(at 120 Hz) that brake control is either
none or a lot.
The challenges are that the
kickback is positive, it needs to be kept
under 200V, and there is a lot of
energy behind the kick from this
motor. I ended up with 100 µF in the
first place because a large (
nonelectrolytic) 10 µF capacitor did very
little to the splash, which almost
immediately would go beyond the
250V rating of the oscilloscope. In
my parts bin are some large diodes
such as DTV32, STPR1020CT ( 2),
STPR2045CT ( 2), TYN058, and others,
if that helps. I've read a little about
snubber circuits, but would like some
technical advice.
#1 If I assume that RLY1 is not
connected to short out the power
supply and is shown de-energized,
when RLY1 is energized, the motor
connections will be reversed and the
motor (acting as a generator) will try
to drive the positive rail of the supply
more positive. C4 will soak up some
of the reverse current and when Q1 is
turned on, it will limit the voltage to
about 12 volts (1 ohm x 12 amps).
Input to the power supply has to be
turned off at this time.
There are two problems: During
the transition time of RLY1 when the
motor is not connected to anything,
the voltage will shoot to the moon,
possibly damaging the motor, and the
delay time in PCB17 may not turn on
Q1 in time to limit the voltage below
200 volts. One solution is to connect
a 12 amp diode and series resistor
across the motor to limit the voltage to
200 volts (R = 200V/12A = 16 ohms).
A 75 watt light bulb would no doubt
work as the resistor.
Russell Kincaid
Milford, NH
#2 Quickly braking an electric
motor requires a circuit that will quickly dissipate quite a bit of energy. Texas
Instruments has produced a paper,
"The Art of Stopping a Motor,"
available at http://e2e.ti.com/blogs_/
b/motordrivecontrol/archive/2013/
10/18/the-art-of-stopping-a-motor.
aspx. TI also has motor control dev
kits that offer braking circuits and
techniques that might help in Tsidqah's
circuit. The kit documentation usually
includes code and circuits. The
Microchip Technology application
note, AN-905, "Brushed DC Motor
Fundamentals," offers a way to provide
a shunt to ground that brakes a motor:
http://ww1.microchip.com/down
loads/en/AppNotes/00905B.pdf. I
suggest the designer replace the 4PDT
relay with a MOSFET H-bridge circuit
that offers more flexibility for various
braking applications. Also, real time
voltage and current measurements
taken during braking with a load
resistance should help determine the
characteristics needed in a braking
load and switching circuits. As for a
testing load, consider a non-electronic
clothes iron or a toaster oven.
Jon Titus
Herriman, UT
#3 I'm not sure what you are doing
with the 4PDT relay as shown, so
maybe it is a drawing error. The way it
is drawn, the motor feed and motor
are both shorted! For motor reversal,
you only need a 2PDT relay. If you are
paralleling contacts for increased
current, that's a bad idea since there is
no guarantee the contacts will open or
close in unison. So, in effect, one
contact will still carry the load.
Paralleling for redundency is also not
good because if a contact welds itself,
bad things can happen when the
parallel contact switches.
I assume — since there is no logic
flow provided — that the TRIG signal
goes low before the EBRK signal goes
high (and what about some protection
for the input to the opto-isolator U6?).
Likely, the spike is exceeding the LED
PRV rating. I would think clamping
diode(s) would help with the CEMF
and perhaps an R/C, as well. Hope
this helps!
Len Powell
Finksburg, MD
[#3142 - March 2014]
Capacitor Forming
I pulled some excellent quality
electrolytic capacitors from the power
supply of an amplifier. The capacitors
are rated at 40 VDC but were used
in a 10V circuit. I wanted to use the
capacitors in a 24 VDC circuit, but I
was told that the capacitors "formed" at
10V and wouldn't work at 24 VDC,
regardless of the original rating. Is this
true?
#1 True, aluminum capacitors will
deform with years of operation at
reduced voltage (or no voltage). The
typical aluminum capacitor will have a
leakage current that increases with
capacitance and voltage. A 40 µF 40V
cap would have leakage in the order
of 2 µA.
To test your capacitor, connect it
through a 10K resistor to 24 volts (or
40 volts if it is available); if the voltage
rises to 24 volts or more, it is okay. If
not, leave it connected until the
voltage does rise above 24 volts. You
can use a smaller resistor to speed up
the process, but don't let the cap get
hotter than 50 degrees C.
Russell Kincaid
Milford, NH
#2 Short answer... maybe.
You can often reform electrolytic
capacitors by slowly ramping the
voltage up — or above — your
expected voltage. Keep a current
meter in series and measure the
charge current (after you have
reached your top voltage). It should be
in the microamp to low milliamp range
for a good capacitor. If it's higher than
a milliamp or two, you have a cap that
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