Stitching Machine Motor Speed Control circuit with explanation

Motor operated stitching machines have a series of carbon buttons in an enclosure operated by foot pedal. As the pressure on the foot pedal is increased or decreased by the foot, these buttons come close or move farther away, their resistance changes and hence the speed of the motor. Even though crude, it seemed to be okay, until my wife complained.

She said that the speed is little too fast even at the minimum pressure on the foot pedal, particularly for some repair work or at embroidery. Most of the housewives also like a little more control over speed of the machine. So here you have it.

The circuit is shown in Schematic 30. This is a standard triac speed control circuit much similar to domestic fan control circuit. Contrary to other triac circuits, you will find that an additional component known as diac is used in this circuit.

Triacsfire more symmetrically when used along with diac in AC power control applications. The diac is a bidirectional trigger diode which does not conduct (except for a small leakage current) until the break over voltage is reached. Its function is designed specifically to trigger a triac or SCR.

In the beginning triac Ql is not conducting; C1 is charged through variable resistor R3. This charge is coupled to Diac through R1, R3. When trigger level of the diac is reached (about 36 V), D1 fires and Triac TR1 is switched on. R3 and C1 combination sets the firing point of the triac from zero crossing along with Rl and R2. LI and C3 combination acts radio frequency filter for the radio interference caused by triac firing.

Entire circuit operates on mains. Care must be exercised when mounting components on normal Veroboard is risky. Remove alternate tracks and mount components. Triac should be mounted on a small heat sink as the triac tends to get hot particularly at lower speeds. All capacitors are polyester or polycarbonate rated at 600V or more. Rl is a preset for minimum speed control. Adjust this according to your requirement. R2 is the linear variable resistor like the volume control in the radios. Use the one with plastic shaft. L1 is a radio interference choke. Take 28 gage winding wire and make 10 turns on a 6 mm former. It can be wound on a round capacitor for C3 and even one end can be soldered to it also.

TDA1151 based Motor Speed Controller circuit with explanation

This motor speed controller uses a TDA1151 monolithic integrated circuit , designed by ST Microelectronics . TDA1151 motor speed controller circuit is designed in a SOT-32 plastic package and can be used for small applications where the space inside is very critical .
Also this circuit diagram can be used as speed regulator for DC motors of record players, tape and cassette recorders, movie cameras, toys or other low cost applications.

This speed controller circuit can provide a high output current up to 800 mA without any additional components and has a low quiescent current of 1.7mA .
The TDA1151 require a low reference voltage ( around 1.2 volts ) and it has a excellent parameters stability versus temperature .
The maximum voltage that can be applied to the TDA1151 is around 20 volts .
As you can see in the circuit diagram the TDA1151 require just few common external components and is very easy to design .

Source: electroniq.net

12V Speed Dimmer / Controller circuit and explanation

This handy circuit can be used as a speed controller for a 12V motor rated up to 5A (continuous) or as a dimmer for a 12V halogen or standard incandescent lamp rated up to 50W. It varies the power to the load (motor or lamp) using pulse width modulation (PWM) at a pulse frequency of around 220Hz.

SILICON CHIP has produced a number of DC speed controllers over the years, the most recent being our high-power 24V 40A design featured in the March & April 2008 issues. Another very popular design is our 12V/24V 20A design featured in the June 1997 issue and we have also featured a number of reversible 12V designs.

Circuit looks like:

For many applications though, most of these designs are over-kill and a much simpler circuit will suffice. Which is why we are presenting this basic design which uses a 7555 timer IC, a Mosfet and not much else. Being a simple design, it does not monitor motor back-EMF to provide improved speed regulation and nor does it have any fancy overload protection apart from a fuse. However, it is a very efficient circuit and the kit cost is quite low.

Parts layout:

Connection diagram:

There are many applications for this circuit which will all be based on 12V motors, fans or lamps. You can use it in cars, boats, and recreational vehicles, in model boats and model railways and so on. Want to control a 12V fan in a car, caravan or computer? This circuit will do it for you.

Circuit diagram:

The circuit uses a 7555 timer (IC1) to generate variable width pulses at about 210Hz. This drives Mosfet Q3 (via transistors Q1 & Q2) to control the speed of a motor or to dim an incandescent lamp.

Halogen lamps:

While the circuit can dim 12V halogen lamps, we should point out that dimming halogen lamps is very wasteful. In situations where you need dimmable 12V lamps, you will be much better off substituting 12V LED lamps which are now readily available in standard bayonet, miniature Edison screw (MES) and MR16 halogen bases. Not only are these LED replacement lamps much more efficient than halogen lamps, they do not get anywhere near as hot and will also last a great deal longer.
Source: Silicon Chip 15 November 2008

Pump Controller For Solar Hot Water System

This circuit optimises the operation of a solar hot water system. When the water in the solar collector is hotter than the storage tank, the pump runs. The circuit comprises two LM335Z temperature sensors, a comparator and Mosfet. Sensor 1 connects to the solar collector panel while Sensor 2 connects to the hot water panel. Each sensor includes a trimpot to allow adjustment of the output level. In practice, VR1 and VR2 are adjusted so that both Sensor 1 and Sensor 2 have the same output voltage when they are at the same temperature. The Sensor outputs are monitored using comparator IC1.

When Sensor 1 produces a higher voltage than Sensor 2, which means that sensor 1 is at a higher temperature, pin 1 of IC1 goes high and drives the gate of Mosfet Q1. This in turn drives the pump motor. IC1 includes hysteresis so that the output does not oscillate when both sensors are producing a similar voltage. Hysteresis comprises the 1MO feedback resistor between output pin 1 and non-inverting input pin 3 and the input 1kO resistor. This provides a nominal 12mV hysteresis so that voltage at Sensor 1 or Sensor 2 must differ by 12mV for changes in the comparator output to occur.

Circuit diagram:

Since the outputs of Sensor 1 and Sensor 2 change by about 10mV/°C, we could say that there is a degree of hysteresis in the comparator. Note that IC1 is a dual comparator with the second unit unused. Its inputs are tied to ground and pin 2 of IC1 respectively. This sets the pin 7 output high. Since the output is an open collector, it will be at a high impedance. Mosfet Q1 is rated at 60A and 60V and is suitable for driving inductive loads due to its avalanche suppression capability. This clamps any inductively induced voltages exceeding the voltage rating of the Mosfet.

The sensors are adjusted initially with both measuring the same temperature. This can be done at room temperature; adjust the trimpots so that the voltage between ground and the positive terminal reads the same for both sensors. If you wish, the sensors can be set to 10mV/°C change with the output referred to the Kelvin scale which is 273K at 0°C. So at 25°C, the sensor output should be set to (273 + 25 = 298) x 10mV or 2.98V.

Note:

The sensors will produce incorrect outputs if their leads are exposed to moisture and they should be protected with some neutral cure silicone sealant. The sensors can be mounted by clamping them directly to the outside surface of the solar collector and on an uninsulated section of the storage tank. The thermostat housing is usually a good position on the storage tank.

Author: John Clarke - Copyright: Silicon Chip Electronics

PWM Discrete Generator Circuit and explanation

PWM waveforms are commonly used to control the speed of DC motors. The mark/space ratio of the digital wave-form can be defined either by using an adjustable analogue voltage level (in the case of a NE555 based PWM generator) or digitally using binary values. Digitally derived PWM waveforms are most often produced by the timer/counter modules in microcontrollers but if you do not want to include a microcontroller in your circuit it’s also quite simple to generate the signals using discrete logic components.

Circuit diagram:

Discrete PWM Generator Circuit Diagram

An extension of the circuit shown can produce two PWM wave-forms from an 8-bit digital input word. Each signal has 15 values. The 8-bit word can be produced for example from an expansion board fitted in a PC or from an 8-bit port of a processor which does not have built-in PWM capability or from a laptop’s printer port. The mark/space ratio is only programmable up to 15/16 rather than 16/16; a binary input of 0000 produces a continuous low on both outputs turning both motors off.

Similar circuits often employ a dedicated ‘enable’ input to turn the motors off but it is not necessary in this design. The diagram shows the circuitry required to produce just one waveform. For the full two channel circuit it is necessary to use an additional 74HC193. The clock signal produced by the HCF4060 generator can be used to drive both channels and the free flip flop in the 74HC74 package can be used for the second channel (the corresponding pin numbers are shown in brackets). Altogether the entire two channel circuit can be built using just four ICs.

Stepper Motor Controller circuit and explanation

Stepper motors are available in several versions and sizes with a variety of operating voltages. The advantage of this general-purpose controller is that is can be used with a wide range of operating voltages, from approximately 5 V to 18 V. It can drive the motor with a peak voltage equal to half the supply voltage, so it can easily handle stepper motors designed for voltages between 2.5 V and 9 V.

The circuit can also supply motor currents up to 3.5 A, which means it can be used to drive relatively large motors. The circuit is also short-circuit proof and has built-in over temperature protection. Two signals are required for driving a stepper motor. In logical terms, they constitute a Grey code, which means they are two square-wave signals with the same frequency but a constant phase difference of 90 degrees.

IC1 generates a square-wave signal with a frequency that can be set using potentiometer P1. This frequency determines the rpm of the stepper motor. The Grey code is generated by a decimal counter in the form of a 4017. Outputs Q0–Q9 of the counter go high in succession in response to the rising edges of the clock signal. The Grey code can be generated from the outputs by using two OR gates, which are formed here using two diodes and a resistor for each gate, to produce the I and Q signals.

Here ‘I’ stands for ‘in-phase’ and ‘Q’ for ‘quadrature’, which means it has a 90-degree phase offset from the I signal. It is common practice to drive the windings of a stepper motor using a pair of push-pull circuits for each winding, which is called an ‘H bridge’. That makes it possible to reverse the direction of the current through each winding, which is necessary for proper operation of a bipolar motor (one whose windings do not have centre taps).

Of course, it can also be used to properly drive a unipolar motor (with centre-tapped windings). Instead of using a push-pull circuit of this sort, here we decided to use audio amplifier ICs (type TDA2030), even though that may sound a bit strange. In functional terms, the TDA2030 is actually a sort of power opamp. It has a difference amplifier at the input and a push-pull driver stage at the output.

Circuit diagram:

Stepper Motor Controller Circuit Diagram

IC3, IC4 and IC5 are all of this type (which is economically priced). Here IC3 and IC4 are wired as comparators. Their non-inverting inputs are driven by the previously mentioned I and Q signals, with the inverting inputs set to a potential equal to half the supply voltage. That potential is supplied by the third TDA2030. The outputs of IC3 and IC4 thus track their non-inverting inputs, and each of them drives one motor winding.

The other ends of the windings are in turn connected to half the supply voltage, provided by IC5. As one end of each winding is connected to a square-wave signal that alternates between 0 V and a potential close to the supply voltage, while the other end is at half the supply voltage, a voltage equal to half the supply voltage is always applied to each winding, but it alternates in polarity according to the states of the I and Q signals.

That’s exactly what we want for driving a bipolar stepper motor. The rpm can be varied using potentiometer P1, but the actual speed is different for each type of motor because it depends on the number of steps per revolution. The motor used in the prototype advanced by approximately 9° per step, and its speed could be adjusted over a range of approximately 2 to 10 seconds per revolution.

In principle, any desired speed can be obtained by adjusting the value of C1, as long as the motor can handle it. The adjustment range of P1 can be increased by reducing the value of resistor R5. The adjustment range is 1:(1000 + R5)/R5, where R5 is given in k.If a stepper motor is switched off by removing the supply voltage from the circuit, it’s possible for the motor to continue turning a certain amount due to its own inertia or the mechanical load on the motor (flywheel effect).

It’s also possible for the position of the motor to disagree with the states of the I and Q signals when power is first applied to the circuit. As a result, the motor can sometimes ‘get confused’ when starting up, with the result that it takes a step in the wrong direction before starting to move in direction defined by the drive signals. These effects can be avoided by adding the optional switch S1 and a 1-k resistor, which can then be used to start and stop the motor. When S1 is closed, the clock signal stops but IC2 retains its output levels at that moment, so the continuous currents through the motor windings magnetically ‘lock’ the rotor in position.

The TDA2030 has internal over temperature protection, so the output current will be reduced automatically if the IC becomes too hot. For that reason, it is recommended to fit IC3, IC4 and IC5 to a heat sink (possibly a shared heat sink) when a relatively high-power motor is used. The tab of the TO220 case is electrically bonded to the negative supply voltage pin, so the ICs can be attached to a shared heat sink without using insulating washers.

Author: Gert Baars - Copyright: Elektor Electronics Magazine

DC Motor Speed Controller

This circuit takes advantage of the voltage drop across bridge rectifier diodes to produce a 5-position variable voltage supply to a DC fan or other small DC motor. It is not as efficient as a switch-mode circuit but it has the virtues of simplicity and no switching hash. The four full-wave bridges are connected so that each has two pairs of series diodes in parallel, giving a voltage drop of about 1.4V, depending on the load current.

Circuit diagram:

DC Motor Speed Controller Circuit Diagram

The rotary switch should have "make before break" contacts which should be rated to take currents up to about an amp or so. For higher currents, higher rated bridge rectifiers and a suitably rugged rotary switch (or solenoids) will be required. If you want smaller voltage steps, you could use the commoned AC inputs on the bridge rectifiers to give intermediate steps on the speed switch.
Author: Stephen Butcher

Motor Generator Stepper

Stepper motors are a subject that keeps recurring. This little circuit changes a clock signal (from a square wave generator) into signals with a 90-degree phase difference, which are required to drive the stepper motor windings. The price we pay for the simplicity is that the frequency is reduced by a factor of four. This isn’t really a problem, since we just have to increase the input frequency to compensate. The timing diagram clearly shows that the counter outputs of the 4017 are combined using inverting OR gates to produce two square waves with a phase difference. This creates the correct sequence for powering the windings: the first winding is negative and the second positive, both windings are negative, the first winding is positive and the second negative, and finally both windings are positive.

Circuit diagram:

Stepper Motor Generator Circuit Diagram

Internally, the 4017 has a divide-by-10 counter followed by a decoder. Output ‘0’ is active (logic one) as long as the internal counter is at zero. At the next positive edge of the clock signal the counter increments to 1 and output ‘1’ becomes active. This continues until output ‘4’ becomes a logic one. This signal is connected to the reset input, which immediately resets the counter to the ‘zero’ state. If you were to use an oscilloscope to look at this output, you would have to set it up very precisely before you would be able to see this pulse; that’s how short it is. The output of an OR gate can only supply several mA, which is obviously much too little to drive a stepper motor directly. A suitable driver circuit, which goes between the generator and stepper motor.

Solar Hot Water Panel Differential Pump Controller circuit and explanation

This circuit optimises the circulation of heated water from solar hot water panels to a storage cylinder. It achieves this by controlling a 12V DC pump, which is switched on at a preset temperature differential of 8°C and off at about 4°C. This method of control has distinct advantages over some systems that run the pump until the differential approaches 0°C. In such systems, the pump typically runs whenever the sun shines. A small 10W solar panel charging a 12V SLA battery is sufficient to run the controller. Most commercial designs use 230VAC pumps, which of course don’t work when there is a power outage or there is no AC power at the site.

Operation:

Temperature sensors TS1 & TS2 are positioned to measure the highest and lowest water temperatures, with one at the panel outlet and the other at the base of the storage cylinder. The difference between the sensor outputs is amplified by op amp IC1d, which is configured for a voltage gain of about 47. As the sensors produce 10mV/°C, a difference of 8°C will produce about 3.76V at the op amp’s output (pin 14). The output from IC1d is fed into the non-inverting input (pin 10) of a second op amp stage (IC1c), which is wired as a voltage comparator. The op amp’s inverting input (pin 9) is tied to a reference voltage, which can be varied by trimpot VR3. When the voltage from IC1d exceeds the reference voltage, the output of the comparator (pin 8) swings towards the positive rail.

A 10MW resistor feeds a small portion of the output signal back to the non-inverting input, adding some hysteresis to the circuit to ensure positive switching action. A third op amp stage (IC1b) acts as a unity-gain buffer. When the comparator’s output goes high, the buffer stage switches the Mosfet (Q1) on, which in turn energises the pump motor. Mosfet Q1’s low drain-source on-state resistance means that in most cases, it won’t need to be mounted on a heatsink. The prototype uses a Davies Craig EBP 12V magnetic drive pump, which draws about 1A when running and is suitable for low-pressure hot water systems only (don’t use it for mains-pressure systems as it may burst!). For mains-pressure systems, the author suggests the SID 10 range of brass-body magnetic drive pumps from Ivan Labs USA.

Circuit diagram:

Solar Hot Water Panel Differential Pump Controller Circuit Diagram

Setup:

Each LM335 temperature sensor and its associated trimpot is glued to a small copper strip using high-temperature epoxy. It is then waterproofed with silicon sealant and encapsulated in heatshrink tubing. Standard twin-core shielded microphone cable can be used for the connection to the circuit board. Before sealing the two units, adjust their trimpots to get 2.98V at 25°C [(ambient temperature x .01) + 2.73V] between the "+" and "-" terminals. When both have been adjusted, clamp them together and allow their temperatures to stabilise for a few minutes. Next, measure the output voltage from the differential amplifier (IC1d), which should be close to 0V. If not, tweak one of the pots until it is.

Separate the two and warm the panel sensor (TS1), monitoring the output of IC1d. You should see a marked increase in voltage, remembering that an 8°C difference between the sensors should give an output of about 3.76V. The pump switch-on point is set by VR3 and can be adjusted over a practical range of about 4-10°C differential (1.88-4.70V). Adjust VR3 to get about 3.8V on pin 9 of IC1c as a starting point. If set too low and the panels are located far from the cylinder, much of the heat will be lost in the copper connecting pipes. On the other hand, if set too high and the weather is mostly cloudy, then the pump will not switch on very often, as the panels will not get hot enough. For best results, use copper pipes for the panel plumbing and insulate them with tubes of closed-cell foam.

As the pipes cool down between pump operations, small diameter pipes of 15mm are more efficient than larger sizes as they contain less static water. In practice, the pump in the author’s setup switches on for about 30 seconds every 4-5 minutes. As the Davies pump shifts 13 litres/minute, it displaces the heated water from a single panel in about 14 seconds. There is a thermal lag in the sensor readings, so after the pump stops, the temperature difference will keep decreasing for 40 seconds or so as the panel sensor cools down and the cylinder sensor heats up.

Author: Mike Scaife - Copyright: Silicon Chip Electronics

Fan Controller with Just Two Components circuit and explanation

The Maxim MAX 6665 (www.maxim-ic.com) provides a complete temperature-dependent fan controller. It can switch fans operating at voltages of up to 24 V and currents of up to 250 mA. The IC is available from the manufacturer in versions with preset threshold temperatures between +40 °C (MAX6665 ASA40) and +70 °C (MAX6665 ASA 70). The device’s hysteresis can be set by the user via the HYST input, which can be connected to +3.3 V, connected to ground, or left open. The following table shows the hysteresis values available:

HYST = Hysteresis
open = 1 °C
ground = 4 °C
+3.3V = 8 °C

Fan Controller Circuit Diagram

The other pins of the SO8 package are the FORCEON input and the status outputs WARN, OT and FANON. The test input FORCEON allows the fan to be run even below the threshold temperature. The open-drain output WARN goes low when the temperature rises more than 15 °C above the threshold temperature, while the open-drain output OT indicates when the temperature is more than 30 °C above the threshold. The push-pull output FANON can be used to indicate to a connected microcontroller that the fan is turned on.

Author: G. Kleine
Copyright: Elektor Electronics

PWM Dimmer/Motor Speed Controller

This is yet another project born of necessity. It's a simple circuit, but does exactly what it's designed to do - dim LED lights or control the speed of 12V DC motors. The circuit uses PWM to regulate the effective or average current through the LED array, 12V incandescent lamp (such as a car headlight bulb) or DC motor. The only difference between the two modes of operation is the addition of a power diode for motor speed control, although a small diode should be used for dimmers too, in case long leads are used which will create an inductive back EMF when the MOSFET switches off.

Photo of Completed PWM Dimmer/Speed Control

The photo shows what a completed board looks like. Dimensions are 53 x 37mm, so it's possible to install it into quite small spaces. The parts used are readily available, and many subsitiutions are available for both the MOSFET and power diode (the latter is only needed for motor speed control). The opamps should not be substituted, because the ones used were chosen for low power and their ability to swing the output to the negative supply rail.

Note that if used as a motor speed controller, there is no feedback, so motor speed will change with load. For many applications where DC motors are used, constant speed regardless of load is not needed or desirable, but it is up to you to decide if this will suit your needs.

Description

First, a description of PWM is warranted. As the pot is rotated clockwise, the input voltage changes linearly with rotation. At first, the voltage is such that the comparator output is just narrow spikes, which turn the MOSFET on for a very short period. Average current is low, so connected LEDs will be quite dim, or a motor will run (relatively) slowly. As the input voltage coming from the pot increases, the MOSFET is on for longer and longer, so increasing power to the load.

Figure 1 - PWM Waveform Generation

Figure 1 shows how the PWM principle works. The red trace is the triangle wave reference voltage, and the green trace is the voltage from the pot. When the input voltage is greater than the reference voltage, the MOSFET turns on, and current flows in the load. Because the frequency is relatively high (about 600Hz), we don't see any flicker from the LEDs, but the tone is audible from a motor that's PWM controlled. The PWM signal is shown in blue. The average current through the load is determined by the ratio of on-time to off-time, and when both are equal, the average current is exactly half of that which would be drawn with DC.

Figure 2 - Dimmer/Speed Controller Schematic

The circuit is shown in Figure 2. U1 is the oscillator, and generates a triangular waveform. R4 and R5 simply set a half voltage reference, so the opamps can function around a 6V centre voltage. U2A is an amplifier, and its output is a 10V peak to peak triangle wave that is used by the comparator based on U2B. This circuit compares the voltage from the pot with the triangle wave. If the input voltage is at zero, the comparator's output remains low, and the MOSFET is off. This is the zero setting.

In reality, the reference triangle waveform is from a minimum of about 1.5V to a maximum of 9.5V, so there is a small section at each end of the pot's rotation where nothing happens. This is normal and practical, since we want a well defined off and maximum setting. Because of this range, for lighting applications, an industry standard 0-10V DC control signal can be used to set the light level. C-BUS (as well as many other home automation systems) can provide 0-10V modules that can control the dimmer.

While a 1N4004 diode is shown for D2, this is only suitable if the unit is used as a dimmer. For motor speed control, a high-current fast recovery diode is needed, such as a HFA15TB60PBF ultra-fast HEXFRED diode. There are many possibilities for the diode, so you can use whatever is readily available that has suitable ratings. The diode should be rated for at least half the full load current of the motor, and the HFA15TB60PBF suggested is good for 15A continuous, so is fine with motors drawing up to 30A.

Construction

While it's certainly possible to build the dimmer on veroboard or similar, it's rather fiddly to make and mistakes are easily made. Also, be aware that because of the current the circuit can handle, you will need to use thick wires to reinforce some of the thin tracks. This is even necessary for the PCB version. Naturally, I recommend the PCB, and this is available from ESP. The board is small - 53 x 37mm, and it carries everything, including the screw terminals. The PCB is double-sided with plated-through holes, and has solder masks on both sides.

The MOSFET will need a heatsink unless you are using the dimmer for light loads only. It is necessary to insulate the MOSFET from the heatsink in most cases, since the case of the transistor is the drain (PWM output). For use at high current and possible high temperatures, the heatsink may need to be larger than expected. Although the MOSFET should normally only dissipate about 2W or so at 10A, it will dissipate a lot more if it's allowed to get hot. Switching MOSFETs will cheerfully go into thermal runaway and self destruct if they have inadequate heatsinking. You may also use an IGBT (insulated gate bipolar transistor) - most should have the same pinouts, and they do not suffer from the same thermal runaway problem as MOSFETs.

As noted above, there are many different MOSFETs (or IGBTs) and fast diodes that are usable. The IRF540 MOSFET is a good choice, and being rated 27A it has a generous safety margin. There are many others that are equally suitable - in fact any switching MOSFET rated at 10A or more, and with a maximum voltage of more than 20V is quite ok.

Testing

Connect to a suitable 12V power supply. When powering up for the first time, use a 100 ohm "safety" resisor in series with the positive supply to limit the current if you have made a mistake in the wiring. The total current drain is about 2.5mA with the pot fully off, rising to 12.5mA when fully on. Most of this current is in the LED, which is also fed from the PWM supply so you can see that everything is working without having to connect a load.

Make sure that the pot is fully anti-clockwise (minimum), and apply power. You should measure no more than 0.25V across the safety resistor, rising to 1.25V with the pot at maximum. If satisfactory, remove the safety resistor and install a load. High intensity LED strip lights can draw up to ~1.5A each, and this dimmer should be able to drive up to 10 of them, depending on the capabilities of the power supply and the size of the heatsink for the MOSFET.

source: http://sound.westhost.com/project126.htm

Electronic Motor Starter circuit and explanation

This motor starter protects single-phase motors against voltage fluctuations and overloading. Its salient feature is a soft on/off electronic switch for easy operation. The transformer steps down the AC voltage from 230V to 15V. Diodes D1 and D2 rectify the AC voltage to DC. The unregulated power supply is given to the protection circuit. In the protection circuit, transistor T1 is used to protect the motor from over-voltage. The over-voltage setting is done using preset VR1 such that T1 conducts when voltages goes beyond upper limit (say, 260V). When T1 conducts, it switches off T2. Transistor T2 works as the under-voltage protector. The under-voltage setting is done with the help of preset VR2 such that T2 stops conducting when voltage is below lower limit (say, 180V). Zener diodes ZD1 and ZD2 provide base bias to transistors T1 and T2, respectively. Transistors T3 and T4 are connected back to back to form an SCR configuration, which behaves as an ‘on’/‘off’ control.

Switch S1 is used to turn on the pump, while switch S2 is used to turn off the pump. While making over-/under-voltage setting, disconnect C2 temporarily. Capacitor C2 prevents relay chattering due to rapid voltage fluctuations. Regulator IC 7809 gives the 9V regulated supply to soft switch as well as the relay after filtering by capacitor C4. A suitable miniature circuit breaker is used for automatic over-current protection. Green LED (LED1) indicates that the motor is ‘on’ and red LED (LED2) indicates that the power is ‘on’. The motor is connected to the normally-open contact of the relay. When the relay energizes, the motor turns on.