Ten Uses for an Autotransformer, or Variac

1) Testing a repaired electronic device.

After performing a repair on a piece of electronic equipment, it is best to slowly power up the device to avoid burning up the replaced parts. You will often discover multiple part failures one at a time. Using the Variac to slowly power up the repaired unit can prevent damaging the replacement parts.

2) Powering up an old radio or amplifier.

When powering up an old radio or amplifier that has not been used for an extended period of time, it is best to apply reduced voltage power to the unit in stages in order to re-condition the electrolytic capacitors.

3) Roasting coffee beans or nuts.

To obtain the freshest coffee possible, coffee connoisseurs prefer to roast and grind their own coffee beans. You can use a Variac to adjust the roasting temperature to the ideal value.

4) Adjusting the temperature of a resistance-type heater.

Resistance-type heating elements can be adjusted by varying the AC power applied.

5) Hot-wire cutting of Styrofoam.

Styrofoam and other types of foam are typically cut to the desired shape using a hot wire cutting system. You can use a Variac to adjust the temperature of the hot wire to the optimal value required for cutting.

6) Dimming incandescent lighting.

Incandescent lights may be dimmed by varying the applied AC voltage. This is not possible with newer LED or fluorescent lamps.

7) Changing fan speed over a narrow range.

The speed of some types of small AC motors may be varied over a narrow range by changing the applied AC voltage. A problem with this application is that the torque of the motor is also reduced which may result in the fan or motor stalling out. Also note that this technique cannot be used with all electric motors.

8) Compensating for line voltage drop in AC power applications.

Long power line runs can often result in voltage drops across the lines, especially in high current draw applications like hot tubs or spas. You can use a Variac to compensate for this voltage loss.

9) High-current unregulated DC power supply.

A high-current adjustable unregulated DC power supply can be constructed using a Variac, high current rectifier, and filter capacitor. Do not attempt this without being aware of the possible safety concerns when using a Variac. Since a Variac is not isolated from the AC power lines, the AC power is directly accessible. The ideal configuration would use an isolation transformer to isolate the AC power mains from the application.

10) Adjusting the voltage of an AC power supply.

The output of an AC power supply, such as an electric train transformer, can be adjusted with a Variac.

Using a Variac to Power Up an Old Radio or Amplifier

It is best to monitor the AC current drawn by the unit as it is powered up, which indicates the “health” of the radio or amplifier. The easiest method is a “clamp-on” AC current meter. In general the AC current draw should be about 50 to 75 % of the rating of the fuse in the radio or amp.


Bring the variac up slowly over a period of several minutes until you reach about 50 volts. This will pre-heat the tube filaments and begin to re-form the electrolytic capacitors. Keep an eye on the current value and be on the lookout for smoke or any malfunctions. The variac should be left running at this voltage level for about 30 minutes.

Next, slowly bring the variac up to around 75 volts. This will allow the power transformer windings in a transformer-equipped set to warm up a bit, driving out any accumulated moisture, and will continue to re-condition the electrolytic capacitors. Leave the variac at this voltage for at least another 30 minutes.

After waiting 30 minutes, raise the voltage to around 90 volts. At this point the radio or amplifier should be functioning. If the unit begins to hum, you will need to replace the electrolytic capacitors. You should probably consider replacing the capacitors on an old piece of equipment even if it doesn’t hum. Old filter caps will fail sooner or later, so you may save yourself some time and trouble later on.

After about 10 to 15 minutes you can raise the variac output to full voltage and verify everything is working correctly. If you notice a sudden increase in current or see or smell smoke, immediately turn everything off.

A variac is an excellent tool for safely powering up an old radio or amplifier that has not been used for an extended period of time. Using this low-priced piece of equipment could save you time and money as well as enable you to bring that old piece of equipment back to life with very little effort.

Measuring Power with a Digital Panel Meter (DPM)

Digital Panel Meters (DPMs) are voltage measuring devices which can be used to measure current using a shunt resistor or hall effect current sensor. When a display of Power (watts) is required, the voltage and current values must be multiplied to produce a voltage that is proportional to the product of these two quantities.

Power (watts) = Voltage X Current

This application note will describe a method to accurately display DC Power values on a DPM.

There are several methods that can be used to calculate power based on voltage and current values. Most methods require a micro-controller to perform the calculation and then display the power level on an LCD or similar indicating device. While this can be an extremely accurate way to display Power, it requires that the analog values of voltage and current be converted to digital values that can be processed by the micro-controller. Furthermore, a firmware program must be written to make it functional. The method described here, uses an analog multiplier IC to perform the calculation directly on the two analog quantities which then produces a third analog value suitable for display on a DPM. This is entirely a hardware solution and requires no software program.

An Analog Devices AD633 device was chosen as the analog multiplier IC due to its excellent accuracy and ease of use. Since it is a 4 quadrant multiplier it functions equally well with any voltage polarity and produces an output voltage that is proportional to the product of the X and Y input voltages. This is the DC Power level that can now be displayed on a DPM.

This application note has shown how a Digital Panel Meter may be used to measure and display DC Power values based on a DC voltage and a DC current applied to the inputs of the circuit. The connection diagram is shown below.



Using FFT in Digital Storage Oscilloscopes (DSO)

Many modern Digital Storage Oscilloscopes (DSOs) have advanced mathematical operations available that typically include varying degrees of FFT analysis of the observed waveform. Fast Fourier Transform (FFT) allows the frequency content of a signal to be displayed along with the amplitude of these various frequency components.

Since the FFT operation is a mathematical operation, it requires a large amount of computing time to properly display the information. For these reasons, FFT operation in a DSO is not equivalent to a spectrum analyzer, but oftentimes it is sufficient for many applications.

FFT analysis of a signal is useful for tracking down EMI/RFI problems and for detecting spurious oscillations in circuitry. It is also useful for displaying the harmonic content of various waveforms and tracking down problems in switching power supplies. Distortion created by nonlinear operation of analog circuitry can easily be shown using FFT analysis. This phenomenon is essentially impossible to view using a standard time domain representation of the signal as is shown with a standard DSO display of the waveform.

FFT analysis is an extremely useful tool available to the technician or engineer. It can provide insight into the operation of a circuit and help determine which components or operating parameters are causing problems. Since this function is entirely a mathematical operation, it is implemented in the firmware of the DSO and therefore does not require additional hardware to implement, and therefore does not increase the cost of the unit.

Reasons for Microstepping a Stepper Motor (Part Two)

Microstepping can replace a mechanical gearbox in certain applications when you need small relative movements or greater step resolution. Even if you have to use a larger stepper motor, this is often a preferable solution in a number of applications. You need to carefully select the appropriate stepper motor to get the best possible results, and you should also consider developing customized sine/cosine profiles.

You can increase stepper motor position accuracy beyond the manufacturer’s specifications by microstepping. One way to accomplish this is by designing a microprocessor-based microstepping system using the motor at two-phase-on stop positions, which are typically the most accurate rotor stop positions. Use an automatic or manual factory calibration process to store a correction value for every stop position on each motor you use.

You use the correction value to send adjusted full-step positions to the stepper motor. These adjusted positions have slightly different current levels in the windings, which compensates for the deviations of the position at the original stop positions. This type of microstepping technique is ideal when optimal step accuracy is the most important design criteria. When you use this technique, the stepper motor system must use a rotor home position indicator for synchronizing the rotor and the compensation profile.

Although the electronics needed to generate microstepping are more complex than the electronics used in half- and full-stepping, the total system complexity — including gearbox, transmission, and stepper motor — is less complex and costly in most applications. In addition, microstepping can simplify or altogether replace gearboxes and mechanics for damping noise and vibrations. Moreover, stepper motor selection is easier and more flexible.

You can use software and PWM-timers or digital-to-analog converters in the microprocessor as a replacement for external stepper motor controllers in microprocessor-based microstepping applications in order to achieve the lowest possible hardware cost, which is to say you can obtain microstepping hardware for about the same price as half- and full-step systems for comparable stepper motor sizes.

Read Part One

Reasons for Microstepping a Stepper Motor (Part One)

Microstepping can improve stepper motor system performance in a number of applications, and it can reduce system cost and complexity relative to half- and full-step driving techniques. In addition, microstepping can help solve noise and resonance problems all while increasing step accuracy and resolution.

A stepper motor system’s natural frequency is determined by holding torque, rotor and load inertia, and the number of full-steps per revolution. When stepper motor system damping is low you risk generating noise or losing steps when the stepper motor operates at or near the resonant frequency depending on damping, total inertia, and the type of stepper motor. These issues can happen at or near integer multiples and fractions of the natural frequency. Typically the frequencies closest to the natural frequency cause the most problems.

The principal source of these resonances is that the stator flux moves in a discontinuous way when you use a non-microstepping stepper motor driver — forty-five or ninety degrees at a time — causing a pulsing energy flow to the rotor, and these pulsations excite the resonance. Using half-steps rather than full-steps reduces the excitation energy to roughly twenty-nine percent of the full-step energy. If we microstep the motor in 1/32-full-step mode only point one percent of the full-step energy remains. You can reduce the excitation energy to a low enough level that all resonances are completely eliminated by microstepping.

However, this is only true of an ideal stepper motor. In practical applications there are additional sources that excite system resonances. Regardless, microstepping improves movement in nearly all applications and, in many cases, microstepping alone will sufficiently reduce noise and vibrations for most applications.

When you run a stepper motor at low frequencies in half- or full-step mode the movement is discontinuous, noise and vibrations are generated, and there will be significant ringing. The frequencies where this occurs are below the stepper motor system’s natural frequency, which is why microstepping offers a safe, simple means of extending noiseless stepping frequencies approaching zero hertz.

You don’t usually need steps smaller than 1/32-full-step — electrical step angles this small are easily absorbed by the stepper motor’s internal friction, meaning the stepping doesn’t generate overshot or ringing. The microstepping positions will deviate from a straight line because of uncompensated sine/cosine profiles.

Unipolar and Bipolar Stepper Motor Selection

There are two basic types of stepper motors for circuit design: unipolar and bipolar. A stepper motor will move one step when the current flow changes direction in the field coil(s), thus reversing the magnetic field of the poles, and this reversal is what separates unipolar and bipolar stepper motors.


Bipolar stepper motor circuits have the advantage of having just one winding and a low winding resistance. The two changeover switches are a disadvantage to using a bipolar stepper motor in that they need more semiconductors than their unipolar counterparts.

Unipolar circuits need just one changeover switch; however, they have the crippling disadvantage of requiring a double bifilar winding, meaning that at a specific bulk factor the wire is thinner and the resistance is considerably higher. Because unipolar motors seem simpler with regard to use with discrete devices, they remain popular today, although bipolar motors can be driven with the same number of components as a result of modern integrated circuits.

A bipolar stepper motor produces the most torque, which is proportional to the intensity of the magnetic field of the stator windings and can be increased with additional windings or by boosting the current; but increasing current can saturate the motor’s iron core and, more importantly, will increase the maximum temperature of the stepper motor. Because of the double cross section of the wire, unipolar motors have the advantage of having half the copper resistance of bipolar motors. At its power loss limit a bipolar stepper motor will still provide roughly forty percent more torque than a unipolar motor built on the same frame.

Current Driving

To keep the power loss of a stepper motor at a reasonable limit you must control the current in the windings. The simplest, most popular solution is to supply only the voltage needed, using the winding’s resistance to limit current. There’s a more efficient and precise way to address this issue, but it is also more complicated than the previous example: include a current generator to achieve independence from the resistance of the winding.

Introduction to Linear & Switching Power Supplies

The main features of a linear power supply are the transformer, large input capacitor, and large transistor with a heat sink . Conventional linear power supplies use a low-frequency transformer consisting of a core made from silicon steel sheet that converts AC mains to the desired voltage and rectifies and filters the voltage to a DC level. Because equipment requires stable DC voltage, the filtered and rectified DC level must be managed with a power regulator, which clamps excess voltage at a specific level and dissipates unwanted voltage as heat.

Linear power supplies should have a limit on the variable range of the AC input voltage in order to maintain the efficiency of the power supply. Linear power supplies clamp unwanted voltage as well as ripple voltage, dissipating both as heat, resulting a very small ripple voltage. Linear power supplies have, as the name implies, linear topology, meaning ripple noise and EMI are not a concern.

A switching power supply improves many of its linear counterparts shortcomings, namely the huge volume, considerable weight, and low efficiency. A switching power supply filters and rectifies the voltage from the AC mains without a low-frequency transformer. Because of the high rectified voltage, the bulk capacitor’s capacitance can be remarkably small. Switching power supplies have high-frequency transistor’s that chop the high DC voltage into high AC voltage, converting it into a specified voltage with a high-frequency transformer, and finally rectify and filter the voltage to a DC level. The high-frequency switching of the diodes and the transistor creates ripple noises at the transient of switching.

Switching power supplies are smaller, lighter, and more efficient than linear power supplies, which is why switching power supplies have become increasingly popular in field applications. In addition, switching power supplies continue to come down in price. However, switching power supplies have the disadvantages of having comparatively complicated circuitry, less stability, greater ripple, slower transient response, and are subject to EMI (which can be reduced with filters). The choice between linear and switching power supplies comes down to whether you will be working in the field.

How to Discharge Capacitors in Switched-Mode Power Supplies

Switched-mode power supplies have multiple large-filter capacitors that can hold hazardous charges, even if the power supply hasn’t been used in days. These filter capacitors typically have values as high as 220uF/250V and 330uF/400V. You must discharge the capacitors before working on power supply circuits so you won’t get shocked.

There are three different ways to discharge large filter capacitors in a power supply: with a screwdriver, the leads of a socketed 100 watt light bulb, and the leads of a high-wattage resistor.

Using a screwdriver to discharge the capacitor is not recommended because you can generate a spark and damage the printed circuit board or circuitry of the power supply. You can even blow the power section. Keep in mind that if you know a capacitor’s stored voltage is relatively low, you can discharge it with a small screwdriver without undue risk.

If the capacitor is holding a heavier charge, discharging the capacitor can melt the screwdriver’s tip as well as the copper of the printed circuit board. A heavy spark is especially dangerous: it can cause small bits of solder lead or copper to shoot from the circuit board, potentially injuring your eyes.

The second method involves placing the leads of a socketed 100 watt electric light bulb on the capacitor’s lead and is used by technicians around the globe. The light bulb acts as an indicator, showing if the capacitor contains a charge. If there is a charge present the light bulb will illuminate and eventually turn off when the capacitor in the switching power supply is discharged.

The last method consists of placing the leads of a high-wattage resistor on the capacitor’s leads. You can use a 2.2k ohm ten-watt resistor to discharge high voltage capacitors in a switched-mode power supply. It is a very simple and effective process, taking mere seconds to completely discharge the capacitor.

There’s really no reason to discharge a capacitor with a screwdriver when all you need is a light bulb or a resistor, so keep that in mind the next time you need to discharge capacitors in a switched-mode power supply.

Switching Power Supply: Preventing Noise

A switching power supply is a remarkably efficient type of power supply, which quickly turns off and on, used to regulate amperage and current. Although all switching power supplies will produce noise, they are usually designed to create noises inaudible to the human ear. If your switching power supply isn’t designed for quiet operation and it’s producing a high- or low-pitched whine or hissing, there are ways to mitigate the noise.

High-Pitched Noise without a Fan 

Check where you’ve plugged in your power supply. High-pitched hisses or whines are typically caused by external electromagnetic interference. Keep your power supply away from anything which produces electromagnetic interference — for example, fluorescent lights, power strips, flat-screen monitors, or large batteries. Simply moving your switching power supply a few feet from objects that produce interference can stop the noise entirely.

If after you’ve moved the power supply and the noise continues, it is likely because you have a dry capacitor that needs replacement. Unplug your power supply and leave it unplugged for three or more hours, because capacitors will hold an electric charge and will need to lose this charge. Do not remove or replace capacitors without electronic training.

Use a soldering iron to remove the capacitors. Purchase identical components, and then solder the replacement capacitors to the circuit board.

High-Pitched Noise with a Fan 

First you need to unplug the device connected to the power supply, and then remove the casing. The fan is most likely oscillating at a frequency that makes the ordinarily high-pitched whine audible to the human ear.

Remove the power supply fan’s protective plating, which is located in the center of the fan. The protective plating will be held on with screws or a thick sticker; remove the screws or peel the sticker back. Place a couple drops of sewing machine oil on the now-exposed bearing. The last step is to put the protective plating back on and replace the power supply casing.

Low-Pitched Hum 

Look for a metal casing that fits over the entire power supply. Place your power supply in the metal casing and screw it shut. Plug the power supply in and turn it on.

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