Selecting a rectifier for plating and electrolysis

Electroplating, anodising and electrolysis all depend on controlled DC: the current passed through the bath sets the rate at which metal is deposited, oxide is grown or a compound is broken down. The rectifier is therefore part of the process, not only a power supply. Selecting it well starts with the chemistry and the work, then moves to the electrical rating.

Start with the process

Current from the work

For plating, the required current is the area being plated multiplied by the current density the process calls for. Current density is set by the plating chemistry and the finish required, and is given by the process supplier in amperes per square decimetre or per square foot. The total area per load, including racks and fixtures, decides the current. The largest load the line will run, not the average, sets the maximum.

Voltage from the bath

The DC voltage needed depends on the bath chemistry, the distance between anode and work, the temperature and the resistance of busbars and connections. Many plating processes run at low voltages, typically in the range of about 6–24 V DC. Anodising, especially hard anodising, needs higher voltages that rise as the oxide layer grows. Electrolysis cells vary widely with the cell design.

A practical specification states the maximum current, the voltage at that current, and the normal operating point.

Electrical points to settle

• Control mode: plating and electrolysis usually run in constant-current mode. Some anodising processes use constant voltage or a programmed ramp. Say which, and whether the operator needs to switch between them.
• Accuracy and resolution: how closely the output must hold its setting and how finely it can be adjusted, particularly at low settings.
• Ripple: some finishes are sensitive to ripple and the process supplier may set a limit. Where low ripple is required, a twelve-pulse rectifier or added filtering is considered.
• Reversal: some processes use periodic current reversal. This changes the rectifier design and must be stated at the start.
• Ramp and timers: soft start, current ramps and ampere-hour or time control are common on plating lines.

Duty and environment

Plating lines often run for long shifts at high load. A rectifier sized exactly to the maximum current and run there continuously has no reserve for hot weather or a dirty filter. A margin on current, commonly 10–20%, gives headroom without oversizing the equipment.

The plating shop environment is usually the deciding factor in construction. Acid and alkaline fumes, humidity and spray attack open electronics and transformer windings. Oil-immersed rectifiers, with the transformer and semiconductors sealed in a tank, are a common answer in such shops. Where the rectifier can stand in a separate, clean room, air cooling becomes practical.

Where the rectifier stands

DC current at thousands of amperes is expensive to carry. Busbars or cables between rectifier and tank lose voltage and produce heat, so the rectifier normally stands close to the tank it serves. The voltage drop in the DC connection is part of the rating: if the bath needs 12 V and the busbar run drops 1 V at full current, the rectifier must deliver 13 V.

Sense leads that measure the voltage at the tank, rather than at the rectifier terminals, let the control compensate for that drop.

Common sizing mistakes

1. Sizing from the average load rather than the largest load the line will run.
2. Leaving out the voltage drop in busbars and connections.
3. Choosing an open, air-cooled design for a shop full of fumes.
4. Overlooking a ripple limit until the finish shows a problem.
5. Adding a large margin on both current and voltage, which oversizes the transformer and wastes energy at the normal operating point.

What the process team should provide

• Process chemistry and the finish or product required.
• Plated area per load, current density and the resulting current.
• Operating voltage, bath temperature and anode arrangement.
• Duty pattern, shift length and planned expansion.
• Shop layout, rectifier location and DC connection length.

Where to go next

• Silicon, Industrial & AC-to-DC Rectifiers
• Industrial rectifier basics
• Industrial power equipment
• Discuss your requirement: share the process data and shop layout.