Shield power dissipation, Motor considerations – Pololu Dual MC33926 User Manual
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use a 9V battery (the rectangular type with both terminals on the same side) as your motor power supply. We
recommend
, lithium-based rechargeable batteries (if you
have a good charger and adequately understand the dangers of using them improperly), or a power adapter with
an appropriate power rating. The current draw is ultimately a function of your motors, your operating voltage, and
your motor load, but the driver is capable of delivering a continuous 6 A (3 A per channel), and it can deliver in
excess of 10 A before the internal current limiting is activated.
Shield Power Dissipation
Each MC33926 motor driver IC has a maximum continuous current rating of 5 A. However, the actual current it
can deliver depends on how well you can keep it cool. The shield’s printed circuit board is designed to draw heat
out of the motor driver chips, but performance can be improved by adding heat sinks.
Unlike other H-Bridges, the MC33926 has a feature that allows it to gracefully reduce current as the current
exceeds 5 A or as the chip temperature approaches its limit. This means that if you push the chip close to its limit,
you will see less power to the motor, but it might allow you to avoid a complete shutdown.
We tested the shield at room temperature with no forced air flow or heat sinks. In our tests, the shield was able
to deliver 5 A to both channels simultaneously for 10 s before the thermal protection started reducing the current.
The shield delivered 4 A on both channels for 37 s, and at 3 A it was able to operate continuously for over 10
minutes without triggering current limiting or thermal protection.
Our tests were conducted at 100% duty cycle; PWMing the motor will introduce additional heating proportional
to the frequency.
This product can get
hot
enough to burn you long before the chip overheats. Take care when handling
this product and other components connected to it.
Motor Considerations
If your motor has a stall current over the driver’s continuous current rating of 3 A per channel, we recommend
you take extra steps to make sure that your motor will not be exposed to loads that will cause it to exceed 3 A for
prolonged periods of time (or you take extra steps to keep the motor drivers cool, such as increasing air flow or
adding heat sinks). Exceeding 3 A for long durations should not damage the shield, but it will eventually activate
the driver’s thermal protection, which might result in inadequate performance for your application.
It is not unusual for the stall current of a motor to be an order of magnitude (10×) higher than
its free-run current. If you do not know your motor’s stall current, you can approximate it by
measuring the current it draws while held stalled at a lower voltage (such as when powered from a
single battery cell) and then scaling that value linearly with voltage. For example, the stall current
of a motor at 6 V is six times the stall current of that motor at 1 V. Another, less accurate method
is to use a multimeter to measure the resistance between the motor terminals and then use Ohm’s
law to compute the stall current I at voltage V: I = V/R. This last method generally is not as reliable
because it can be difficult to measure such small resistances accurately.
Occasionally, electrical noise from a motor can interfere with the rest of the system. This can depend on a number
of factors, including the power supply, system wiring, and the quality of the motor. If you notice parts of your
system behaving strangely when the motor is active, first double-check that your power supply is adequate, then
Pololu Dual MC33926 Motor Driver Shield User's Guide
© 2001–2013 Pololu Corporation
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