Plating Rectifiers | REXPLATING

Initially, electroplaters used generators to provide the direct current needed for the electroplating process till they were replaced by plating rectifiers which convert alternating current AC to direct current DC.

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The plating rectifier has a transformer that reduces the voltage, diodes responsible for the conversion, a control system for both the voltage and the current, a device for cooling the rectifier and overload protection.

These rectifiers have different waveforms which you must consider before you place an order. It may be a full-wave current or half-wave but the best to use for electroplating process is the full-wave to get the best results for processes like chromium plating and other.

Rectifiers have good characteristics that make them a better choice than generators. Some of the excellent ones are, energy saving, compact and lightweight, easy modulation, output stability, etc. They also have many applications and comes with many voltage power supply types.

Plating Explained

Plating is all about a process through which a conductive surface is coated with a metal. The reasons for electroplating metals is to make them more attractive looking, to inhibit corrosion, to reduce friction, to harden the metals, enhance their paint adhesion, improve their capacity for solder and many other reasons.

The direct current DC which is essential for a successful plating process can come from a low-voltage generator or a rectifier. Initially, the generator was the main source of direct plating current, but because these generators were very costly to acquire and maintain, rectifiers became the cheaper alternative.

Rectifiers are so versatile that many devices make use of them. Some of the devices that work with rectifiers are:

1. Radio detectors or signals
2. DC power supplies
3. High-voltage DC power transmission systems
4. Household appliances like laptops, video games. Notebooks and televisions
5. Electroplating units.

Plating rectifier is an important part of the electroplating process. Some plating forms require the use of electrical current for depositing the coating material onto the metal part. These conductive parts are dipped into the plating tank or bath, and then, direct current DC will be applied by the rectifier that is between the electrode and the parts.

The application of the direct current DC causes an attraction between the electric-field-driven coating and the conductive part of the metal. This attraction results in the coatings depositing on the parts. The coating requirements necessitate the controlling and filtering of both ripple voltage and DC voltage to achieve a high-quality finish.

A rectifier is responsible for converting alternating current (AC) to direct current (DC). This electrical device is made up of one or multiple diodes that are responsible for the conversion from AC to DC.

This part of the rectifier called a diode is a one-way valve that enables current to flow in just one direction. When current begins to flow in that one direction, the process is called rectification. There are few important parts of the rectifier which you have to know:

• Transformer; a rectifier transformer is responsible for reducing the voltage. It has both primary and also secondary magnetic cores which are wound with the conductive wires.
• To know the rate at which the transformer steps down the voltage that
  passes through it, it will be, the number of wire turns on the primary magnetic cores to the total wire turns around the rectifier&#;s secondary magnetic cores.
• You can depict this mathematically as Primary=400 turns, Secondary is 10 turns, therefore, the ration becomes, 40:1.
• This means that if the input to primary is 480v, the secondary output will be 12v (480/40) However, most electroplating process makes use of 6-18v, and you can easily get a rectifier that will produce direct current at 50-10,000 amperage or more.

Rectification and the control system

When the alternating current produces a wave on an oscilloscope which oscillates both below and above the horizontal, the rectifier will filter out the lower part of the wave. When this filtration occurs, half-way rectification occurs.

Modern conductors make use of a semiconductor to carry out the filtration process, and when AC is removed, direct current will now flow through. The diode is one of the devices used by a rectifier to carry out the conversion.

Silicon also works as a controller for each electroplating rectifier. These devices are called thyristors or SCR &#;silicon-controlled rectifiers.

Although they are diodes, they require a control signal at a &#;gate&#; or terminal. The SCR works as a regulator and a rectifier depending on if it is used on either the primary or secondary side of the rectifier transformer.

The designing of the SCR on any of these two sides will depend on what you are using the rectifier for, i.e., the application, the efficiency of your unit, the cost implication and the reliability.

Before offering any rectifier, it will be good for the supplier to consider what you need and give you a detailed explanation of which one will suit the needs.

Ripple

SCR might be effective, but they have one problem which is producing a direct current that is contorted by &#;Ripple.&#; Some of the larger plating rectifiers produce ripple that is five percent or below at their rated capacity. If this is the case with your rectifier, it is never a good idea to operate it because, as you power the unit down, ripple increases.

If you want to electroplate precious metals, chromium, alloys, etc. your process requires a lower ripple rate.

However, if the ripple is higher than necessary, you can add a filter, and it will smoothen the rectifier wave. Although when it comes to electroplating, the ripple is not that important. Therefore, if you want to specify a plating rectifier, tell the suppliers that it must come with a five percent ripple or lower than five percent to ensure that the plating processes will run smoothly.

Waveform

There are also some waveforms which a rectifier has:

1. Full Wave; this wave reverses the AC wave negative part and joins it with positive
2. Half Wave; it allows either the negative or the positive to flow through while blocking the other one.
3. Single-Phase AC; a center-tap transformer leads to the formation of a full-wave rectifier by two diodes.
4. Three-Phase AC; makes use of three pairs of diodes.

Another important factor you must chip into the supplier is the current waveform of the plating rectifier. Although a rectifier can come in any of the above waveforms, the appropriate waveform for many of the plating processes is a full-wave rectifier.

This factor is critical because if you are handling a chromium electroplating process, it will not work with a half-wave rectifier.

There are other parts of a plating rectifier like a voltage and current control, overload protection and also a cooling device.

The commonly used material in a rectifier diode is Silicon. To handle the load, several of these diodes are used. When the rectifier is in full operation, it generates a lot of heat and to remove this heat; there is always a water cooling system or a fan.

All electroplating rectifiers make use of variable output control. If you have either an infinitely variable control or tap switches, you can use it. There are fixed output rectifiers available, and they are more affordable but not suitable because they may break down as a result of load factors if the plating shop is always busy. There are other controls as well as current density control, automatic voltage and current which make the work of the plater very easy by providing a consistent output.

A rectifier can come in different shapes of many physical forms like it can be a solid-state diode, mercury physical forms, vacuum tubes diodes, silicon-controlled rectifiers and many semiconductor switches that are silicon-based.

Characteristics of a Rectifier

There are many important characteristics of a rectifier which makes them more suitable than a low-voltage generator for electroplating processes:

• Plating rectifiers save energy; it adopts a high-frequency switching power supply transformer and as a result, the efficiency at which it converts current improves.
• It is lightweight and also very small; if you have a thyristor rectifier, the size and weight will be 1/5-1/10. This weight and size make it very easy for you to expand, plan, install, maintain and move as the case may be.
• Easy modulation; Rectifier output waveform can be modulated. Due to high frequency, the cost of output waveform adjusting processing is low and can be easily changed depending on what the users require. As a result, there will be room to
  improve the efficiency on site and also improve the quality of processing product.
• Stability of output; the system has a high response speed, and as a result, the rectifier can adapt strongly to load changes and electricity network. This characteristic will ensure the accuracy of more than 1%. Also, the switching power supply works very efficiently to enable high precision control which will, in turn, improve the products quality.

Application features of a rectifier

There are many rectifier application features like:

1. Reduction of porosity; as the rate at which nuclei forms is greater than the rate of growth, it leads to nuclei refinement.
2. Improve the binding force; rectifier enable the film to break down, and as a result, there will be a strong bond between the coating film and substrate.
3. It enhances the capacity for coverage and dispersion
4. It helps to reduce the stress on the coating; a rectifier help to reduce internal coating stress and as a result, there will be a reduction of additive and an achievement of a crack-free coating film.
5. Rectifier helps a lot if you want to achieve an alloy coating composition that is stable
6. With a rectifier, there will not be a need for anode activator since it improves the dissolution of an anode

Plating rectifiers improve coating mechanical properties by increasing density, reducing volume and surface resistivity, improved wear resistance, toughness, corrosion resistance and also enable the controlling of the electroplating coating hardness.

Rectifier Placement

The right place to keep the rectifier is as close to the electroplating tank as possible. If you place the rectifier too far from the plating tank, it will make the bus run longer resulting in loss of current and maintenance. The main reason for keeping the
rectifier close to the tank is because the components of the rectifier need cooling and if you allow them to cool under the air, it will draw corrosives faster and also shorten the life of the rectifier.

The best and easiest way to solve the rectifier placement problem is to keep it over a wall that is near so that it will be closer to the plating tank. You can still keep the rectifier controls next to your electroplating station. However, if you cannot keep
it on the other side, you can use air-tight or water-cooled units directly or through heat exchange.

Also, you may need to use water to cool your rectifier if the ambient air is above the required temperature.

Maintaining and repairing a rectifier

A rectifier has only pumps or fans as moving parts which are used to cool it. If you install and operate your rectifier at the specified temperature levels, it will last for a very long time. As for maintenance,
you can do it regularly like other equipment. Just clean the air filters regularly or change them as the need arises. Make sure that you remove any object like boxes or other objects that may inhibit the flow of air.

Always check the fan blades to make sure that they are secure. Run maintenance exercise on filters and pumps. Clean the heat sinks and the semiconductors clean to enhance cooling. Thoroughly check all the instrumentation for control like buttons, lights, etc. on a regular basis and change any of them whenever you discover malfunctioning.

A good brand of rectifier comes with an onboard diagnostics that indicate areas of trouble on the digital display. Voltmeters, Ammeters, and oscilloscopes help the unit to show the trouble area. There is no special skill or a degree in engineering needed before repairing a faulty rectifier. The only thing you should have is an understanding of electrical equipment and also knowledge about the dangers of high currents and
high voltages.

The good news is that the people who supply rectifiers will always include a detailed manual and the list of parts so that you can easily repair the rectifier and reduce the number of downtimes you experience.

Things to consider before buying a plating rectifier

There are certain things you must consider before you buy any rectifier. There are some things you are noticing about your current rectifier which you may want to improve or maintain your next purchase. Also, your supplier may also make some reasonable suggestions on the features that will improve your work efficiency.

On a general note, you should check out the following before you buy a rectifier:

1. Ease of installation
2. Maintenance costs
3. Repeatability
4. Reliability
5. Warranty
6. Conversion Efficiency

Although there is no way of knowing how superior a rectifier is, you can ask others about their experience and also maintain a trust relationship with your supplier so that he will always tell you the truth.

Today&#;s rectifier operates on the same principles used for decades. Sure, there have been many incremental improvements in manufacturing methods and materials, but the basic principles are the same. The most noteworthy improvements are those associated with electronics and computerization.

All rectifiers have 1) a transformer that lowers a utility&#;s high voltage to the 6&#;18 volts normally used by electroplaters or to the somewhat higher voltages required by anodizers; 2) a rectification system that changes alternating current (AC) to direct current (DC); 3) power regulators; 4) controls and 5) a means of cooling these components. Other than the cooling section, there are no moving parts.

Transformers

A transformer has primary and secondary sets of magnetic cores wound with conductive wire. The ratio of the number of wire turns around the primary versus the number around the secondary regulates how much the voltage is stepped down. For example, if the primary has 400 turns and the secondary 10 turns, the ratio is 40:1. Therefore if the input to the primary is 480 v, the secondary&#;s output is 12 v (480/40). Most common plating processes require 6-18 v, and rectifiers producing this at amperages of 50&#;10,000 or more are readily available.

Rectification and Control

AC produces a wave on an oscilloscope that oscillates above and below the horizontal. The rectifier &#;filters out&#; the lower portion of the wave, as shown in Fig. 1a. This is &#;half-wave rectification.&#; It pulsates. Fig. 1b shows full-wave, single-phase rectification, which still has pulsation, but at closer intervals. Fig. 1c shows full-wave, three-phase rectification. It still has slight pulsation, but there is a variation (ripple) of less than 5%, and that is satisfactory for most (not all) electroplating processes.

The modern rectifier uses a semiconductor (usually silicon) to &#;filter out&#; the AC and allow only DC to flow through it. The simplest of these devices is known as a &#;diode.&#;

Silicon also can be part of controlling a rectifier. These devices are known as &#;thyristors&#; or &#;silicon-controlled rectifiers&#; (SCR). They are basically diodes, but require a control signal at a terminal or &#;gate.&#; They are used both for rectification and regulation of output, depending on the design. The difference is in whether the SCR is used on the primary or secondary side of the transformer. There are good reasons for either design, based upon your applications, the unit&#;s efficiency, reliability and cost. Your rectifier supplier should consider your needs and explain which will work better for you and why.

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Ripple

A problem with SCRs is that they produce DC distorted by &#;ripple&#;&#;the remainder of the fluctuating AC sine wave. Most larger rectifiers produce ripple of five percent or less at rated capacity. Operating a rectifier at far less than its capacity is not a good idea, since ripple increases as the unit is powered down. Plating chromium, precious metals, alloys and certain other metals requires minimum ripple, and if ripple is too high, it&#;s easy to add a ripple filter that smoothes the wave. Ripple is not very important in electrocleaning, electropolishing and anodizing.

Other Control Methods

The tap switch is a simple alternative to solid-state controls. It relies on switching between varying numbers of wire windings on the primary and secondary transformer sections. If, for example, one has 400 windings on the primary and 10 on the secondary, with an output of 12 v, voltage will be cut to six if the windings on the primary are doubled to 800. A switch contacting larger or smaller numbers of windings on the primary controls the voltage. A tap-switch control produces DC with less than five percent ripple regardless of the output voltage. A disadvantage, however, is that voltage fluctuations are normal, a problem if plating thickness is critical.
A similar control is the variable autotransformer, which works on the same principle, but has no &#;taps,&#; and is continuously variable.

Switch-Mode Power

Another type of rectifier is the switching transistor-type. An EMI filter rectifies and filters incoming line power to produce DC. An inverter changes it back to AC at approximately 1,000 times the line frequency. This high-frequency AC is then stepped down to plating voltages, and rectified and filtered again to produce a high-current, low-voltage output, with very low ripple. Plating voltages and amperages come from a totally sealed, water-cooled enclosure. The small footprint and light weight allow it to hang on a wall or fit where conventional units won&#;t.

Computerization

The most striking technology changes have been those made possible by electronic components&#;microprocessors, computers, etc. Rectifiers with digital controls regulate amperage and voltage more precisely. They allow the current to be &#;ramped up&#; as the parts enter plating or anodizing tanks, and turned off at precisely the right times. They can provide pulse plating, control ampere-hours, reverse current, and be programmed for complex wave forms. They may have computer interfaces.

Computerization (some of it from personal computers equipped with special software) and microprocessors can control a number of rectifiers, the movements of hoists and conveyors, additions of brighteners and other chemicals, solution temperatures, agitation and other variables. You can apply exactly the thicknesses of metals specified, in the sequence desired, time after time.

Computerization tells the operator at a remote location when something isn&#;t right. That cuts costs, reduces rejects, produces a part that is the same every time, and eliminates labor. The computer can control power so exactly that none is wasted, thus cutting power bills. Its memory records what has been done to each part and under what conditions. If there is a reject, it&#;s easier to trace what happened.

Anodizing

Anodizers use rectfiers very similar to those used by electroplaters, except that higher voltages are required&#;as high as 150 for hard-coat anodizing. Some color-anodizing processes such as electrolytic coloring require AC power supplies (still referred to as &#;rectifiers&#;) as well as DC and instrumentation that allow adjusting voltages and positive-negative cycles rapidly and according to a predetermined program.

Pulse Plating and Periodic Reverse

Pulse plating involves application of rapidly cycled interrupted direct current. It may be combined with periodic reverse, in which the current is cycled between forward and reverse, for preset times in each direction. In electronics applications pulse plating produces deposits that are denser, have finer crystal structure, can be applied more rapidly, are smoother, and require less plating-solution additives. Rectifiers for these applications are programmable for a range of currents measured in milliamps to perhaps 1,000 amperes, delivered as forward, reverse or pulsed current, or combinations thereof. They are more expensive, but so is gold. Pulse plating and interrupted-current techniques have moved beyond the electronics industry to such applications as electropolishing, anodizing, electrocleaning and barrel plating.

Rectifier Placement

The rectifier should be as close as possible to the plating tank. The farther away, the longer the bus runs and the greater the current losses and maintenance. The problem is that you have to cool the rectifier components to prolong their life, and if the rectifier is air cooled, it may be drawing in corrosives, shortening its life. Sometimes the problem can be solved by installing the rectifier on the other side of a nearby wall. The controls can still be next to the plating station. But if that&#;s not feasible, there are air-tight water-cooled units, either direct or heat-exchange. Rectifiers are rated to be operated at certain maximum temperatures, and if the ambient air exceeds those temperatures, water cooling may be mandatory.

Maintenance and Repair

Rectifiers have no moving parts other than the fans or pumps used for cooling. Properly installed and operating at prescribed temperatures, they should last for a long time. As with any other piece of equipment, regular maintenance pays. Change or clean air filters regularly and keep away boxes or other items that interfere with air flow. Be sure that fan blades are secure. Maintain water pumps and filters. Keep semiconductors and heat sinks clean to aid in cooling. Check all the control instrumentation&#;buttons and lights, etc.&#;regularly, and replace if necessary.
Rectifiers often have onboard diagnostics that show trouble spots on their digital displays. Ammeters, voltmeters and oscilloscopes help to isolate troubles. You don&#;t have to be an electrical engineer to repair a rectifier, but you do have to understand electrical equipment and be aware of the dangers of high voltages and currents. Suppliers of rectifiers provide detailed manuals and parts lists to aid in repairing and minimizing downtime.

Buying a Rectifier

You know some of the things you like about the rectifiers you have now. Your chemical supplier may suggest features that would improve the processes used in your shop. Hopefully this brief article triggers thinking about other ideas you&#;d like to explore.
What makes a good rectifier? What should you look for when considering a purchase? According to Ray Dargis1 , the list includes&#;

• Reliability
• Repeatability
• Conversion Efficiency
• Low Maintenance Costs
• Excellent Warranty

• Ease of Installation

Expandability

You can&#;t look inside a rectifier to see its superior qualities. Most innards look alike. But there are some important choices in manufacturing materials and methods hidden in those cabinets. Your job is to establish trust with your supplier, and possibly to ask your fellow platers about their experiences. You may want to add to the above list your own &#;wish list&#; of items that would improve your ability to produce a quality finish with ease and assurance, and to do some things you can&#;t do now.

1. Pretreatment for Electroplating, by Ray Dargis, page 11, PF Directory.

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