Power supply board of an injection molding machine

The power supply board of an injection molding machine turns the cabinet supply into the regulated DC voltages that the controller boards, sensors and valve coils run on. Typical outputs are 24 V DC for the I/O, the sensors and the solenoid valves, and 5 V DC for the logic circuits; our power supply board supplies 24 V DC and 5 V DC. When the supply weakens, the whole control system misbehaves, so a power supply fault often looks like the failure of another board.

Where the power supply board sits

The power supply board is mounted in the electrical cabinet with the CPU, I/O and drive interface boards, which it feeds through connectors or a distribution bus. On some machines, separate DIN-rail power supplies feed the 24 V circuits and the controller's own board produces the logic voltages. The wiring diagram of your machine shows which arrangement it uses.

How a power supply board works

Most modern power supply boards are switch-mode supplies:

1. The input is rectified and smoothed by electrolytic capacitors.
2. A switching transistor chops it at high frequency through a small transformer, which also isolates the outputs from the input.
3. The outputs are rectified, filtered and regulated, so they stay at the set voltage as the load changes.
4. Fuses and protection circuits limit or shut down the outputs in case of a short circuit or overload.

The electrolytic capacitors in steps 1 and 3 are the parts that age; the capacitors guide explains why heat shortens their life.

Signs of a failing power supply board

• The controller or the operator panel does not start, or restarts at random.
• Restarts or communication errors when many valves switch on at once, because the output voltage dips under load.
• Sensors that switch erratically, or several unrelated I/O faults at the same time.
• A fuse that blows again after replacement.
• Bulging or leaking capacitors, burnt or darkened areas on the board, or a whining noise.

Causes of failure

• Heat: blocked cabinet filters and failed fans shorten the life of the capacitors and other components.
• Overloads and short circuits in the field wiring, such as a damaged sensor cable shorting the 24 V supply to the machine frame.
• Voltage surges and an unstable mains supply.
• Dust, oil mist and condensation on the board.

Testing and replacing a power supply board

Measuring live circuits and replacing the board are jobs for qualified electricians. Replace the board only with the machine switched off at the main isolator and locked out, after the capacitors have had time to discharge.

1. With the machine in a safe state, measure each output at the board under its normal load. An oscilloscope shows ripple and short dips that a meter misses.
2. If an output is low, disconnect its loads one group at a time to find out whether the supply is weak or a load is shorted.
3. Isolate and lock out the machine, wait for the capacitors to discharge, and check that no voltage is present.
4. Photograph the board and label the connectors. Wear a grounded wrist strap and handle the board by its edges.
5. Fit a board of the same model and output ratings, reconnect it, and measure the outputs before running the machine.

What to send for a replacement quote

• The machine brand, model and serial number, and the controller model printed on the operator panel and the boards.
• The board's model number or label, its input and output voltages if they are printed, and photos of both sides with the connectors.
• The symptom: dead, restarting, low output voltage or a blown fuse.

The controller guide and the I/O module guide cover the boards it feeds, and the electrical system guide gives the overview of the cabinet.