Few questions from Xylotex's 3 axis data sheet
November 28, 2006
I have a few questions from the Data Sheets that Xylotex includes with their 3 axis board.
This quote from the data sheet:
“The on-board drive circuitry limits the source/sink current to the motor without the need for external power resistors.”
I was trying to find out what source/sink current is and I came upon this analogy on the web:
"Current sources and sinks are local electrical currents that flow from a location where they can’t be detected into a location where they can be detected (current source) or vice versa (current sink). By analogy to the flow of water, a current source would be like a mountain spring. A current of water flows from a hidden location underground to the surface where it can be easily seen. A sink on the other hand would be like water flowing down a drain. It flows from where you can see it to where you can’t. Electrical current can have the same basic properties. Current flowing from the “live” wires of an electrical outlet would be a source, while current flowing back along the common would be a sink (although with AC circuits this distinction gets a little murkey)”
Does this quote explain the boards use of source/sink current? If so could someone draw a parallel for me between this quote and how it would apply to the above quote taken from the data sheet for the 3 axis board?
Also, in my research about power resistors, I concluded that a power resistor is just a resistor that can maintain a voltage given its resistance and the current, but at the same time, can dissapate more heat than say a smaller resistor with the same resistance. So these resistors are usually larger than say a Ľ watt or ˝ watt resistor type. Does a source / sink current create heat? And is the circuitry designed to avoid this heat? Is this propriatary to the design or could some briefly explain how this is accomplished?
I noticed on the three axis board there is a connector at the bottom with the following markings: GND, DIRA, STEPA, Vcc. I’m assuming that this would be for a connection to a fourth axis such as Xylotex’s 1 axis board which has a connector with markings
G, D, S, +5.
Most older driver boards had simple switched outputs. The were simply on or off at full voltage which meant you could easily exceed the current rating of the motors. Therefore, you had to limit the current flow using resistors.
The newer current limiting driver boards use current limited outputs instead of resistors. This is done by an internal amplifier. The current value will never rise above the preset regulated value that you determine. This value is usually set to be a value equal to or less than the rated value of the motor. The solid state device acts like an electronic resistor for the most part and disipates the energy as heat which is why these driver boards have fairly large heat sinks on the driver chips.
The easiest way of thinking of it is the source/sink current is the controlled current through the I.C's output and the motor winding. This board uses PWM (Pulse Width Modulation) in the IC, in place of resistence or a transistor acting as a resistor. Resistence limits current by dropping the voltage (which limits the performance), ohms law proves this. PWM puts full voltage on the windings and limits the current by controlling the ON time of a series of very fast pulses. PWM is now used in most small efficent power supplies which deliver high current for their size, Computers, lap tops, chargers... (usually called Switching power supplies) and many other applications. PWM is much more efficent and delivers more power to the shaft in steppers than is possible by analog (resistive type) current limiting.
IIRC 'Jones on Stepper motors' had a great explanation, and Marris (Gecko) covered it in a white paper on his site a couple of years ago.
The heat is generated in the IC during the transition (switching) phase of the pulse train, and in higher current levels requires a Heat Sink to disapate it. The heat sinks on that board are no where near large enough to disapate the heat in a resistive current limiting circuit using transistors in the IC.
A power resistor is a resistor with a high power rating, IOW a 100 watt resistor is called a power resistor. Some old drives used power resistors to limit the current, but to limit the current with a 24 volt feed and a 2 volt motor, 22 volts are being absorbed by the resistor, at 2 anps thats 4 watts going to the motor and 44 being disapated by resistence (OUCH!). Actually with inrush current you get some (but not near as much as with PWM) advantage over just feeding the motor with 2 volts.