Protection circuitry for op-amp driven motor

[Jay]

Hi,

I have a very small DC motor that I am driving with a single-supply op-
amp which is in-turn driven by a high-precision DAC circuit. Steady-
state the motor draws < 10 mA current. FWIW I'm going this route over a
MOSFET/PWM due to the low PWM frequency that results with 14 to 16-bit
desired resolution.

Anyway, I realize I should probably add some kind of protection
circuitry. However, I'm not sure exactly what to add. So far, I've put
down a schottky "freewheeling" diode across the motor leads.

Any other suggestions?

My only other idea was a schottky in series with the op-amp output and
the motor's + lead to prevent back-EMF from forcing the op-amp to sink
current (i.e. if the op-amp is commanded to reduce the output voltage
and the motor's inertia allows to motor to act like a generator).

Best regards,
Jay.

 

[Spehro Pefhany]

On Thu, 25 Mar 2004 12:52:47 -0600, the renowned Jay
<localhost@127.0.0.1> wrote:

Hi,

I have a very small DC motor that I am driving with a single-supply op-
amp which is in-turn driven by a high-precision DAC circuit. Steady-
state the motor draws < 10 mA current. FWIW I'm going this route over a
MOSFET/PWM due to the low PWM frequency that results with 14 to 16-bit
desired resolution.

Anyway, I realize I should probably add some kind of protection
circuitry. However, I'm not sure exactly what to add. So far, I've put
down a schottky "freewheeling" diode across the motor leads.

Any other suggestions?

My only other idea was a schottky in series with the op-amp output and
the motor's + lead to prevent back-EMF from forcing the op-amp to sink
current (i.e. if the op-amp is commanded to reduce the output voltage
and the motor's inertia allows to motor to act like a generator).

Then the op-amp won't be able to brake the motor. If you put Schottky
diodes to the rails then worst case you'll have the equivalent of a
short on the op-amp output.

Best regards,
Spehro Pefhany
--
"it's the network..." "The Journey is the reward"
speff@interlog.com Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog Info for designers: http://www.speff.com