CCD Controller Board Designs

CCD Controller Fan Failure Detector board, EL-1305

The Fan Failure board gives an indication of the operation of the connected brushless DC fans. If a fan does fail, the corresponding front panel LED will illuminate.

The CCD controller chassis has four 12V brushless fans installed to keep the electronics cooled. This board was designed and added to the controller to help aide in troubleshooting. The board is designed around a Maxim MAX6684 fan failure device. (Datasheet here) The device monitors the current draw of the fan to determine 'excessive under speed or rotor lock'. If this occurs, the device pulls the fail output pin low. This output is then routed to pin 4 of the output connector. Each fan that connects to the board is wired with it's positive lead to pin 2 and it's negative lead to pin 3 of it's connector. At the same time, a jumper wire must be wired from the connector's pin 1 to it's pin 4. This wire provides the 'cable connected' function by routing the output of the MAX6684 through pin 4 to pin 1 and then on to the rest of the circuit. The circuitry on the board then applies this signal to one gate of a 74LS04 hex inverter. If there is no fan connected to a particular channel the input of the inverter is pulled high via a 10K pull-up resistor. This signal is also wires to the 4 input AND gate at the bottom left corner of the drawing. This gate ANDs the output of each of the four fan failure circuits and is wired to the 4N35 optical isolator. The isolated output signal is wired to pins 5 and 6 of J1 and is available for monitoring by other equipment. Note: if the failure mode of a connected fan is a locked rotor, the output of the MAX6684 chip will switch from high to low at about a 0.5 to 1 hertz rate. This happens because the chip turns off it's output for a short time and then reapplies the power in an attempt to restart the fan.

The signal returning from the fan connector, again, drives the input of a hex inverter, U8, who's output drives the base of a 2N3904 transistor. The transistor is used to turn on an LED on the front panel to indicate a failure. There are four circuits and thus, four LEDs on the front panel that indicate which fan, 1, 2, 3, and/or 4, has failed.

The MAX6684 also has an OFF input that can be used to turn the fans off via a remote source. The OFF signal is optically isolated via another 4N35. The signal is made available to a jumper block for each of the four circuits. Jumpers can be set to invoke this remote control on a per channel basis. For each channel, a shorting block is used to connect the remote signal or connect directly to +5V. If the +5V circuit is used, the fans will be on at all times. Beside connecting to the jumper blocks, the DISABLE signal is wired to pin 13 of the 74LS04 hex inverter. The output of the inverter. is wired to a 2N3904 transistor that turns the front panel DISABLED LED on when the fans are turned off remotely.

Sheet 2 shows the remaining two circuits.