Discuss this article in the Nuts & Volts forums at http://forum.nutsvolts.com.
QUESTIONS & ANSWERS
AI am sure you are aware that there are mechanical push on/push off switches; Mouser part 112-R13-512B2-BR is an example. An IC that comes to mind for the job is the D flip-flop
(CD4013); see Figure 5.
The NOT-Q output is connected to the data input,
and the data is transferred to the Q output on the rising
edge of the clock input. Each time the button is pushed,
the output toggles. I paralleled the two flip-flops to give
more drive capability, then had to do something with the
I looked at your circuit in Figure 6, and it ought to
work. C1 is initially charged to nine volts so when the
switch is closed, Q1 is turned on which turns on Q3
which, in turn, turns on Q2 and Q1. The Q1-Q2 loop is
latched with the output low.
With Q2 on, C1 discharges through R2 with a time
constant of 1/2 second. If the switch is held closed for
more than one second, Q1 is turned off and the circuit
goes back to the beginning.
I can’t imagine that saturation storage time is the
problem, but if you replace the bipolar transistors with
enhancement mode MOSFETs that problem will go away.
■ FIGURE 5.
DC SERVO QUESTION
QI want to control my 3 HP DC motor using PWM with a microcontroller. The motor is Baldor CD1803R
and I will be controlling it with knobs
and switches. I want to do these
• Control the direction of
rotation (CW and CCW)
• RPM control
• LCD display (all of the above
It will not decrease speed when
loaded, hence I want to use speed
regulation, standard feedback, and
— Selahattin ŠADOGLU
AI can’t produce a detailed circuit for free, but I may be able to suggest a method. It has been nearly
60 years since I studied servos and
the industry has changed a lot. The
datasheet only tells me that the
motor is rated 3 HP, 180 volts; the
field is rated 100/200 volts (I don’t
know what that means); and the field
current is nominal one amp. That is
not nearly enough to do a proper
design, but I can offer some
generalities. I have not mastered LCD
displays, so I won’t talk about that.
■ FIGURE 6.
enough to not produce an annoying
sound. All you need to do to reverse
the direction of rotation is reverse the
field voltage; refer to Figure 7. Some
means of removing power if the field
current drops to near zero will be
needed to prevent a runaway motor.
If it were me, I would do it all
analog; a generator on the shaft to
supply the field and a rheostat on the
generator field to control the
■ FIGURE 7.
April 2013 23