I have a pair of motors and the datasheet says that typical peak effective current is 177mA, max rated current is 218mA, typical no load is 53mA.

Now thats all fine but it also says typical start current 1230mA.

My question is how important is this number...

If 1230mA cant be supplied does that mean the motor will not start? Or just start slower?

Im looking at a H bridge IC and its maximum output current per bridge is 1A. Does this mean the motor will not start at all (or slower?) also would it burn out the chip? Or because its only a start current and not an average current the sudden spike above 1A will not affect the chip (just aslong as its not above 1A for too long?)

The motor is 12V

  • \$\begingroup\$ The L293 H bridge says it's rated for 1A but can supply peaks of 2A. Having said that if you are running on a low voltage supply for the motor (such as 5V the L293 is a poor choice these days) \$\endgroup\$
    – Andy aka
    Jan 23 '14 at 18:39
  • \$\begingroup\$ Oh Yes! I went back to look for a peak value and it is there! 2A for 5ms? Thanks for that, ti.com/lit/ds/symlink/sn754410.pdf, So according to this, I actually CAN use it for my motor? My motor is 12V. I was trying to avoid the L298N due to its bulkyness and weird pins. \$\endgroup\$ Jan 23 '14 at 19:00
  • \$\begingroup\$ The L293D seems more avalible than the L293, however its max peak current is 1.2mA so just short by 30mA. Would I be able to get away with that? Which is better out of the L293 and SN754410 since it is being sold as a drop in replacement to the L293 (which I guess means its newer so better?) \$\endgroup\$ Jan 23 '14 at 19:15
  • \$\begingroup\$ The problem with both those devices is that when conducting, they drop 1 or 2 volts in the H bridge pass transistors (each transistor) - this means you might see 9V (typically) at 1A or maybe 10V at 0.5A across your motor. This does not help starting torque and overcoming stiction (static friction). I'd be looking for a H bridge with decent FET output stages. What ampage is your 12v capable of supplying? \$\endgroup\$
    – Andy aka
    Jan 23 '14 at 19:24
  • \$\begingroup\$ The power supply hasn't been designed yet fully, im just working inside out so I know all my power requirements first. However it is going to be battery powered as its for a small robot micro mouse, it only needs to last 1 hour or so between battery swaps. \$\endgroup\$ Jan 23 '14 at 19:53

Torque is directly proportional to current. If the current to the motor is limited, then it's torque will be limited. Whether it can start with reduced torque is only something you can say. It depends on what the torque versus speed profile of the load looks like.

If you don't allow the motor to produce full torque on startup, it might not spin at all, therefore trying to draw the startup current forever. It is not a good idea to plan a motor controller that can only deliver the stall current for a short time. If not, you should have some mechanism in there to shut the motor off after the full current times time limit has been reached. You can do some computation in the processor to simulate temperature in both the motor and the driver, and shut things down when you think either gets too hot. You can also outright measure temperature, but due to lag in getting heat from where it is produced to the sensor, it may not tell you of a over temperature condition in time to prevent damage.

  • \$\begingroup\$ Sir, he is not referring to the steady state , but zero state response of the motor. I think he is asking how does he compensate zero state response. \$\endgroup\$ Jan 23 '14 at 19:41
  • \$\begingroup\$ @StandardSandun And what happens if the motor is stalled bu mechanical means? Is a bit of smoke okay? Something that should be considered, as Olin does. \$\endgroup\$ Jan 23 '14 at 21:15

I'd be looking for a much more "capable" H-bridge driver than the L292 or SN754410 - they can't provide the current you need without dropping probably about 3V in total from your 12V supply. This is likely to mean problems getting the motor spinning under load conditions where most torque is needed to overcome stiction.

The DRV8800 can supply 2.8A and uses output MOSFETs which are typically 0.35 ohms on resistance - that's 0.7 ohms in total and, at 1A this loses 0.7V - better than the other devices: -

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