# A Discriminating Circuit?

Discussion in 'Electronic Basics' started by Randy Gross, Nov 29, 2003.

1. ### Randy GrossGuest

Greetings,

I am searching for a circuit that either senses the rectified feed to a
12vdc motor, or its speed, and, if the voltage, or speed, falls below a
preset, then it either trips a breaker else energizes a relay that will
supply an alternative (a battery to boost rpms).

Here's the catch. That same circuit would have to sense that the motor feed
has regained the correct voltage and disengage the booster.

A good analogy here would be a motor driving an alternator. When the motors
speed drops below that necessary to maintain the desired output, because
the primary (rectified 12vdc) feed has been interrupted, the feed is
disengaged and an alternative feed (battery) is engaged until the primary
feed has regained, at which time the secondary feed is disengaged and the
primary resumes.

This is an experiment and the process is trail and error. I know very
little about digital circuitry and this system needs that kind of
precision.

an Inquiring mind,
Randy Gross

2. ### John PopelishGuest

usually, a current measurement is used as a means to determine an
overload condition, since torque is roughly proportional to current.
A suggestion: Think instead, in terms of continuous regulation of the
input voltage via some switching regulator with continuous feedback of
the generator speed (frequency). A PID controller (proportional,
integral, derivative) that compares the measured speed (frequency) to
a fixed setpoint. You tune the gain coefficients on the three terms
can be made with one or three opamps and the ones in many voltage
regulator PWM control chips might well fill the bill. Keeping all
this decision making in the analog realm makes things work smoother.
The over current limit works just like an over current override on a
regulated supply.

3. ### Randy GrossGuest

<>...
: Randy Gross wrote:
<Snip>
:
: > Here's the catch. That same circuit would have to sense that the motor
feed
: > has regained the correct voltage and disengage the booster.
:
: A suggestion: Think instead, in terms of continuous regulation of

You're right! Continuous regulation makes more sense than the intermittent
approach I described. Maintaining the correct frequency is my main concern.
My programming professor had a term she tried to hammer into our skulls
everyday, and that term was K.I.S.S.

K eep I t S imple S tupid,

sometimes I forget;-)

Thanks John

the
: input voltage via some switching regulator with continuous feedback of
: the generator speed (frequency). A PID controller (proportional,
: integral, derivative) that compares the measured speed (frequency) to
: a fixed setpoint. You tune the gain coefficients on the three terms
: can be made with one or three opamps and the ones in many voltage
: regulator PWM control chips might well fill the bill. Keeping all
: this decision making in the analog realm makes things work smoother.
: The over current limit works just like an over current override on a
: regulated supply.
:
:
: --
: John Popelish
: