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current sensing

Posted: Tue Jun 17, 2008 6:57 pm
by danbob
I am building a screwdriver antenna. I have the motor and built a simple 555 PWM supply for it. I want to be able to sense the current increase at the end of the travel at both ends to turn off the motor. I have a bunch of 5 watt .1 ohm resistors and was going to use one to sense the current but after playing with it I don't think I can do it that way because the motor draws different currents at different speeds. Any ideas would be appreciated. Dan n7nbw

Posted: Tue Jun 17, 2008 10:02 pm
by MrAl
Hi there,

Yeah, the start current would be about the same as the stall current,
so you'd have to start the motor and then wait some period of time
later before measuring the current in order to determine if the
motor has stalled. Depending on the motor you may have to wait
1 second or so, but the duty cycle would have to be limited too
in order to keep the motor from heating up too much.

Re: current sensing

Posted: Tue Jun 17, 2008 10:02 pm
by Bob Scott
danbob wrote:I am building a screwdriver antenna. I have the motor and built a simple 555 PWM supply for it. I want to be able to sense the current increase at the end of the travel at both ends to turn off the motor. I have a bunch of 5 watt .1 ohm resistors and was going to use one to sense the current but after playing with it I don't think I can do it that way because the motor draws different currents at different speeds. Any ideas would be appreciated. Dan n7nbw
Try to incorporate end-of-travel sensor switches instead. eg: Micro switches or toggle.

Posted: Tue Jun 17, 2008 11:46 pm
by rshayes
An alternate method would be to sense the motor "back EMF". This is the voltage generated by the rotation of a DC motor. It is approximately proportional to the motor's speed.

The voltage across the motor terminals is the sum of the motor current times the armature resistance and the voltage induced in the armature by its rotation in the magnetic field generated by the field magnets (assuming a permanent magnet motor). A bridge circuit can be used to subtract the voltage due to the armature resistance and current from the motor voltage. The output of the bridge is then proportional to the speed of the motor. This voltage would drop suddenly when the motor ran into a physical stop.