field of the effective shielding.
2) the so-called grounding is to establish a conduction passage between two points in order to connect the electronic equipment or components to a reference point called “ground”. Grounding is an important method for the suppression of electromagnetic interference in the switching power supply equipment, and the power supply can play a part in restraining the interference.. In circuit system design should follow the principle of “one point grounding, if the formation of multi point grounding, closed the ground loop, when the magnetic flux through the loop will produce magnetic induction noise. Is actually very difficult to achieve “grounding”. Therefore, to decrease the grounding impedance, eliminate influence of distributed capacitance and adopt planar or multipoint earthing, using a conductive plane as a reference, grounding of the nearest received the reference to needs. To further reduce the pressure drop of the grounding loop, the bypass capacitor can reduce the amplitude of the return current. In the low frequency and high frequency coexistence circuit system, the low frequency circuit, the high frequency circuit, the power circuit of the ground wire are separately connected, and then connected to the public reference point..
3) filtering is an effective method to inhibit the conduction interference, and it has a very important role in the design of electromagnetic compatibility of the equipment or system.. As an important unit of power line conducted interference suppression of EMI filter can be inhibited from interference from the power grid to the power supply itself against, can also inhibit and feed back to the grid in the interference generated by switching power supply. In the filter circuit, the dedicated a lot of filter elements, such as feedthrough capacitor, a three terminal capacitor, ferrite magnet ring bodies, they can improve the filtering characteristics of the circuit. Properly designed or selected filters, and properly installed and used filters, are important parts of the anti-jamming technology.
Select filter when you note the following:
(1) the frequency of the interference frequency and the interference frequency are clearly close to that of the two frequencies, and the frequency characteristic of the filter can be applied to the two frequencies.;
(2) guarantee that the filter can work reliably under high pressure;
(3) the filter is continuously energized at a maximum current rating when its temperature rise is lower, in order to ensure that the working performance of the components in the filter is not destroyed when the rated current is continuously working;
(4) for making the filter frequency characteristic of the work match the design value of the signal source impedance and the load impedance of the signal which is connected with it is equal to the specified value of the design:
(5) the filters must have shielding structure, the shielding box cover and the body to have good electrical contact, filter capacitor lead should be as short as possible, the best selection of short lead low inductance the feedthrough capacitance;
(6) have a high work reliability, because of the protection, electromagnetic interference filter, the fault often fault than the other components of the more difficult to find.
When installing the filter, note the following:
(1) the power supply line filter shall be located in the place where the power source is close to the power source, and do not allow the power cord to be left in the device frame without the filter;
(2) the capacitor lead in the filter should be as short as possible, so as to avoid lead inductance and capacitive reactance at low frequency resonance;
(3) the grounding wire of the filter has a large short-circuit current passing through, will cause additional electromagnetic radiation, so the filter element itself is good shielding and grounding;
Filter (4) input and output should not cross the line, otherwise it will crosstalk caused by the filter input and output capacitance coupling pathway, thereby reducing the filter characteristics, usually is input and output between the partition or shielding layer.