Voltage divider

[andreboos]

What is a safe way to divide voltage on an FRC robot?

I'd like a relay to drive a motor at far less than 12 volts. According to the wikipedia page, the voltage of a resistive voltage divider would vary with the load, and I'd like to keep it stable. Additionally, the resistors would have to be rated for multiple hundreds of watts to be FRC legal, and those that I can find are expensive and bulky.

Edit: Does R47 prohibit resistive loads between a relay module and a motor?

[Jon Stratis]

A few rules you should check:

R49

Motors, servos, and electric solenoids used on the Robot shall not be modified in any way, except as follows:

The mounting brackets and/or output shaft/interface may be modified to facilitate the physical connection of the motor to the Robot and actuated part.
The electrical input leads may be trimmed to length as necessary.
The locking pins on the window motors (PN 262100-3030 and 262100-3040) may be removed.
The connector housings on the window motors (PN 262100-3030 and 262100-3040) may be modified to facilitate lead connections.

R65

All outputs from sensors, custom circuits and additional electronics shall connect to only the following:

other custom circuits,
additional COTS electronics,
input ports on the Digital Sidecar,
input ports on the Analog Breakout,
the RS-232 DB-9 RS-232 port on the cRIO,
the Ethernet network connected to either Port 1 or Port 2 of the cRIO,
the CAN-bus if and only if all Jaguar speed controllers on the CAN-bus are wired in full compliance with Rules [R61] and [R62], or
the sensor inputs on the Jaguar speed controller.

R66

A noise filter may be wired across motor leads or PWM leads. For the purposes of Inspecttion and rules compliance, such filters will not be considered custom circuits, and will not be considered a violation of Rule [R49] or Rule [R65]. Acceptable signal filters must be fully insulated and are:

A one microfarad (1 µF) or less non-polarized capacitor may be applied across the power leads of any motor on your Robot (as close to the actual motor leads as reasonably possible).
A resistor may be used as a shunt load for the PWM control signal feeding a servo.

A voltage divider like you want to use would be considered a custom circuit, and does not fall under the exemption in R66 - thus, it would be illegal.

Hook the motor up to a Victor or Jaguar, and use the code to slow it down to the point you want.

[dellagd]

You could drive two motors in series then use a gearbox to put their outputs together.

Now, they both receive 6 volts. Ta da!

[EricVanWyk]

Originally Posted by dellagd

You could drive two motors in series then use a gearbox to put their outputs together.

Now, they both receive 6 volts. Ta da!

The OP was looking for FRC legal methods.

Putting two brushed motors in series is illegal for FRC, and will not actually have the effect you claim it will.

[Al Skierkiewicz]

Eric is referring to
R46
Each power-regulating device (speed controller or relay module) shall control one and only one electrical load (motor, actuator, electric solenoid, or compressor).

Another reason this is a bad idea is that the current will vary with the load on the motor. If the load is non-constant, the voltage drop across the external resistance will also vary.