Version 1.6 - May 28, 2012
9
pin and changing its output level between logic high and logic low will switch the B-side power
device either on (connected to Batt +) or off (connected to Batt -). This works fine for lights,
fans and solenoids or various full-power or off type loads. This frees the scarce timer
resources for other uses.
As with the Enable input, it is possible for multiple MegaMotos to use the same PWM source.
For example, if two motors always operate together, identical jumpers for Enable and PWM-
A/B can be installed on both MegaMotos. This obviously saves pins and allows more motors
to be controlled from a single Arduino. In fact, there is no real limit to the number of
MegaMoto units controlled in this way. The three MegaMotos per Arduino limit is only for
completely independently controlled loads. If you have five motors that always operate
together or in identical groups there is no reason why five or more MegaMoto units cannot be
stacked up. This might prove unwieldy from a physical standpoint but electrically it will work.
The Enable input is also used to reset the power chip protection circuits in the case of an over
temp or over current shutdown. The power chips are designed such that these faults are
latched and the chip will remain off for safety until reset.
To reset the power chips after a
fault lower the Enable input for at least four microseconds (4
µ
S) then raise to logic
high again.
Current Sense:
The current sensor outputs are different than the logic-level inputs such as
Enable or PWM. These outputs source a current that is converted to a voltage by a resistor.
As such they are much more sensitive to being connected in parallel between units or
between A-side and B-side power chips. The bridged connection in H-bridge mode works
because the current sense output provides zero current when the low-side of the power chip is
passing current. In various half-bridge modes or between stacked units, the current sensors
should not be connected in parallel.
A secondary function of the current sense output is to indicate if the device has gone into
protective shutdown. The Arduino can monitor the current sensor voltage to detect safety
shutdown.
If the current sense output is giving a steady logic high a fault condition is
indicated.
A short pulse on the Enable pin will reset the protection circuits and allow the
device to operate again assuming the fault condition has cleared. In particular over
temperature shutdown may take several seconds to clear as the device cools.
In some cases operator attention is required if the device enters shutdown i.e. some serious
or dangerous fault is indicated. In that case the power chips will remain latched off until the
power chip voltage source is cycled. The Arduino power source does not need to be cycled in
that case just the voltage source connected to the power chips.
Power connections:
The power connections Batt + and Batt – are the primary voltage
source for the power chips and hence the motor outputs MA and MB. The power chips are
internally powered and no external voltage source is required to power the input or protection
circuits. The power chips are not connected to the 5V or 3.3V connections from the Arduino.
The exception is the 5V jumper on the Enable block. However, this is simply to provide a logic
high to the Enable input pins and no significant current is drawn from the Arduino 5V supply.
When multiple MegaMoto units are stacked on one Arduino base it is important that the power
connections be closely tied. Since the negative (ground) connections are common between
the Arduino and the MegaMoto there is a possibility of a “ground loop” in which the negative
rail connections have a slightly different voltage. This is commonly due a very long or thin
battery negative wire or to daisy chaining the power rails from one unit to the next in a chain.
Since the design of the MegaMoto assumes that the ground connections will be at the same
level it is important to avoid ground loops. The best practice to ensure this does not happen is
to connect the negative battery connection from each unit directly to the battery negative