On 06/11/2012 11:56 PM, Jeshua Lacock wrote: >> But that is also a 15V part IIRC. But that's ok - the only reason the >> > Makerbot uses a PC power supply is because they are dirt cheap and TUV >> > certified. You can use any supply you want. I use 15V supplies. > OK, that was my main concern. I just don't understand how the Gecko will > "know" not to give it more than 5v? > Modern stepper motor drivers use the inductance of the stepper motor coil to their advantage. If you turn the 15 DC volts on to your stepper motor, the coil current starts out at 0 amps. A fast motherboard parallel port could be issuing a step pulse to the stepper motor controller before the fastest stepper I've seen could get to 1 amp from a fifteen volt supply. Check out this link to see how long a typical fast stepper motor takes for the current to build up:
http://wiki.linuxcnc.org/cgi-bin/wiki.pl?Stepper_Motor_Speed_Limitations Most modern stepper motor controllers switch the full voltage on to the stepper motor coil, and monitor a series resistor to see what the current through the coil is. When the current through the coil exceeds it's set point, the switch is turned off, and the coil goes into fast/slow decay mode. It then either waits a preset time, or looks for the current to dip below the preset level by a certain amount. I don't know the circuitry in the Gecko drives, but they may have a low level current source drive current through the current set resistor. I believe that the G540 drives 1 amp for each 1000 ohms of set resistor resistance. It may do that by supplying a fixed current to the set resistor, then comparing the voltage across the resistor to the voltage across the resistor in series with the motor coil. Lets look at a hypothetical situation. With a 1 ohm coil series resistor, and 1mA going through the setpoint resistor, if the setpoint resistor was 1.5 Kohms, there would be 1.5 volts across that resistor. If you compare the voltage across the coil series resistor, and make it match the voltage across the setpoint resistor, then there would be 1.5 volts across the 1 ohm coil series resistor too. That would correspond to a current level of 1.5 Amps. So the circuit would turn on the full 15V to the coil. When the current through the coil reaches 1.5 A (assuming a setpoint resistance of 1.5 Kohms) The circuit would turn the voltage off, and let the coil current fall for a time, then if it was back below the setpoint, the circuit would turn the voltage back on. For more than you care to know about driving stepper motors, look up "Jones on stepper motors" http://www.divms.uiowa.edu/~jones/step/ <http://www.divms.uiowa.edu/%7Ejones/step/> If you use the Gecko, be sure to read the manual about setting the current level a few times, so you don't overdrive your motors. ------------------------------------------------------------------------------ Live Security Virtual Conference Exclusive live event will cover all the ways today's security and threat landscape has changed and how IT managers can respond. Discussions will include endpoint security, mobile security and the latest in malware threats. http://www.accelacomm.com/jaw/sfrnl04242012/114/50122263/ _______________________________________________ Emc-users mailing list Emc-users@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/emc-users
