22-03-2011, 11:05 AM
[attachment=10714]
PHASE-ANGLE CONTROL OF SCR USING AT89C51
Silicon-controlled rectifiers (SCR) are solidstate semiconductor devices that are usually used in power switching circuits. SCR controls the output signal by switching it ‘on’ or ‘off’, thereby controlling the power to the load in context. The two primary modes of SCR control are phase-angle fired-where a partial waveform is passed to regulate the power.
In the phase-angle controller, the firing pulse is delayed to turn on the SCR in middle of every half cycle. This means that every time a part of an AC cycle is cut, the power to the load also gets cut. To deliver more or less power to the load, the power to the load also gets cut. To deliver more or less power to the load. The phase angle is increased or decreased, thereby controlling the throughput power.
There are several ways to control the firing angle of SCR. This article
Describes a microcontroller AT 89C51-based-angle controller. A microcontroller can be programmed to fire SCR over the full range of half cycles from 0 to 180º - to get a good linear relationship between the phase angle and the delivered output power.
Basically, the zero-crossing detector circuit interrupts the microcontroller after every 10 ms. This interrupt commands the microcontroller to generate some delay (in the range of 1 ms to 9ms). The user can increase or decrease the delay in intervals of 1 ms using switches. The SCR is then fired through the opto-coupler. This repeats after every 10 ms.
CIRCUIT DESCRIPTION
The complete circuit is divided into two sections:
1. The zero-cross detector section
2. The control section
The zero-cross detector section. Fig.1 shows the circuit diagram of the zero-crossing detector and the power supply. The main section of the circuit are a rectifier, regulated power supply and zero-crossing detector. The 230V AC mains is stepped down by transformer X1 to deliver the secondary output of 9V, 500mA. The transformer output of 9v, 300mA. The transformer output is rectified by a full-wave bridge rectifier comprising diodes D1 through D4
And then regulated by IC 7805 (ic3). Capacitors C2 and C3 are used forpassing the ripples present in the regulated 5V power supply. A capacitor above 10µF is connected across the output of the regulator IC, while diode D6 protects the regulator IC in case their input is short to ground. LED5 acts as the Power-on indicator and resistor R5 limits the current through LED5.
This regulated 5V is also used as biasing voltage for both transistors (T1and T2) and the control section. A pulsating DC voltage is applied to the base of transistor T1 through diode D5 and resistors R1 and R2. When the pulsating voltage goes to zero, the collector of transistor T1 goes high. This is used for detecting the pulse when the voltage is zero. Finally, the detected pulse from ‘C’ is fed to the microcontroller of the control section.
The control section. Fig 2 shows the circuit diagram of the control section for the phase-angle control of SCR. It comprises a microcontroller AT89C51, opto-coupler MCT2E, LED module and a few discrete components. Port 0 (P0.0 though P0.7) of AT89C51 is used for interfacing data input pins D0 through D7 of the LCD module. Port pins P2.6, P2.5 and P2.7 of the microcontroller control the registers select (RS), read/write (R/W) and enable (E) input pin of the LED module, respectively. Preset VR1 is used for controlling the contrast of the LCD module. Push –to-on switches S1, S2 and S3 are connected with the pins P1.0 P1.1 and P1.2 through diodes D9, D10 and D11, respectively. External interrupt pin INT0(P3.2) of the microcontroller is connected to S1,S2 and S3 THROUGH d12, D13 and D14, respectively. The role of different switches is shown in Table 1.
The output of the zero-crossing detector from ‘C’ is fed to the external interrupt pin INT1(P3.3) of the microcontroller.
Port pin P2.0 is connected with pin 2 of the opt-coupler(MCT2E).The output pin 5 of MCT2E is used for triggering the gate of SCR TYN604. The
Anode of SCR IS CONNECTED TO THE LOAD (BULB) WITH THE 230v AC supply.
A 12MHz crystal along with capacitors C5 and C4 are connected to the microcontroller pins 18 and 19 to provide the basic clock to the microcontroller. Power on reset is derived by using capacitor C6 and resistor R6. Switch S4 is used for a manual reset.
The operation
The complete operation can be well understood with the help of waveform in Fig.3.
1. The waveform at point ‘A’ is a fully rectified wave that is fed to the base of T1.
2. When the base voltage falls below 0.7V, transistor T1 is switched off, pulling the output higher. This results in a very short positive pulse, which is available at the collector, (at point ‘B’) as shown in the second waveform.
3. As this positive pulse is inverted by transistor T2, it produces one negative pulse of the same width at ‘C’. This is shown as the thied
Waveform.
4. This negative pulse is fed to the interrupt pin (INT1) of the microcontroller, which acts as an interrupt for the microcontroller. The microcontroller then generates a positive pulse on P2.0 (at point ‘D’) after some delay. This turns ‘off’ the internal LED of the opto-coupler (MCT2E) AND A POSITIVE PULSE IS PRODUCED AT OUTPUT ‘E’. This is used for triggering (fire) SCR1.
5. Depending on the time delay in between the interrupt and the pulse on port pin P2.0 of the microcontroller, the SCR is fired in the middle of the half-wave cycle.
6. Two different waveforms one for 4 ms delay and the other for 8 ms delay – are shown in fig.3. In the case of 4ms delay, the output positive cycle of the AC wave is 60 per cent of the input. Therefore, nearly 60 per cent of the power is delivered to the load (the dotted line shows part of waveform that has been cut). In the second case of 8 ms delay, the output cycle is 20 per cent of the input cycle, so only 20 per cent of the power is delivered to the load.
This change in delay is done using switches S1 and S2. Different LEDs are used for indicating different functions as shown in Table II.
The diodes D12 through D14 are connected in such a manner that whenever any of the three push-to-on switches are pressed, it generates an external interrupt INT0.
When switch S1 is pressed for the first time, it enables external interrupt INT1 and displays the message ‘SCR on.’ So after every 10 ms, external interrupt 0 and the message ‘SCR on.’ So after every 10ms, external interrupt INT1 is generated which starts the entire operation. Pressing switch S1 again disables external interrupt 0 and the message ‘SCR off’ is displayed. The complete SCR operation gets shut off.
On pressing S2, the delay increases by 1 ms (firing angle will shift by 18º) and firing of SCR is delayed by 1 ms. The power delivered to the load is also decreased by 10 per cent. The maximum delay that can be applied is 9 ms which will delay firing by an angle of 162º.When the limit is reached, it is indicated by LED3 and a message ‘Max. phase angle’ is displayed on the LCD. The glowing of the bulb goes off.
Similarly, when S3 is pressed, the delay is decreased by 1 ms and the load current increases by 10 percent. The minimum delay is 0 ms, which means a full positive cycle is applied. However, when the limit is reached, it is indicated by LED4 and a message and a message ‘Min phase angle’ is displayed.
An actual-size, single-side PCB for phase-angle control using SCR is
Shown in Fig. 4 and its component layout in Fig.5.
