Showing posts with label power supply. Show all posts
Showing posts with label power supply. Show all posts

Circuit Amp with Tone Controls Soft Switching Schematic Diagrams

Circuit Amp with Tone controls Soft switching schematics Circuit Electronics,

The soft switching is enabled by a BD131 transistor wired as a switch in emitter follower configuration. The collector is wired to a permanent supply voltage, the 2H series inductor serves only to filter out power supply hum. This inductor is not too important and may be omitted if the DC supply is adequately smoothed. The control voltage is applied to the BD131 base terminal, the 10u capacitor and 10k resistor having a dual purpose:-
i) a gradual charge of the 10u capacitor ensures that the transistor will switch linearly from 0 volts to full supply, and
ii) serves as a hum filter ensuring a very smooth dc supply to the amplifier and tone controls.
LED1 will light when the amplifier is on. The control voltage should ideally be 0 volts when the amplifier is off and full supply voltage when on. The LM380 is shown driving two 8 ohm loudspeakers, the load is therefore 4 ohms. The 4u7 capacitor acts as a crude crossover, lower frequencies are impeded and so this loudspeaker may be a "tweeter" type.
Schematics for Amp with Tone controls Soft switching Circuit Electronics
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Circuit Op-Amp Mic Preamp Schematic Diagrams

Circuit Op-Amp Mic Preamp schematics Circuit Electronics,
A high quality microphone preamplifier using a single power supply, suitable for dynamic or electret microphones. The op-amp used can be any low noise, high performance type, e.g. NE5534,TL071, OPA 371 etc
Schematics for Op-Amp Mic Preamp Circuit Electronics
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Circuit 6 Zone Alarm Schematic Diagrams

Circuit 6 Zone Alarm schematics Circuit Electronics,

Circuit Notes:
All zones Z1 to Z6 use normally closed alarm contacts. Zone 1 is a timed zone which must be used as the entry and exit point of the building. Zones 2 to 6 are immediate zones, which will trigger the alarm with no delay. Some RF immunity is provided for long wiring runs by the input capacitors, C1 - C6. The key switch, S1 acts as the Set and Reset/Unset switch. For best security this should be the metal type switch with a key.
All IC's except IC6 are CMOS types with buffered outputs, these are denoted by the suffix "B". Unbuffered CMOS IC's have a suffix starting "U" and will not work in this circuit. IC6 is a 5 Volt regulator providing power to the main CMOS IC's, the alarm power supply can be any suitable 12 to 15V power supply.
In operation the DPDT switch S2 is set to the "run" position. When keyswitch S1 is turned reset, this is the unset (off) state of the alarm. In this condition capacitor C8 will discharge via D9, R1 and Z1 and capacitor C7 will have discharged via D8, R17 and S1. Relay RLY1 will not be energized and all CMOS IC's and the display will have no power.

When S1 is turned to set all CMOS IC's receive 5 Volt power. C11 will briefly charge and apply a low input signal to one half of U7A a CMOS4001B, a dual input OR gate. The output of U5A will also be low (make sure all windows and doors zones 2 to 5 are shut) and the output of U7A is high. The output of U7A is then inverted by U7B and fed back via R18 to U7A's input keeping the circuit latched. The output of U7B is low and so Q1 and the relay RLY1 is off and no alarm will sound.

Also when S1 is set, C8 slowly charges via R13. C8 and R13 form the exit timer and allow time to vacate the building. The delay is approximately 1.1 x the value of C8 (in uF) or about 52 seconds with values shown. During the exit delay zone switch Z1 can be opened and closed without triggering the alarm. After the exit time ends, C8 will be charged and one half of the 2 input AND gate, U4A will now be high. Any opening of zones 2 through 6 will cause the alarm to trigger and relay RLY1 will energize. If an intruder attempts a break-in via zone 4 for example, the output of U1D will change state from low to high. When this happens, the high signal is forwarded by U2C a triple input OR gate CMOS4075 and is sent to input C on the CMOS4511 BCD to Decimal display driver. The binary code for four is 100 and input C is high, A abd B are low, and the LED display will illuminate digit 4. The high output from U2C is also forwarded to U5A, again a triple input OR gate. The output of U5A is now sent via S2 to the input of U7A. U7A and U7B form a bistable latch, the change in state causing the output of U7A to change to low, the output of U7B to become high and fed back via R18 to the input of U7A again. The circuit is now latched in the high state. The high output of U7B does two things. First it switches on Q1 and relay RLY1 sounding the alarm. Secondly the high output at U7B is applied to the blanking input of the CMOS4511B via S2 and also to the enable latching pin. The display will now continually show the number of the triggered zone, even if the zone switch is opened or closed again. It is a similar process for any of the other immediate zones.

As this alarm uses 6 zones, the CMOS 4511 BCD to decimal decoder must count to 6 which is 110 in binary. Therefore only inputs A,B and C are required, D is simply tied to ground. The pinout for the CMOS 4511 is shown below.
CMOS 4511
If the building is entered via Zone 1 then the entry timer starts. The output of U1A (in the set condition ) is low. Entry via Z1 triggers U1A to become momentarily high as the door is opened. U4A then produces a high output, as does U4B. The high signal is now passed via D7 to the input of U1A latching it in the high state. C7 then charges via R15. This is the entry delay and is approximately 1.1 x 0.47 x C7 or about 24 seconds with values shown. Once charged U4C will become high, trigger the alarm and cause the LED display to be latched, as per the preceding paragraph.

switch S2 is normally used in the run position. However in the test position, this allows a useful "walkthrough" test of the alarm. In the test position the input of U7A is always low and will not trigger the alarm, also the blanking input is also high, meaning that the 7 segment display is always illuminated. With all zones closed, open any zone, the corresponding number will be shown on the display. Note that if two zones are opened the diplay will not necessarily indicate the correct zone, this is not a fault, just the way the circuit is designed. When in run mode, the first zone to trigger the alarm is "caught" and latched and will be displayed until the alarm is reset.

Schematics for 6 Zone Alarm Circuit Electronics
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Circuit MotorCycle Alarms 5 6 Schematic Diagrams

Circuit MotorCycle Alarms 5 6 schematics Circuit Electronics,
Circuit Notes
Any number of normally-open switches may be used. Fit the mercury switches so that they close when the steering is moved or when the bike is lifted off its side-stand or pushed forward off its centre-stand. Use micro-switches to protect removable panels and the lids of panniers etc. When one of the trigger-switches is closed - the relay will energize and the siren will sound.
You can choose what happens next. If you build the circuit as shown, the siren will continue to sound until you turn it off - or until the battery is exhausted. But, if you leave out D3 - the siren will stop sounding immediately the trigger-switch is re-opened.
While you're within earshot of your machine - the former configuration is best. You can always turn off the alarm yourself. But if you are going to be away from your bike for any length of time - and you don't want to cause a nuisance - then the latter configuration is probably more suitable. If you include a SPST switch in series with D3 - you can select the behaviour that best suits the circumstances at any given time.

Schematics for MotorCycle Alarms 5 6 Circuit Electronics
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Circuit Alarm Pintu Schematic Diagrams

Circuit Alarm Pintu schematics Circuit Electronics,
Circuit Notes
Figure 1 represents a cheap and simple Gate Alarm, that is intended to run off a small universal AC-DC power supply.
IC1a is a fast oscillator, and IC1b a slow oscillator, which are combined through IC1c to emit a high pip-pip-pip warning sound when a gate (or window, etc.) is opened. The circuit is intended not so much to sound like a siren or warning device, but rather to give the impression: "You have been noticed." R1 and D1 may be omitted, and the value of R2 perhaps reduced, to make the Gate Alarm sound more like a warning device. VR1 adjusts the frequency of the sound emitted.
IC1d is a timer which causes the Gate Alarm to emit some 20 to 30 further pips after the gate has been closed again, before it falls silent, as if to say: "I'm more clever than a simple on-off device." Piezo disk S1 may be replaced with a LED if desired, the LED being wired in series with a 1K resistor.
Figure 2 shows how an ordinary reed switch may be converted to close (a "normally closed" switch) when the gate is opened. A continuity tester makes the work easy. Note that many reed switches are delicate, and therefore wires which are soldered to the reed switch should not be flexed at all near the switch. Other types of switches, such as microswitches, may also be used.

Schematics for Alarm Pintu Circuit Electronics
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Circuit CMOS Toggle Flip Flop Using Push Button Schematic Diagrams

Circuit CMOS Toggle Flip Flop Using Push Button schematics Circuit Electronics,

The circuit below uses a CMOS dual D flip flop (CD4013) to toggle a relay or other load with a momentary push button. Several push buttons can be wired in parallel to control the relay from multiple locations.
 A high level from the push button is coupled to the set line through a small (0.1uF) capacitor. The high level from the Q output is inverted by the upper transistor and supplies a low reset level to the reset line for about 400 mS, after which time the reset line returns to a high state and resets the flip flop. The lower flip flop
section is configured for toggle operation and changes state on the rising edge of the clock line or at the same time as the upper flip flop moves to the set condition. The switch is debounced due to the short duration of the set signal relative to the long duration before the circuit is reset. The Q or Qbar outputs will only supply about 2 mA of current, so a buffer transistor or power MOSFET is needed to drive a relay coil, or lamp, or other load. A 2N3904 or most any small signal NPN transistor can be used for relay coil resistances of 250 ohms or more. A 2N3053 or medium power (500 mA) transistor should be used for coil resistances below 250 ohms. The 47 ohm resistor and 10uF capacitor serve to decouple the circuit from the power supply and filter out any short duration noise signals that may be present. The RC network (.1/47K) at the SET line (pin 8) serves as a power-on reset to ensure the relay is denergized when circuit power is first applied. The reset idea was suggested by Terry Pinnell who used the circuit to control a shed light from multiple locations.
Schematics for CMOS Toggle Flip Flop Using Push Button Circuit Electronics
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Circuit Improved 3 Transistor Audio Amp (80 milliwatt) Schematic Diagrams

Circuit Improved 3 Transistor audio Amp (80 milliwatt) schematics Circuit Electronics,
The original circuit in the radio used a 300 ohm resistor where the 2 diodes are shown but I changed the resistor to 2 diodes so the amp would operate on lower voltages with less distortion. The transistors shown 2n3053 and 2n2905 are just parts I used for the other circuit above and could be smaller types. Most any small transistors can be used, but they should be capable of 100mA or more current. A 2N3904 or 2N3906 are probably a little small, but would work at low volume. 

The 2 diodes generate a fairly constant bias voltage as the battery drains and reduces crossover distortion. But you should take care to insure the idle current is around 10 to 20 milliamps with no signal and the output transistors do not get hot under load. 

The circuit should work with a regular 8 ohm speaker, but the output power may be somewhat less. To optimize the operation, select a resistor where the 100K is shown to set the output voltage at 1/2 the supply voltage (4.5 volts). This resistor might be anything from 50K to 700K depending on the gain of the transistor used where the 3904 is shown.

Schematics for Improved 3 Transistor audio Amp (80 milliwatt) Circuit Electronics
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Circuit Operational Amplifier (Op-Amp) Basics Schematic Diagrams

Circuit Operational amplifier (Op-Amp) Basics schematics Circuit Electronics,

The op-amp is basically a differential amplifier having a large voltage gain, very high input impedance and low output impedance. The op-amp has a "inverting" or (-) input and "noninverting" or (+) input and a single output. The op-amp is usually powered by a dual polarity power supply in the range of +/- 5 volts to +/- 15 volts. A simple dual polarity power supply is shown in the figure below which can be assembled with two 9 volt batteries. 

Inverting amplifier:
The op-amp is connected using two resistors RA and RB such that the input signal is applied in series with RA and the output is connected back to the inverting input through RB. The noninverting input is connected to the ground reference or the center tap of the dual polarity power supply. In operation, as the input signal moves positive, the output will move negative and visa versa. The amount of voltage change at the output relative to the input depends on the ratio of the two resistors RA and RB. As the input moves in one direction, the output will move in the opposite direction, so that the voltage at the inverting input remains constant or zero volts in this case. If RA is 1K and RB is 10K and the input is +1 volt then there will be 1 mA of current flowing through RA and the output will have to move to -10 volts to supply the same current through RB and keep the voltage at the inverting input at zero. The voltage gain in this case would be RB/RA or 10K/1K = 10. Note that since the voltage at the inverting input is always zero, the input signal will see a input impedance equal to RA, or 1K in this case. For higher input impedances, both resistor values can be increased. 

Noninverting amplifier:
The noninverting amplifier is connected so that the input signal goes directly to the noninverting input (+) and the input resistor RA is grounded. In this configuration, the input impedance as seen by the signal is much greater since the input will be following the applied signal and not held constant by the feedback current. As the signal moves in either direction, the output will follow in phase to maintain the inverting input at the same voltage as the input (+). The voltage gain is always more than 1 and can be worked out from Vgain = (1+ RB/RA). 

Voltage Follower:
The voltage follower, also called a buffer, provides a high input impedance, a low output impedance, and unity gain. As the input voltage changes, the output and inverting input will change by an equal amount.

Schematics for Operational amplifier (Op-Amp) Basics Circuit Electronics
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Circuit Capacitor Discharge Ignition Circuit (CDI) Schematic Diagrams

Circuit Capacitor Discharge Ignition Circuit (CDI) schematics Circuit Electronics,

The CDI ignition circuit produces a spark from an ignition coil by discharging a capacitor across the primary of the coil. A 2uF capacitor is charged to about 340 volts and the discharge is controlled by an SCR. A Schmitt trigger oscillator (74C14) and MOSFET (IRF510) are used to drive the low voltage side of a small (120/12 volt) power transformer and a voltage doubler arrangement is used on the high voltage side to increase the capacitor voltage to about 340 volts. A similar Schmitt trigger oscillator is used to trigger the SCR about 4 times per second. The power supply is gated off during the discharge time so that the SCR will stop conducting and return to it's blocking state. The diode connected from the 3904 to pin 9 of the 74C14 causes the power supply oscillator to stop during discharge time. The circuit draws only about 200 milliamps from a 12 volt source and delivers almost twice the normal energy of a conventional ignition circuit. High voltage from the coil is about 10KV using a 3/8 inch spark gap at normal air temperature and pressure. Spark rate can be increased to possibly 10 Hertz without losing much spark intensity, but is limited by the low frequency power transformer and duty cycle of the oscillator. For faster spark rates, a higher frequency and lower impedance supply would be required. Note that the ignition coil is not grounded and presents a shock hazard on all of it's terminals. Use CAUTION when operating the circuit. An alternate method of connecting the coil is to ground the (-) terminal and relocate the capacitor between the cathode of the rectifier diode and the positive coil terminal. The SCR is then placed between ground and the +340 volt side of the capacitor. This reduces the shock hazard and is the usual configuration in automotive

Schematics for Capacitor Discharge Ignition Circuit (CDI) Circuit Electronics
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Circuit Generating -5 Volts From a 9 Volt Battery Schematic Diagrams

Circuit Generating -5 Volts From a 9 Volt battery schematics Circuit Electronics,

A 555 timer can be used to generate a squarewave to produce a negative voltage relative to the negative battery terminal. When the timer output at pin 3 goes positive, the series 22 uF capacitor charges through the diode (D1) to about 8 volts. When the output switches to ground, the 22 uF cap discharges through the second diode (D2) and charges the 100 uF capacitor to a negative voltage. The negative voltage can rise over several cycles to about -7 volts but is limited by the 5.1 volt zener diode which serves as a regulator. Circuit draws about 6 milliamps from the battery without the zener diode connected and about 18 milliamps connected. Output current available for the load is about 12 milliamps. An additional 5.1 volt zener and 330 ohm resistor could be used to regulate the +9 down to +5 at 12 mA if a symmetrical +/- 5 volt supply is needed. The battery drain would then be around 30 mA.

Schematics for Generating -5 Volts From a 9 Volt battery Circuit Electronics
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Circuit 1.5 Volt LED Flashers Schematic Diagrams

Circuit 1.5 Volt LED Flashers schematics Circuit Electronics,

The LED flasher circuits below operate on a single 1.5 volt battery. The circuit on the upper right uses the popular LM3909 LED flasher IC and requires only a timing capacitor and LED.

The top left circuit, designed by Andre De-Guerin illustrates using a 100uF capacitor to double the battery voltage to obtain 3 volts for the LED. Two sections of a 74HC04 hex inverter are used as a squarewave oscillator that establishes the flash rate while a third section is used as a buffer that charges the capacitor in series with a 470 ohm resistor while the buffer output is at +1.5 volts. When the buffer output switches to ground (zero volts) the charged capacitor is placed in series with the LED and the battery which supplies enough voltage to illuminate the LED. The LED current is approximately 3 mA, so a high brightness LED is recommended. 

In the other two circuits, the same voltage doubling principle is used with the addition of a transistor to allow the capacitor to discharge faster and supply a greater current (about 40 mA peak). A larger capacitor (1000uF) in series with a 33 ohm resistor would increase the flash duration to about 50mS. The discrete 3 transistor circuit at the lower right would need a resistor (about 5K) in series with the 1uF capacitor to widen the pulse width.

Schematics for 1.5 Volt LED Flashers Circuit Electronics
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Circuit LED 12 Volt Lead Acid Battery Meter Schematic Diagrams

Circuit LED 12 Volt Lead Acid battery Meter schematics Circuit Electronics,
In the circuit below, a quad voltage comparator (LM339) is used as a simple bar graph meter to indicate the charge condition of a 12 volt, lead acid battery. A 5 volt reference voltage is connected to each of the (+) inputs of the four comparators and the (-) inputs are connected to successive points along a voltage divider. The LEDs will illuminate when the voltage at the negative (-) input exceeds the reference voltage. Calibration can be done by adjusting the 2K potentiometer so that all four LEDs illuminate when the battery voltage is 12.7 volts, indicating full charge with no load on the battery. At 11.7 volts, the LEDs should be off indicating a dead battery. Each LED represents an approximate 25% change in charge condition or 300 millivolts, so that 3 LEDs indicate 75%, 2 LEDs indicate 50%, etc. The actual voltages will depend on temperature conditions and battery type, wet cell, gel cell etc. Additional information on battery maintenance can be found at:

Schematics for LED 12 Volt Lead Acid battery Meter Circuit Electronics
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Circuit 3.6 Volt cell phone battery meter Schematic Diagrams

Circuit 3.6 Volt cell phone battery meter schematics Circuit Electronics,

This is a similar circuit to the above and provides a 4 LED bar graph indicating the voltage of a common 3.6 volt Lithium - Ion recharable cell phone battery. The reference voltage is provided by a TL431 programmable voltage source which is set to 3.9 volts where the TL431 connects to the 1K resistor. The lower reference for the LED at pin 14 is set with the 5K adjustable resistor. 

The programmed voltage of the TL431 is worked out with a voltage divider (10K 5.6K). The adjustment terminal or junction of the two resistors is always 2.5 volts. So, if we use a 10K resistor from the adjustment terminal to ground, the resistor current will be 2.5/10000 = 250uA. This same current flows through the upper resistor (5.6K) and produces a voltage drop of .00025 * 5600 = 1.4 volts. So the shunt regulated output voltage at the cathode of the TL431 will be 2.5 + 1.4, or 3.9 volts. 

Working out the LED voltages, there are three 390 ohm resistors in series with another adjustable (5K) resistor at the bottom. Assuming the bottom resistor is set to 2K ohms, the total resistance is 390+390+390+2000 = 3170 ohms. So, the resistor current is the reference voltage (3.9) divided by the total resistance, or about 3.9/ (390 + 390 + 390 + 2000) equals 1.23 mA. This gives us about .00123*2000= 2.46 volts for the bottom LED, and about .00123*390 = .48 volts for each step above the bottom. So, the LEDs should light at steps of 2.46, 2.94, 3.42, and 3.9. A fully charged cell phone battery is about 4.2 volts. You can adjust the 5.6K resistor to set the top voltage higher or lower, and adjust the lower 5K resistor to set the bottom LED for the lowest voltage. But you do need a 6 to 12 volt or greater battery to power the circuit.

Schematics for 3.6 Volt cell phone battery meter Circuit Electronics
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Circuit Low Voltage, High Current Time Delay Circuit Schematic Diagrams

Circuit Low Voltage, High Current Time Delay Circuit schematics Circuit Electronics,

In this circuit a LM339 quad voltage comparator is used to generate a time delay and control a high current output at low voltage. Approximatey 5 amps of current can be obtained using a couple fresh alkaline D batteries. Three of the comparators are wired in parallel to drive a medium power PNP transistor (2N2905 or similar) which in turn drives a high current NPN transistor (TIP35 or similar). The 4th comparator is used to generate a time delay after the normally closed switch is opened. Two resistors (36K and 62K) are used as a voltage divider which applies about two-thirds of the battery voltage to the (+) comparator input, or about 2 volts. The delay time after the switch is opened will be around one time constant using a 50uF capacitor and 100K variable resistor, or about (50u * 100K) = 5 seconds. The time can be reduced by adjusting the resistor to a lower value or using a smaller capacitor. Longer times can be obtained with a larger resistor or capacitor. To operate the circuit on higher voltages, the 10 ohm resistor should be increased proportionally, (4.5 volts = 15 ohms).

Schematics for Low Voltage, High Current Time Delay Circuit Circuit Electronics
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Circuit 2 Watt Switching Power Supply Schematic Diagrams

Circuit 2 Watt switching power supply schematics Circuit Electronics,
In this small switching power supply, a Schmitt trigger oscillator is used to drive a switching transistor that supplies current to a small inductor. Energy is stored in the inductor while the transistor is on, and released into the load circuit when the transistor switches off. The output voltage is dependent on the load resistance and is limited by a zener diode that stops the oscillator when the voltage reaches about 14 volts. Higher or lower voltages can be obtained by adjusting the voltage divider that feeds the zener diode. The efficiency is about 80% using a high Q inductor.

Schematics for 2 Watt switching power supply Circuit Electronics
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Circuit 2 Cell Lithium Ion Charger Schematic Diagrams

Circuit 2 Cell Lithium Ion Charger schematics Circuit Electronics,

This circuit was build to charge a couple series Lithium cells (3.6 volts each, 1 Amp Hour capacity) installed in a portable transistor radio. 

The charger operates by supplying a short current pulse through a series resistor and then monitoring the battery voltage to determine if another pulse is required. The current can be adjusted by changing the series resistor or adjusting the input voltage. When the battery is low, the current pulses are spaced close together so that a somewhat constant current is present. As the batteries reach full charge, the pulses are spaced farther apart and the full charge condition is indicated by the LED blinking at a slower rate. 

A TL431, band gap voltage reference (2.5 volts) is used on pin 6 of the comparator so the comparator output will switch low, triggering the 555 timer when the voltage at pin 7 is less than 2.5 volts. The 555 output turns on the 2 transistors and the batteries charge for about 30 milliseconds. When the charge pulse ends, the battery voltage is measured and divided down by the combination 20K, 8.2K and 620 ohm resistors so when the battery voltage reaches 8.2 volts, the input at pin 7 of the comparator will rise slightly above 2.5 volts and the circuit will stop charging. 

The circuit could be used to charge other types of batteries such as Ni-Cad, NiMh or lead acid, but the shut-off voltage will need to be adjusted by changing the 8.2K and 620 ohm resistors so that the input to the comparator remains at 2.5 volts when the terminal battery voltage is reached. 

For example, to charge a 6 volt lead acid battery to a limit of 7 volts, the current through the 20K resistor will be (7-2.5)/ 20K = 225 microamps. This means the combination of the other 2 resistors (8.2K and 620) must be R=E/I = 2.5/ 225 uA = 11,111 ohms. But this is not a standard value, so you could use a 10K in series with a 1.1K, or some other values that total 11.11K

Schematics for 2 Cell Lithium Ion Charger Circuit Electronics
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Circuit Variable Voltage and Current Power Supply Schematic Diagrams

Circuit Variable Voltage and Current power supply schematics Circuit Electronics,

Another method of using opamps to regulate a power supply is shown below. The power transformer requires an additional winding to supply the op-amps with a bipolar voltage (+/- 8 volts), and the negative voltage is also used to generate a reference voltage below ground so that the output voltage can be adjusted all the way down to 0. Current limiting is accomplished by sensing the voltage drop across a small resistor placed in series with the negative supply line. As the current increases, the voltage at the wiper of the 500 ohm pot rises until it becomes equal or slightly more positive than the voltage at the (+) input of the opamp. The opamp output then moves negative and reduces the voltage at the base of the 2N3053 transistor which in turn reduces the current to the 2N3055 pass transistor so that the current stays at a constant level even if the supply is shorted. Current limiting range is about 0 - 3 amps with components shown. The TIP32 and 2N3055 pass transistors should be mounted on suitable heat sinks and the 0.2 ohm current sensing resistor should be rated at 2 watts or more. The heat produced by the pass transistor will be the product of the difference in voltage between the input and output, and the load current. So, for example if the input voltage (at the collector of the pass transistor) is 25 and the output is adjusted for 6 volts and the load is drawing 1 amp, the heat dissipated by the pass transistor would be (25-6) * 1 = 19 watts. In the circuit below, the switch could be set to the 18 volt position to reduce the heat generated to about 12 watts.

Schematics for Variable Voltage and Current power supply Circuit Electronics
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Circuit Variable 3 - 24 Volt / 3 Amp Power Supply Schematic Diagrams

Circuit Variable 3 - 24 Volt / 3 Amp power supply schematics Circuit Electronics,

This regulated power supply can be adjusted from 3 to 25 volts and is current limited to 2 amps as shown, but may be increased to 3 amps or more by selecting a smaller current sense resistor (0.3 ohm). The 2N3055 and 2N3053 transistors should be mounted on suitable heat sinks and the current sense resistor should be rated at 3 watts or more. Voltage regulation is controlled by 1/2 of a 1558 or 1458 op-amp. The 1458 may be substituted in the circuit below, but it is recommended the supply voltage to pin 8 be limited to 30 VDC, which can be accomplished by adding a 6.2 volt zener or 5.1 K resistor in series with pin 8. The maximum DC supply voltage for the 1458 and 1558 is 36 and 44 respectively. The power transformer should be capable of the desired current while maintaining an input voltage at least 4 volts higher than the desired output, but not exceeding the maximum supply voltage of the op-amp under minimal load conditions. The power transformer shown is a center tapped 25.2 volt AC / 2 amp unit that will provide regulated outputs of 24 volts at 0.7 amps, 15 volts at 2 amps, or 6 volts at 3 amps. The 3 amp output is obtained using the center tap of the transformer with the switch in the 18 volt position. All components should be available at Radio Shack with the exception of the 1558 op-amp.

Schematics for Variable 3 - 24 Volt / 3 Amp power supply Circuit Electronics
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Circuit Parallel Port Relay Interface Schematic Diagrams

Circuit Parallel Port Relay Interface schematics Circuit Electronics,

Below are three examples of controlling a relay from the PC's parallel printer port (LPT1 or LPT2). Figure A shows a solid state relay controlled by one of the parallel port data lines (D0-D7) using a 300 ohm resistor and 5 volt power source. The solid state relay will energize when a "0" is written to the data line. Figure B and C show mechanical relays controlled by two transistors. The relay in figure B is energized when a "1" is written to the data line and the relay in figure C is energized by writing a "0" to the line. In each of the three circuits, a common connection is made from the negative side of the power supply to one of the port ground pins (18-25).
There are three possible base addresses for the parallel port You may need to try all three base addresses to determine the correct address for the port you are using but LPT1 is usually at Hex 0378. The QBasic "OUT" command can be used to send data to the port. OUT,H0378,0 sets D0-D7 low and OUT,H378,255 sets D0-D7 high. The parallel port also provides four control lines (C0,C1,C2,C3) that can be set high or low by writing data to the base address+2 so if the base address is Hex 0378 then the address of the control latch would be Hex 037A. Note that three of the control bits are inverted so writing a "0" to the control latch will set C0,C1,C3 high and C2 low.

Schematics for Parallel Port Relay Interface Circuit Electronics
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Circuit LM317T Voltage Regulator with Pass Transistor Schematic Diagrams

Circuit LM317T Voltage Regulator with Pass Transistor schematics Circuit Electronics,

The LM317T output current can be increased by using an additional power transistor to share a portion of the total current. The amount of current sharing is established with a resistor placed in series with the 317 input and a resistor placed in series with the emitter of the pass transistor. In the figure below, the pass transistor will start conducting when the LM317 current reaches about 1 amp, due to the voltage drop across the 0.7 ohm resistor. Current limiting occurs at about 2 amps for the LM317 which will drop about 1.4 volts across the 0.7 ohm resistor and produce a 700 millivolt drop across the 0.3 ohm emitter resistor. Thus the total current is limited to about 2+ (.7/.3) = 4.3 amps. The input voltage will need to be about 5.5 volts greater than the output at full load and heat dissipation at full load would be about 23 watts, so a fairly large heat sink may be needed for both the regulator and pass transistor. The filter capacitor size can be approximated from C=IT/E where I is the current, T is the half cycle time (8.33 mS at 60 Hertz), and E is the fall in voltage that will occur during one half cycle. To keep the ripple voltage below 1 volt at 4.3 amps, a 36,000 uF or greater filter capacitor is needed. The power transformer should be large enough so that the peak input voltage to the regulator remains 5.5 volts above the output at full load, or 17.5 volts for a 12 volt output. This allows for a 3 volt drop across the regulator, plus a 1.5 volt drop across the series resistor (0.7 ohm), and 1 volt of ripple produced by the filter capacitor. A larger filter capacitor will reduce the input requirements, but not much.

Schematics for LM317T Voltage Regulator with Pass Transistor Circuit Electronics
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