20-11-2012, 02:39 PM
AUTOMATIC CURRENT CONTROL
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PROJECT DESCRIPTION
The project Current detector cum controller is very much useful for controlling the load in any industry. In this project we are measuring the current consumed by all the loads connected in a house. If the load current exceed the set value of current the load will be disconnected immediately. The heart of the project is microcontroller AT89S51 and current sensing transformer. The current sensing transformer is used to sense the current consumed by load. The current sensed by the current sensor is converted into voltage and feed to the ADC0804 for analog to digital conversion. The digital equivalent of the current is read by microcontroller AT89S51 from the ADC0804. The digital value of current is processed my microcontroller and displayed on LCD. We have provided a 16x2 LCD display for displaying the value of load current and set current. For changing the value of set current there are two keys called UP/DOWN keys. The UP/DOWN Keys can be used to increase or decrease the value of set current. One key is provided to reset the load supply after an over current trip. Five different loads are connected for testing purpose. The load supply can be can be switched ON/OFF through a relay controlled by microcontroller. We have used 5V regulated supply for microcontroller AT89S51, ADC0804, LCD and 12V unregulated supply for relay circuit.
Current Sensor/Transformer:-
Current transformers can perform circuit control, measure current for power measurement and control, and perform roles for safety protection and current limiting. They can also cause circuit events to occur when the monitored current reaches a specified level. Current monitoring is necessary at frequencies from the 50 Hz/60 Hz power line to the higher frequencies of switchmode transformers that range into the hundreds of kilohertz.
The object with current transformers is to think in terms of current transformation rather than voltage ratios. Current ratios are the inverse of voltage ratios. The thing to remember about transformers is that Pout = (Pin — transformer power losses). With this in mind, let's assume we had an ideal loss-less transformer in which Pout = Pin. Since power is voltage times current, this product must be the same on the output as it is on the input. This implies that a 1:10 step-up transformer with the voltage stepped up by a factor of 10 results in an output current reduced by a factor of 10. This is what happens on a current transformer. If a transformer had a one-turn primary and a ten-turn secondary, each amp in the primary results in 0.1A in the secondary, or a 10:1 current ratio. It's exactly the inverse of the voltage ratio — preserving volt times current product.
How can we use this transformer and knowledge to produce something useful? Normally, an engineer wants to produce an output on the secondary proportional to the primary current. Quite often, this output is in volts output per amp of primary current. The device that monitors this output voltage can be calibrated to produce the desired results when the voltage reaches a specified level.
A burden resistor connected across the secondary produces an output voltage proportional to the resistor value, based on the amount of current flowing through it. With our 1:10 turns ratio transformer that produces a 10:1 current ratio, a burden resistor can be selected to produce the voltage we want. If 1A on the primary produces 0.1A on the secondary, then by Ohm's law, 0.1 times the burden resistor will result in an output voltage per amp.
Many voltage transformers have adjusted ratios that produce the desired output voltage and compensate for losses. The turns-ratios or actual turns aren't the primary concern of the end-user. Only the voltage output and possibly regulation and other loss parameters may be of concern. With current transformers, the user must know the current ratio to use the transformer. The knowledge of amps in per amps out is the basis for use of the current transformer. Quite often, the end users provide the primary with a wire through the center of the transformer. They must know what secondary turns are to determine what their output current will be. Generally, in catalogues, the turns of the transformers are provided as a specification for use.