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As mentioned in part 1, it is possible to rearrange the circuit to provide a constant frequency as the pulse width is varied.  The additional effort involves having to run three wires from the pot rather than just two, so it's not really arduous.  As shown below, R1 and R2 can be used to set the maximum and minimum speeds, and can be made variable if specific max/min speeds are needed.

Note: There is a project (and PCB) for a motor speed controller/ LED dimmer - see Project 126 for details.

The original circuit can also be easily rearranged to only use 3 of the 6 Schmitt triggers, and this connection method is shown below.  Feel free to mix and match between the two versions - for example the oscillator from Figure 1 can be used in the Figure 2 version or vice versa.

Figure 2 - Alternative Motor Speed Controller

To set a maximum speed, vary R1, and vary R2 to set the minimum.  Trimpots can be used if your requirement is critical, but with no feedback, there will always be some variation anyway.

With the values shown, the on and off times will change, but the period (for a complete on/off cycle) remains fairly constant regardless of pot setting.  There will be some variation if R1 and R2 are not equal, but the frequency change will not have any effect on operation.  The oscillator frequency is again approximately 560Hz, and this can be changed by making C1 larger (lower frequency) or smaller (higher frequency).

MOSFET and diode requirements are unchanged from the Figure 1 version, and can be selected according to your requirements or what you have available - provided that the devices are rated sufficiently for the load.

Like the previous version, this controller can also be used as a DC lamp dimmer, heater controller, or any other application that lends itself to PWM operation.

Part 1

Main Index

App. Notes Index