Very simple current chopper for my 3 axis PIC. The files: HOWTO.TXT (Yes... You found that allready!) PCB.PDF for toner Xfer or transparent film. The coard says TOP but it is the bottom. Mistake. Layout.PDF. Component placement. Schematic.PDF The schematic. 2 pics of the board i use myself. Note the first resistor on the top picture. Adjusted the pot fo far to the vcc causing the device to work as a toaster!!! Explanation on this subject below!!! First, Iīm danish so please bare with my english if i use strange phrases!!! You will hopefully understand the meaning of my words anyway... Here we go... I decided that the era of giant power resistors on my controller had to end!!! The big current limiting resistors get very hot, and they are quite expensive too. Beside that they donīt have ajusting capabality, so i often had to fiddle with the values to make my steppers run smoothly. I made this small circuit to replace the resistors. This is how it works: The 2 transistor circuit measures the voltage over the power resistor. As current in the stepper winding rises, so does the voltage over the power resistor. The transistor BD 135 is mounted across the resistor (base-emitter) and when voltage reaches apx. 0.6 volts the transistor closes. The collector of the BD 135 is connected to the much larger power transistorīs base and pulls down the base of the MJ15003 causing it to open. The voltage on the emitter will drop and reduce the current flowing in the power resistor and stepper windings. The pot is mounted to fine tune the currentflow. No additional safety circuit is added, so if you turn the potmeter to far to the vcc you will DEFINATELY fry your BD 135 and probably the resistor too!!! So it is for fine tuning of current only!!! You can add a 560 ohm resistor between VCC and potmeter to make the circuit fool proof... The size of the resistor determines the current that the device delivers. The resistor is placed close to the BD 135 and as the heat rises in the resistor the BD 135 draws more current causing the circuit to lower the current. An example: Using a resistor of 0.39 ohms : 0.6V/0.39 = 1.53 amps for the entire motor. Using a resistor of 0.1 ohms : 0.6V/0.1 = 6 amps for the entire motor. The resistor must be able to handle the wattage applied to it. Example: A current of 6 amps equals 3.6 watts. (6 amps * 0.6 volts) In this case i would use a 5 watt resistor to be safe. And donīt forget to add a heatsink to the Power transistors. They can get very hot. I had the MJ15003 in stock from a power amplifier project i made yeas ago. They are capable of pulling a beefy 250 watts per motor!!! Overkill... You might as well use the much cheaper 2N3055. They are 117 Watt devices and work like a charm! No need to add solder to the traces of the PCB. if you exceed the current and burn these traces... PLEASE SEND ME A PICTURE OF YOUR MOTORS!!! Got to see them!!! This board is definately NOT a 100% proof current chopper, but it does the job. Other peoble at CNCZone made circuits far more advanced, but also more expensive and difficult to make. *************************************************************************************************************** Parts used: 3 X MJ15003 NPN power transistor (2N3055 or similar will do, but it MUST be NPN!!!) 3 X resistor of X ohms (Calculate value using the formula below) 3 X BD 135 NPN transistor (Any other NPN with same pinout will work as well) 3 X 1K potmeters A drilled PCB and a 4 pin connector. (I had a connector with screw terminals in stock, so i used that) A heatsink large enough to deflect the heat from the 3 power trannies... *************************************************************************************************************** I hope this info will do for now... The circuit is simple and easy to build, but feel fre to pop a mail or a line in the forum if any questions skould come up... Regards... Alexander (666) CCL@Cyberchrist.dk