Aftermarket JOBO CPE2 Motor Speed Controller:

The balky performance of the motor in the JOBO CPE2is a frequent topic on usenet. Often the motor will not rotate the drum at all in the low speed setting. The relay will just clack away as the circuit becomes overloaded. The usually recommended fix is a new motor from the folks at Jobo. This is an expensive "fix" since the motor is an expensive German unit. This article presents an alternative solution to the JOBO CPE2 Motor Problem borrowing from modern motor control techniques.

Background:

The original JOBO CPE2 Motor Controller (shown below) 

uses a switch to select one of two voltages to determine motor speed. Unfortunately, the torque of a motor is directly proportional to the current flowing through the motor coils. If you cut the voltage to the motor in half, you also cut the current (and torque) in half. A better way of controlling the motor's speed would be to borrow from robotics technology and use Pulse Width Modulation (PWM) technology and an "H-Bridge" Motor Controller. The duty cycle (time on divided by cycle time) is adjusted, yet the voltage applied to the coils is always 24 VDC. The "H Bridge" is merely two pairs of transistors, logic controlled to alternate "+" and "-" to the motor coils, changing the motor direction without placing a high current load through the contacts of a switch or relay.  The duty cycle will vary from about 10% to almost 100%. The design used in this article uses off-the-shelf parts to construct a continuously variable PWM Controller.

The new controller fits into of the original space. In fact, the layout is pretty generous and a "next" project will be to incorporate a temperature sensor and timer in the available space. The new controller is shown below:

I have replaced the spade connectors on the original controller with 0.100" Molex connectors. The original connectors could have been salvaged from the original board, but I didn't it deem it worth the effort. The board can be fastened to the original ABS pillars with 4-40 screws.

Click on the button to the right to see the schematic:   

The heart of the controller is the SG3525 PWM chip. The capacitor and resistor networks attached to pins 5,6 and 7 control the frequency of the controller. The small potentiometer is necessary to trim the controller frequency to a high level so that there is no audible singing of the motor. With better shielded motors this  is unnecessary, but in the case of the original JOBO it's a must. The voltage divider from pin 15 (the 5.1 Volt on board reference) to pin 1 controls the duty cycle by adjusting the voltage on Pin 1. The outputs on pin 14 and 11 are connected via 1N4148 diodes to the PWM input of the LMD18201 H-Bridge Chip. Note the 1 uF/100 Volt ceramic capacitor across pins 6 and 7. This is necessary to prevent Radio Frequency Interference.

The muscle of the controller is the National Semiconductor www.nsc.com LMD18201 "H Bridge" motor controller. This chip provides inputs for direction, brake and speed. The physical layout of the chip is a little different from conventional (the lead spacing is 0.067 inch) so a footprint had to be built for the printed circuit board. The LMD18201 is capable of handling loads far larger than those presented by the JOBO's motor, but it takes up little space and is very easy to design in. In fact, the LMD18201 is capable of handling loads far greater than presented by the JOBO Motor, up to 3.0 amps (the JOBO motor draws only a few hundred milliamps). The LMD18201's slightly more expensive brother, the LMD18200 has the capability of "Current Sense" but this feature was deemed to be of little value in this application. At the time of this writing, the LMD18201 cost about $12.50 at Digikey A discrete "H Bridge" could be built but would result in a little more complexity on the circuit board and higher parts count.

Motor direction is controlled via the DIR pin on the LMD18201. The input to the DIR pin is either 5.1 volts derived from the VRef on pin 15 or ground. A current limiting resistor is included to prevent accidental overload to the SG3525. The foil side of the PCB layout may be seen by clicking the button below:

    :

The topside (silkscreen) is illustrated below:

H1 and H2 are mounting holes. These fit exactly for my JOBO, but you may need to alter their location for your unit.

To implement the controller in an existing JOBO CPE2, I removed the original board, and using aircraft metal shears cut through the board to separate the "On/Off" portion from the "Speed Control" portion. The original "On/Off" portion is used in its original form. I attached it to the motor base by making a small wooden pillar which is epoxied to the base. The pillar is tapped for a 4-40 screw which will hold the "On/Off" switch in place.

I clipped the original connectors and replaced them with Molex connnectors. As illustrated above, the A.C. input uses a 2-pin female connector, the potentiometer uses a 4-pin connector and the motor control and reversing mechanism an 8-pin connector. The latter is preferable a "female" PCB mount connector since voltages are present on its terminals.

A note on the motor speed potentiometer. You can use a small Allen Bradley "mod pot" which will clear the PCB. To use the original knob you have to file the 1/4" shaft to accomodate the smaller diameter of the original which was about 6mm and you will have to enlarge the mounting hole. Alternatively, you can use a US dimensioned potentiometer with a 1/8" shaft. With a little ingenuity you can still use the original knob.

A Note on the JOBO CPE2 Carcase:

The JOBO controller is housed in ABS plastic. The top and bottom are secured via screws into threaded brass inserts in the ABS pillars. The screws that JOBO uses are the moral equivalent of 4-40's, not up to the task by any stretch. This method of affixing the top and bottom  is not satisfactory arrangement since ABS plastic is very prone to cracking. I would suggest that you modify the the method of attachment by removing the brass inserts, enlarging the holes, tapping the bottom for a 1/4-20 thread and using 1/4-20 stainless steel Allen bolts.

Next Steps:

For now, I am content to let the motor run continuously. This draws about 7 watts. By making the BRAKE input of the LMD18201 high, the H-Bridge is disabled and the motor will be switched off. This is just one mod of the printed circuit board.