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I am building a CNC engraver/Router. I have a Chinese 12-48 VDC, 400 W PM spindle. The spindle speed is from 3000 RPM at 12 V input to 12000 RPM at 48 V. I want to drive the spindle with a MC-60 treadmill controller. The MC-60 has a max output of 95-100 VDC. I want to limit the output to 48 VDC, and thus the RPM to 12000. How can I do this? By adding a resistor to the output of the speed control potentiometer?

I am familiar with the MC-60, and have 2 currently in service running machines in my shop. One is a tabletop Mill, and one is a sand muller. Many years ago, I was an avionics technician, but I've forgotten most of what I knew then...

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If you just want to limit the range of the controller output to a safe value by preventing the pot being turned up to far then a resistor will probably suffice.

schematic

simulate this circuit – Schematic created using CircuitLab

Figure 1. Modification.

Make R2 about three times the value marked or measured on the pot to reduce the range to 0 to 1/4 maximum.

How can I do this? By adding a resistor to the output of the speed control potentiometer?

Note how we modify the maximum voltage of the potentiometer rather than "adding a resistor to the output".

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had luck with the resistor divider? I am new to the site but not the MC60 or most other readily available treadmill control boards(s). This MC60 originally used an interface board that took a PWM from the console, to the interface, thru an opto, then plugged onto those 3 tabs on the MC60. This gave isolation to the user/control circuits. The MC60 is an SCR/Triac fired controller that is very ugly in noise, and usually had a large inductor and optional large cap in the motor drive circuits to eliminate a lot of that noise. You are currently using the MC60's elsewhere, how are you operating them? The opto waveform across the Tabs(resistors) produced an average dc-level, so the divider is the right solution, although, beware, that motor rail is "not" isolated from the line, meaning the motor- (Vbulk-) is not earth-ground but hot. This means that resistor network is also riding a hot-ground so beware of shocks if trying to use a pot. That hasn't gone well for others trying to vary DC-Drives by tacking-in a divider or pot. Good Luck!

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