80 July 2015
When the circuit in Figure 3 is
first energized, C charges to 1/2 Vcc
through voltage divider R1/R2. When
the button S1 is pushed, C discharges
through the relay coil. It pulls in, and
the relay is held closed by the current
through R1 and the relay coil. When
the button is pushed again, C, (now
fully discharged by R2), goes across
the relay coil making it drop out
and returning the circuit to where it
La Crescenta, CA
#6 Use a pushbutton switch to
toggle a JK flip-flop.
[#5154 - May 2015]
I have a drill press that has a five
amp fuse that recently has started to
blow on just about every project. I am
not working on harder material and
the bits are sharp, so I don’t think its
increased torque from friction which
leaves the electronics. As this is simply
a motor and a power switch, I can’t
imagine what might be causing the
increased current draw. Suggestions?
#1 You don’t say what type of
fuse you are using, but if it is a buss
type glass fuse you might want to
make sure you are using a slow-blow
type fuse for your drill press.
#2 Worn bearings could be
allowing the armature to rub on the
stator. Remove the belt and try to
move the motor shaft; if it moves at
all, it is bad.
Russell L. Kincaid
#3 If this is just an on/off switched
drill press, then you definitely have
an excessive friction issue. If a good
lubrication doesn’t help, then the
brushes or commutator in the motor
are worn and binding.
La Quinta, CA
#4 Does the dill press sound any
different? Maybe the bearing in the
motor is wearing out, causing the
What about the cord? Has it been
pinched/damaged in any way? Or
even the outlet that it is plugged into,
are there visible signs of scorching?
Lastly, how old is it? Could the
motor be electrically failing? Things
to consider beyond stresses due to
King City, CA
#5 Mr. Olivo is experiencing
nuisance fuse blowing in his drill
press. The problem may well be that
the fuses that he has been using
cannot tolerate the inrush current
demand of the motor. At the instant
of power application, the magnetic
state of the motor core may be such
that the first half-cycle of utility power
drives it into saturation.
Assuming that the fuse is a 1/4”
x 1-1/4” cartridge fuse, a Bussmann
MDL- 5 fuse will open at five amperes,
but will tolerate more than
100 amperes during a one-half cycle of utility power.
See the time-current curves
for this family on datasheet
#2004, which is downloadable
elx_1_ 4_x_1-1_ 4_/mdl-v_mdl.
If the fuse is another size,
check with your local electrical
jobber for an appropriate fuse
having similar inrush tolerance.
Peter A. Goodwin
n FIGURE 3
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