A rectifier is an electrical device used to convert alternating current (AC) into direct current (DC) by allowing a current to flow through the device in one direction only. Diodes work like one-way valves within the rectifier to maintain this flow of current. This process is generally known as “rectification.”
While rectifiers have many uses, they are most often used as the primary components of DC power supplies and high-voltage direct current transmission systems. In an industrial setting, rectifiers are typically spec’d based on voltage applied, current needed in the process, quality of the power, and how the control will be arranged.
Ripple is an important measurement for determining the efficiency and quality of a rectifier. Ripple is the clarity of the power (how clean it is) expressed in a percentage. AC divided by DC equals the percent of ripple.
There are a wide variety of industrial rectifiers, including SCR, powerstat, tapswitch, switch mode, IGBT chopper, and thyristor rectifiers. Tapswitch and powerstat rectifiers offer little to no control for operators. They have a very low ripple reading, low amperage, and low cost, yet they are typically very expensive to repair.
Alternatively, SCR, SMPS, and IGBT chopper rectifiers offer seemingly infinite control for operators; they can easily control voltage from top to bottom between zero and 100 percent.
SCR Rectifiers
An SCR rectifier is a semiconductor power supply that meters electricity by opening electrical “valves” that work together to rectify electricity. The longer the “valve” is open, the higher the voltage leaving the rectifier will be.
RapidX Series SCR Rectifier
SCR rectifiers are variable voltage DC power supplies that are low frequency, high ripple systems. These systems are rugged and have a history of durability in the market— a number of our customers have systems still running after 40 or 50 years. They regulate and react steadily, carry a lot of power in large copper windings, and are fairly easy to troubleshoot with large, easy-to-identify components.
Switch Mode Power Supplies
A switch mode power supply (SMPS) is an electronic power supply that uses a switch from AC to DC, back to AC, then once again back to DC. This is all done at high frequency allowing for the internal parts to be smaller. They utilize a high primary voltage—where 480VAC input units typically see 700VDC—switching voltages inside the transformer.
With smaller parts, tighter windings, smaller footprints, and an even tinier tolerance, these precision pieces of equipment offer good space efficiency per watt, modern computer interfaces, and intuitive controls.
Switch mode power supplies have been around for roughly 40 years. Traditionally, these were used in smaller current applications and only in the last 10 years or so have large-scale switch modes been successfully deployed.
Rectifiers can be air cooled, water cooled, oil cooled or hybrid cooled. Traditionally, most rectifier manufacturers offer air or water cooled rectifiers. Air cooled units are typically larger per watt of power because they need more surface area to dissipate the heat, while water cooled units are typically smaller because surface area is not a factor in cooling these units.
Although extremely durable and long-lasting, rectifiers do occasionally need repairs. The majority of rectifier issues are caused by the overheating of electrical components, like a circuit board. The root cause of overheating could be corrosion, bad cooling lines, a wrong component put in during a repair, breakdown of organic varnish in the transformer, or poor bussing connections.
Good heat management is important for long-term rectifier maintenance. The general rule of thumb to minimize heat is: clean, flat, and tight:
Keep all contacts clean.
Ensure all electrical surfaces are mounted flat to one another.
All electrical surfaces must be tight.
Often customers confuse transformers and power supplies when they are searching for one or the other. These two technologies are related, but are completely different in their applications. This post will look at the similarities and differences and help you decide if you are in need of a transformer or power supply and the options available to you.
These devices typically have two modes and a function that all apply to alternating current applications. They are used for stepping up AC voltage, stepping down AC voltage, and isolating high voltage from low voltage in either mode. In the simplest terms, a transformer is a pair of special inductors coiled around an electromagnet. The ratio of how many turns in the inductive coil on one side compared to the other side determines the mode of step up versus step down. If there are more turns on the higher voltage side and fewer coils on the other, the transformer is acting in step-down mode.
In short, transformers do not convert AC to DC.
There are several types of technologies for converting AC power to DC power. Here are the basic steps for producing this effect (this may vary based on technology, but the ideas still apply):
AC Voltage is stepped down to a safe level using a transformer or similar technology.
The reduced voltage is then passed through a full wave rectifier to produce a positive supply that is still going between 0 Volts and the stepped voltage minus the average forward voltage of the rectifier.
The bouncing power is not stable for most applications, so this must be regulated.
Regulation often changes based on technology more than the rectification and transformation, the general first step is to add a capacitor to reduce the ripple current to a more steady line.
After the right capacitor is chosen, there is still too much ripple in most cases. Further regulation design is implemented to get the output to the designed output voltage at nearly DC levels with minimal ripple as possible. This design will change based on the type of power supply.
Power supplies and converters are more process based and convert AC to DC through use of multiple parts, including transformers at times. So technically, these devices may have transformers, but are not transformers by nature of operation. Here are some photos of supplies and converters that we have for example:
There are many types of power supply. Most are designed to convert high voltage AC mains electricity to a suitable low voltage supply for electronics circuits and other devices. A power supply can by broken down into a series of blocks, each of which performs a particular function.
For example a 5V regulated supply:
Transformer - steps down high voltage AC mains to low voltage AC.
Rectifier - converts AC to DC, but the DC output is varying.
Smoothing - smooths the DC from varying greatly to a small ripple.
Regulator - eliminates ripple by setting DC output to a fixed voltage.
Power supplies made from these blocks are described below with a circuit diagram and a graph of their output:
The low voltage AC output is suitable for lamps, heaters and special AC motors. It is not suitable for electronic circuits unless they include a rectifier and a smoothing capacitor.
Transformers convert AC electricity from one voltage to another with little loss of power. Transformers work only with AC and this is one of the reasons why mains electricity is AC.
Step-up transformers increase voltage, step-down transformers reduce voltage. Most power supplies use a step-down transformer to reduce the dangerously high mains voltage (230V in UK) to a safer low voltage.
Transformers waste very little power so the power out is (almost) equal to the power in. Note that as voltage is stepped down current is stepped up.
Photograph © Rapid Electronics
The input coil is called the primary and the output coil is called the secondary. There is no electrical connection between the two coils, instead they are linked by an alternating magnetic field created in the soft-iron core of the transformer. The two lines in the middle of the circuit symbol represent the core.
Rapid Electronics: Transformers
Transformer circuit symbol
The ratio of the number of turns on each coil, called the turns ratio, determines the ratio of the voltages. A step-down transformer has a large number of turns on its primary (input) coil which is connected to the high voltage mains supply, and a small number of turns on its secondary (output) coil to give a low output voltage.
turns ratio = Vp = Np VsNs power out = power in Vs × Is = Vp × Ip
Vp = primary (input) voltage
Np = number of turns on primary coil
Ip = primary (input) current
Vs = secondary (output) voltage
Ns = number of turns on secondary coil
Is = secondary (output) current
There is more information about power supplies and transformers on the Electronics in Meccano website.
There are several ways of connecting diodes to make a rectifier to convert AC to DC. The bridge rectifier is the most important and it produces full-wave varying DC. A full-wave rectifier can also be made from just two diodes if a centre-tap transformer is used, but this method is rarely used now that diodes are cheaper. A single diode can be used as a rectifier but it only uses the positive (+) parts of the AC wave to produce half-wave varying DC.
A bridge rectifier can be made using four individual diodes, but it is also available in packages containing the four diodes required. It is called a full-wave rectifier because it uses all the AC wave (both positive and negative sections). Alternate pairs of diodes conduct, this changes over the connections so the alternating directions of AC are converted to the one direction of DC.
1.4V is used up in a bridge rectifier because there is 0.7V across each diode when conducting and there are always two diodes conducting, as shown in the diagram.
Bridge rectifiers are rated by the maximum current they can pass and the maximum reverse voltage they can withstand. Their voltage rating must be at least three times the RMS voltage of the supply so the rectifier can withstand the peak voltages. Please see the Diodes page for more details, including pictures of bridge rectifiers.
Bridge rectifier
Output: full-wave varying DC
(using all the AC wave)
A single diode can be used as a rectifier but this produces half-wave varying DC which has gaps when the AC is negative. It is hard to smooth this sufficiently well to supply electronic circuits unless they require a very small current so the smoothing capacitor does not significantly discharge during the gaps. Please see the Diodes page for some examples of rectifier diodes.
Single diode rectifier
Output: half-wave varying DC
(using only half the AC wave)
Smoothing is performed by a large value electrolytic capacitor connected across the DC supply to act as a reservoir, supplying current to the output when the varying DC voltage from the rectifier is falling. The diagram shows the unsmoothed varying DC (dotted line) and the smoothed DC (solid line). The capacitor charges quickly near the peak of the varying DC, and then discharges as it supplies current to the output.
Note that smoothing significantly increases the average DC voltage to almost the peak value (1.4 × RMS value). For example 6V RMS AC is rectified to full wave DC of about 4.6V RMS (1.4V is lost in the bridge rectifier), with smoothing this increases to almost the peak value giving 1.4 × 4.6 = 6.4V smooth DC.
Smoothing is not perfect due to the capacitor voltage falling a little as it discharges, giving a small ripple voltage. For many circuits a ripple which is 10% of the supply voltage is satisfactory and the equation below gives the required value for the smoothing capacitor. A larger capacitor will give less ripple. The capacitor value must be doubled when smoothing half-wave DC.
Rapid Electronics: Electrolytic Capacitors
Smoothing capacitor, C, for 10% ripple:
C = 5 × Io Vs × f
where:
C = smoothing capacitance in farads (F)
Io = output current in amps (A)
Vs = supply voltage in volts (V), this is the peak value of the unsmoothed DC
f = frequency of the AC supply in hertz (Hz), this is 50Hz in the UK
There is more information about smoothing on the Electronics in Meccano website.
Voltage regulator ICs are available with fixed (typically 5, 12 and 15V) or variable output voltages. They are also rated by the maximum current they can pass. Negative voltage regulators are available, mainly for use in dual supplies. Most regulators include some automatic protection from excessive current ('overload protection') and overheating ('thermal protection').
Many of the fixed voltage regulator ICs have 3 leads and look like power transistors, such as the 7805 +5V 1A regulator shown on the right. They include a hole for attaching a heatsink if necessary.
For low current power supplies a simple voltage regulator can be made with a resistor and a zener diode connected in reverse as shown in the diagram. Zener diodes are rated by their breakdown voltage Vz and maximum power Pz (typically 400mW or 1.3W).
The resistor limits the current (like an LED resistor). The current through the resistor is constant, so when there is no output current all the current flows through the zener diode and its power rating Pz must be large enough to withstand this.
For more information about zener diodes please see the Diodes page.
These are the steps for choosing a zener diode and resistor:
The zener voltage Vz is the output voltage required
The input voltage Vs must be a few volts greater than Vz
(this is to allow for small fluctuations in Vs due to ripple)
The maximum current Imax is the output current required plus 10%
The zener power Pz is determined by the maximum current: Pz > Vz × Imax
The resistor resistance: R = (Vs - Vz) / Imax
The resistor power rating: P > (Vs - Vz) × Imax
The example shows how to use these steps to choose a zener diode and resistor with suitable values and power ratings.
For example
If the required output voltage is 5V and output current is 60mA:
Vz = 4.7V (the nearest value available)
Vs = 8V (a few volts more than Vz)
Imax = 66mA (current plus 10%)
Pz > 4.7V × 66mA = 310mW, choose Pz = 400mW
R = (8V - 4.7V) / 66mA = 0.05kΩ = 50Ω,
choose R = 47Ω
Resistor power rating P > (8V - 4.7V) × 66mA = 218mW, choose P = 0.5W
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