Software Programmable Extended Range PWM MOSFET Braking - A New
Concept in Brake Control
The Extended Range PWM brake profile uses Pulse Width Modulation to control
brake strength. The control is completely linear, allowing the user to vary the
pulse width (and resulting brake strength ) from 100% to 0% in a linear ramp
without any cliffs or steep drop offs.
Although it controls brake strength like a conventional controller, pulse width
modulated brakes work on a completely different principle than resistance
controlled brakes...giving the user complete control of brake strength regardless
of what class of car is being raced.
When a slot car motor is braking it behaves like a generator, its back EMF driving
a current backwards through the armature to slow it down. Reducing this current
by introducing a resistance into the brake line reduces the strength of the brakes.
Conventional controllers use variable resistors called potentiometers (aka pots)
to control braking current. However, the pots need to be matched to the motor's
impedance to provide a useful range of adjustment. If the pot isn't matched
properly, the brakes will be impossible to adjust. They will either be completely
on or completely off, with no intermediate settings.
For example, 1/24th scale commercial slot car controllers generally use a 2 to 5
ohm pot, 1/32nd scale controllers a 25 to 50 ohm pot, and HO controllers a 50 to
100 ohm pot, depending on the manufacturer. If you try to use a 2 to 5 ohm pot to
control a 1/32nd scale car's braking, the pot will always appear to be a dead
short compared to the motor's impedance, regardless of where it is set. The pot
won't reduce braking current so the brakes will always be full-on (dropping off to
full-off if a pro-style pot is used).
Conversely, a 25 or 50 ohm pot is much too large to control a 1/24th scale car's
brakes. After the first few degrees of rotation it appears as an open circuit to a
1/24th scale slot car motor, reducing current flow so much that the brakes go
from full-on to full-off with no intermediate settings.
Instead of introducing resistance into the brake circuit, a PWM controlled brake
reduces current flow by rapidly cycling the brakes on and off. Braking current is
directly related to PWM duty cycle, regardless of motor impedance. An 80% duty
cycle will always provide 80% of the motor's maximum braking current for any
given motor speed...and a 50% duty cycle always cuts the current in half.
Since the brake strength is directly related to PWM duty cycle, Extended Range
PWM braking provides a very wide useful range of adjustment for all
scales...validated by track testing HO, 1/32nd and 1/24th scale cars.
Concept in Brake Control
The Extended Range PWM brake profile uses Pulse Width Modulation to control
brake strength. The control is completely linear, allowing the user to vary the
pulse width (and resulting brake strength ) from 100% to 0% in a linear ramp
without any cliffs or steep drop offs.
Although it controls brake strength like a conventional controller, pulse width
modulated brakes work on a completely different principle than resistance
controlled brakes...giving the user complete control of brake strength regardless
of what class of car is being raced.
When a slot car motor is braking it behaves like a generator, its back EMF driving
a current backwards through the armature to slow it down. Reducing this current
by introducing a resistance into the brake line reduces the strength of the brakes.
Conventional controllers use variable resistors called potentiometers (aka pots)
to control braking current. However, the pots need to be matched to the motor's
impedance to provide a useful range of adjustment. If the pot isn't matched
properly, the brakes will be impossible to adjust. They will either be completely
on or completely off, with no intermediate settings.
For example, 1/24th scale commercial slot car controllers generally use a 2 to 5
ohm pot, 1/32nd scale controllers a 25 to 50 ohm pot, and HO controllers a 50 to
100 ohm pot, depending on the manufacturer. If you try to use a 2 to 5 ohm pot to
control a 1/32nd scale car's braking, the pot will always appear to be a dead
short compared to the motor's impedance, regardless of where it is set. The pot
won't reduce braking current so the brakes will always be full-on (dropping off to
full-off if a pro-style pot is used).
Conversely, a 25 or 50 ohm pot is much too large to control a 1/24th scale car's
brakes. After the first few degrees of rotation it appears as an open circuit to a
1/24th scale slot car motor, reducing current flow so much that the brakes go
from full-on to full-off with no intermediate settings.
Instead of introducing resistance into the brake circuit, a PWM controlled brake
reduces current flow by rapidly cycling the brakes on and off. Braking current is
directly related to PWM duty cycle, regardless of motor impedance. An 80% duty
cycle will always provide 80% of the motor's maximum braking current for any
given motor speed...and a 50% duty cycle always cuts the current in half.
Since the brake strength is directly related to PWM duty cycle, Extended Range
PWM braking provides a very wide useful range of adjustment for all
scales...validated by track testing HO, 1/32nd and 1/24th scale cars.
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Software Programmable for the Ultimate in Flexibility
The Linear 200's braking is controlled by software programmed brake control
profile chips, socketed so that users can try completely different brake profiles as
they become available. The chips are highly integrated microcontrollers, software
programmable single-chip computers with analog-to-digital converters and on-
chip timers. They monitor the brake pot and trigger positions, converting the
analog voltages to digital data that can be acted upon by the brake profile
program. The brake profile program calculates the length of the brake pulse, then
loads the timers to create the precision timed brake pulses.
This design approach provides significant benefits to Linear 200 owners. By
plugging in a different brake chip, they can dramatically change the braking
behavior...for example changing the controller brake from a Brake N' Release™
brake control to a control type that adjusts the brake's strength, much like in a
conventional electronic controller.
Current Sensing Circuit Protection For High Performance Brakes
MOSFET brakes are incredibly strong, but they need to be well protected against
mis-wiring and short circuits. Automotive type ATO blade fuses may not always
blow fast enough to protect a MOSFET and PTCs (the resettable fuse used in
many electronic controllers) have a higher resistance than fuses which
increases even more after they have been tripped.
The Linear 200 uses the same MOSFET protection techniques found in high-rel
military and automotive electronics...current sensing control circuits to protect
against shorts and polarity protection to protect against mis-wiring.
Short circuit protection is provided by a fully electronic breaker circuit with
automatic reset. The circuit introduces an on-resistance into the brake line
comparable to a 10-AMP ATO blade fuse and reacts within a few
microseconds…tens of thousands of times faster than the fuse. This is more
than sufficient to protect the MOSFET against the instantaneous surges of
hundreds of amps that high-amp-output power supplies can put out before their
own current limiting safety circuits kick in.
It has a much lower resistance than the PTCs used in other electronic controllers
with the most important difference being that it’s on-resistance will not change
after the circuit breaker has been tripped. By comparison, not only is a PTC's on-
resistance is much higher than this circuit initially (about 5 times higher, at least),
it increases after it's been tripped and reset (not returning to it's original value).
The electronic breaker uses a high precision current-sensing resistor in the
brake path, whose value doesn’t change. The voltage drop across the resistor is
monitored…and if it exceeds a predetermined amount, the brake MOSFET is
shut off. The retry duty cycle is short enough that the MOSFET does not heat up
appreciably even if the duration of the shorted condition exceeds several minutes.
The circuit also includes a reverse polarity protection MOSFET in the brake line to
protect the controller if the brake wire is connected to the white power post. This
MOSFET acts like a "perfect" diode, blocking current flow in one direction without
introducing the 0.7V diode drop in the forward direction that would hurt braking
performance. The combined on-resistance of ALL of the components in the
brake path, including the brake FET and all circuit protection components, is less
than 13 milliohms…far less than the on-resistance of a PTC alone.
The benefit to you is that you’ll have a controller with incredibly strong brakes, no
pesky brake fuses to replace, and brakes that will not deteriorate due to an
increase in PTC resistance or dirty brake contacts.
See how easy it is to change control chips on the Linear 200 family of controllers:
Linear 200 Brake Chip Installation
The Linear 200's braking is controlled by software programmed brake control
profile chips, socketed so that users can try completely different brake profiles as
they become available. The chips are highly integrated microcontrollers, software
programmable single-chip computers with analog-to-digital converters and on-
chip timers. They monitor the brake pot and trigger positions, converting the
analog voltages to digital data that can be acted upon by the brake profile
program. The brake profile program calculates the length of the brake pulse, then
loads the timers to create the precision timed brake pulses.
This design approach provides significant benefits to Linear 200 owners. By
plugging in a different brake chip, they can dramatically change the braking
behavior...for example changing the controller brake from a Brake N' Release™
brake control to a control type that adjusts the brake's strength, much like in a
conventional electronic controller.
Current Sensing Circuit Protection For High Performance Brakes
MOSFET brakes are incredibly strong, but they need to be well protected against
mis-wiring and short circuits. Automotive type ATO blade fuses may not always
blow fast enough to protect a MOSFET and PTCs (the resettable fuse used in
many electronic controllers) have a higher resistance than fuses which
increases even more after they have been tripped.
The Linear 200 uses the same MOSFET protection techniques found in high-rel
military and automotive electronics...current sensing control circuits to protect
against shorts and polarity protection to protect against mis-wiring.
Short circuit protection is provided by a fully electronic breaker circuit with
automatic reset. The circuit introduces an on-resistance into the brake line
comparable to a 10-AMP ATO blade fuse and reacts within a few
microseconds…tens of thousands of times faster than the fuse. This is more
than sufficient to protect the MOSFET against the instantaneous surges of
hundreds of amps that high-amp-output power supplies can put out before their
own current limiting safety circuits kick in.
It has a much lower resistance than the PTCs used in other electronic controllers
with the most important difference being that it’s on-resistance will not change
after the circuit breaker has been tripped. By comparison, not only is a PTC's on-
resistance is much higher than this circuit initially (about 5 times higher, at least),
it increases after it's been tripped and reset (not returning to it's original value).
The electronic breaker uses a high precision current-sensing resistor in the
brake path, whose value doesn’t change. The voltage drop across the resistor is
monitored…and if it exceeds a predetermined amount, the brake MOSFET is
shut off. The retry duty cycle is short enough that the MOSFET does not heat up
appreciably even if the duration of the shorted condition exceeds several minutes.
The circuit also includes a reverse polarity protection MOSFET in the brake line to
protect the controller if the brake wire is connected to the white power post. This
MOSFET acts like a "perfect" diode, blocking current flow in one direction without
introducing the 0.7V diode drop in the forward direction that would hurt braking
performance. The combined on-resistance of ALL of the components in the
brake path, including the brake FET and all circuit protection components, is less
than 13 milliohms…far less than the on-resistance of a PTC alone.
The benefit to you is that you’ll have a controller with incredibly strong brakes, no
pesky brake fuses to replace, and brakes that will not deteriorate due to an
increase in PTC resistance or dirty brake contacts.
See how easy it is to change control chips on the Linear 200 family of controllers:
Linear 200 Brake Chip Installation
Programmable brake profile
chips convert analog pot
settings to precise timing
pulses
chips convert analog pot
settings to precise timing
pulses
Brake profile chip socket on transistor module
Low resistance brake MOSFET with current sense and
reverse polarity protection
reverse polarity protection
Linear 200 HO - 1/32nd Controller with SOFTouch
throttle control and Extended Range PWM MOSFET
Brakes
throttle control and Extended Range PWM MOSFET
Brakes
Linear 200 PWM LP linear response 1/24th scale
controller with Extended Range PWM MOSFET Brakes
controller with Extended Range PWM MOSFET Brakes