Showing posts with label regulator. Show all posts
Showing posts with label regulator. Show all posts
Wednesday, October 9, 2013
Thrifty Voltage Regulator
One of the drawbacks of a three-pin voltage regulator is that the input voltage needs to be 2.5–3 V higher than the output voltage. This makes these integrated regulators unsuitable for battery power supplies. If, for instance, the output voltage is 5 V, a 9 V battery could be discharged to 7.5 V or thereabouts only. On top of this, most of these regulators draw a current of about 2 mA. Special low-drop versions sometimes offer a solution, but they are not ideal either. The regulator described here is rather thriftier: it draws a current of only 300 µA and the difference between its input and output is only 100–200 mV In the circuit diagram, T1 is arranged as a series regulator, which means that the difference between input voltage and output voltage is limited to the transistor’s saturation potential.
Therefore, a 9 V battery can be discharged to about 5 V, which is quite an improvement on the situation with an integrated regulator. Diodes D1-D2-D3, or a suitable zener diode (D4), in conjunction with R5 and P1, form a variable reference voltage source, which is used as the (output-dependent) base potential of T3. If the output voltage drops below a desired level, the base potential of T3 also drops. The transistor then conducts less hard and its collector voltage rises. The base voltage of T2 also rises, so that T1 is driven harder. This results in the near-instantaneous restoration of the output voltage.
The design of the reference voltage source is clearly of paramount importance. The current through the LEDs or the zener diode is of the order of only 100 µA. This means that thedrop across a 5.1 V zener diode is only 4.3 V and across each LED, only about 1.43 V. For a wanted output voltage of 4.8 V, the three LEDs proved very effective, whereas the zener did not. It may well be necessary, if a zener diode is used, to try one rated at 4.7 V. If, however, an output voltage of 5 V is wanted, it will be necessary to carefully select a zener diode. When the battery voltage has dropped to a level where it is only marginally higher than the wanted output voltage, T1 and T2 conduct hard.
A further drop in the battery voltage will cause the collector potential of T2 to drop rapidly to 0 V, since T2 tries to make T1 conduct hard. The large drop in the collector potential of T2 may be used to drive a BATT-LOW indicator. This may be done in three ways as shown in Figure 2. When network a is connected between terminals A and B, transistor T4 will normally be held cut off by divider R6-R7a. If then the voltage at B drops suddenly, T4 conducts, where-upon D5 indicates that the battery is nearly flat. The network in b is similar to that in a, but is intended for a liquid-crystal display of BATT-LOW.
The collector of T4 is linked to the IC that drives the decimal point and the BAT-LOW segment of the display. Network c may be used if there is an unused inverter or gate in the circuit to be powered. The high value of resistor R7b prevents the internal protection diodes of the IC being damaged. When the regulator has been built, connect it to a variable power supply via a multimeter set to the mA range and set P1 roughly at its mid-position. Turn P1 slowly until the desired output voltage is obtained.
If with an output voltage of 4.8 V the regulator draws a current of more than 250–300 µA, the three LEDs or zener diode must be replaced. The regulator can provide a current of up to about 25 mA. With a fresh 9 V battery, the dissipation of T1 does not exceed 100 mW. If the input voltage is higher, it may be necessary to mount the transistor on a suitable heat sink or replace it by a power transistor, for instance, a Type BD138.
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Therefore, a 9 V battery can be discharged to about 5 V, which is quite an improvement on the situation with an integrated regulator. Diodes D1-D2-D3, or a suitable zener diode (D4), in conjunction with R5 and P1, form a variable reference voltage source, which is used as the (output-dependent) base potential of T3. If the output voltage drops below a desired level, the base potential of T3 also drops. The transistor then conducts less hard and its collector voltage rises. The base voltage of T2 also rises, so that T1 is driven harder. This results in the near-instantaneous restoration of the output voltage.The design of the reference voltage source is clearly of paramount importance. The current through the LEDs or the zener diode is of the order of only 100 µA. This means that thedrop across a 5.1 V zener diode is only 4.3 V and across each LED, only about 1.43 V. For a wanted output voltage of 4.8 V, the three LEDs proved very effective, whereas the zener did not. It may well be necessary, if a zener diode is used, to try one rated at 4.7 V. If, however, an output voltage of 5 V is wanted, it will be necessary to carefully select a zener diode. When the battery voltage has dropped to a level where it is only marginally higher than the wanted output voltage, T1 and T2 conduct hard.
A further drop in the battery voltage will cause the collector potential of T2 to drop rapidly to 0 V, since T2 tries to make T1 conduct hard. The large drop in the collector potential of T2 may be used to drive a BATT-LOW indicator. This may be done in three ways as shown in Figure 2. When network a is connected between terminals A and B, transistor T4 will normally be held cut off by divider R6-R7a. If then the voltage at B drops suddenly, T4 conducts, where-upon D5 indicates that the battery is nearly flat. The network in b is similar to that in a, but is intended for a liquid-crystal display of BATT-LOW.
The collector of T4 is linked to the IC that drives the decimal point and the BAT-LOW segment of the display. Network c may be used if there is an unused inverter or gate in the circuit to be powered. The high value of resistor R7b prevents the internal protection diodes of the IC being damaged. When the regulator has been built, connect it to a variable power supply via a multimeter set to the mA range and set P1 roughly at its mid-position. Turn P1 slowly until the desired output voltage is obtained.If with an output voltage of 4.8 V the regulator draws a current of more than 250–300 µA, the three LEDs or zener diode must be replaced. The regulator can provide a current of up to about 25 mA. With a fresh 9 V battery, the dissipation of T1 does not exceed 100 mW. If the input voltage is higher, it may be necessary to mount the transistor on a suitable heat sink or replace it by a power transistor, for instance, a Type BD138.
Monday, September 23, 2013
High Input Voltage Linear Regulator
Commonly used 3-pin linear voltage regulators, for example the LM317, cannot handle input voltages in excess of about 30V. The LR8A from Supertex Inc is a new, adjustable three pin regulator that can accept input voltages up to 450V and can supply an output current of 0.5mA to 10mA. Using this device it is possible to work with rectified 230VAC. The LR8 has a wide input voltage range of +12 V to +450V. Two external resistors (R1 and R2) allow the output voltage to be adjusted from 1.20 V to 440 V provided that the input voltage is at least 10 V greater than the output voltage. The LR8 adjusts the voltage difference between the Vout and ADJ pins to a nominal value of 1.20V.
This 1.20V is amplified by the external resistor ratio of R1 and R2. An internal constant bias current of 10µA is connected to the ADJ pin so that Vout is increased by a constant voltage of 10µA times R2. The formula for calculating the output voltage is given next to the circuit diagram. To ensure stable operation of the regulator a minimum output current of 500µA is necessary and a bypass capacitor of minimum 1.0µF should be used. Protection circuits in the LR8 limit the output current to 15mA typically and temperature protection ensures that the device temperature will not exceed 125oC.
When the device reaches its temperature limit, the output voltage/current will decrease to keep the junction temperature within limits. The two circuit diagrams show the LR8 used as a voltage regulator and as a constant current source. The current source can be used to a drive an LED. This configuration would give an LED with super-wide input voltage range, i.e., from +12V to +450V. The LR8 was originally designed to be used for switch mode supply start-up applications so it incorporates a feature which shuts down the LR8 when the output voltage exceeds the input voltage. Diode D1 is therefore necessary in the voltage regulator circuit diagram to prevent the output voltage exceeding the input voltage at any time.
The minimum value of the input capacitor C1 can be calculated from the following formula: C1(min) = (IL t ) / (Vpk – Vout – 10V) Where IL is the load current, and t the period between two voltage peaks. At 50 Hz, using one rectifying diode this will give a value t = 20 ms. Vpk is the peak input voltage, while Vout is the selected output voltage. The LR8 is available in two package outlines. The LR8N8 is a SOT89 SMD package while the LR8N3 is the familiar TO92 Transistor outline (e.g. BC 238). The TO-92 package can dissipate a maximum of 0.74W while with suitable heatsinking, the SMD package can dissipate 1.6W.
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This 1.20V is amplified by the external resistor ratio of R1 and R2. An internal constant bias current of 10µA is connected to the ADJ pin so that Vout is increased by a constant voltage of 10µA times R2. The formula for calculating the output voltage is given next to the circuit diagram. To ensure stable operation of the regulator a minimum output current of 500µA is necessary and a bypass capacitor of minimum 1.0µF should be used. Protection circuits in the LR8 limit the output current to 15mA typically and temperature protection ensures that the device temperature will not exceed 125oC.
When the device reaches its temperature limit, the output voltage/current will decrease to keep the junction temperature within limits. The two circuit diagrams show the LR8 used as a voltage regulator and as a constant current source. The current source can be used to a drive an LED. This configuration would give an LED with super-wide input voltage range, i.e., from +12V to +450V. The LR8 was originally designed to be used for switch mode supply start-up applications so it incorporates a feature which shuts down the LR8 when the output voltage exceeds the input voltage. Diode D1 is therefore necessary in the voltage regulator circuit diagram to prevent the output voltage exceeding the input voltage at any time.
The minimum value of the input capacitor C1 can be calculated from the following formula: C1(min) = (IL t ) / (Vpk – Vout – 10V) Where IL is the load current, and t the period between two voltage peaks. At 50 Hz, using one rectifying diode this will give a value t = 20 ms. Vpk is the peak input voltage, while Vout is the selected output voltage. The LR8 is available in two package outlines. The LR8N8 is a SOT89 SMD package while the LR8N3 is the familiar TO92 Transistor outline (e.g. BC 238). The TO-92 package can dissipate a maximum of 0.74W while with suitable heatsinking, the SMD package can dissipate 1.6W.Friday, March 29, 2013
5 A Constant Voltage Constant Current Regulator Circuit
This is a design circuit for constant voltage constant current (CVCS) regulator doesn’t mean a system with a constant load, since there will be no regulation in such case. What we call CVCS regulator is a regulator with two modes. This is the figure of the circuit;
The first mode is constant voltage, where the regulator trying to regulate the supply to a variable load at a constant voltage. In this mode off course the current will be variable depending on the load. The current will vary a range that is limited to a predetermined maximum current level. When the load draw more current than this level then the regulator will switch to constant current mode, where the regulator keep the current at a fixed maximum value. Any attempt of the load to draw more current will cause the regulator to decrease the voltage as a reaction to keep it constant, so it acts like a current limiter. Here is the schematic diagram of the CVCS regulator.
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