>>>READER-TO-READER QUESTIONS AND ANSWERS
[#12082 - December 2008]
PV vs. EMR
Since solar panels are inherently a
diode, would they survive the electro-magnetic burst of an EMR weapon?
Semiconductor survivability under
EMP (electro-magnetic pulse) depends
on the design which must somehow
limit the current when all junctions are
conducting. In the case of solar
panels, the cells will be destroyed by
the reverse current from the charged
battery unless there is a series resistor
or thermal breaker. The diode that
protects against reverse current will be
turned on also, so that is no help. A
shielded reverse current relay might
work but I would rather rely on the
thermal breaker.
S1 DPDT Switch
S2 N.O. Push Button Switch
LED 1 is the NPN Output
LED 2 is the PNP Output
Q1 2N4401
Q2 2N4403
Figure 3
relay as soon as luminance is black.
Russ Kincaid
Milford, NH
[#12088 - December 2008]
Testing Components
How can you differentiate a bad
transistor from a good one?
The circuit in Figure 3 will allow
you to test if a transistor is good and
what type it is, NPN or PNP.
When you test a transistor, LED 1
or LED 2 will light showing the type of
transistor it is. If you press S2 and
neither LED lights, switch to S1 and try
again. You should get one of the LEDs
to light. If not, the transistor is bad.
Allen Resignalo
Pittsburgh, PA
ACS755 current sensor. These are
available up to 200 amps and provide
full isolation from the measured
current because they are Hall-effect
devices. Lower current applications
can use the ACS715. A single ADC
input on your microcontroller is all
that is needed to measure the current
sensed by the Allegro device.
T. Black
Folsom, CA
[#1092 - January 2009]
Three-Phase Power
Does anyone know of a simple,
inexpensive, but very accurate way to
monitor three-phase current in brushless motor circuits? The method should
require, at most, one channel of ADC.
#1 The simplest way to determine
the total current consumed by the
brushless motor is to measure the DC
input to the motor controller, rather
than measure and accumulate the
current on its three distinct outputs.
Although a traditional current
shunt resistor method can be used
with your microcontroller's ADC, I
would recommend the Allegro
94 February 2009
#2 You can always monitor the
current in the DC-Intermediate Loop.
A modern set-up has the following
topology: a rectifier (synchronous or
conventional), power factor correction
switcher, DC intermediate loop (may
need braking resistors to avoid high
voltages), and an H-bridge, motor. The
rectifier is not necessary if the supply
is DC. Monitoring the current in
the DC-Loop will give you a good
indication of the overall motor current,
especially after the filter caps. However,
there are some downsides, especially
if the drive is regenerative and can
feed back into the power grid. This is
a necessity for drives which have a
significant amount of braking to do.
Consider start/stop cycles, and some
drives run almost entirely as brakes.
Walter Heissenberger
Hancock, NH
[#1094 - January 2009]
Svideo Luminance
I have a DVD player with Svideo
out; I need to build a simple circuit to
use the luminance pins to trigger a
The solution to this problem
breaks down to these basic functions:
In Figure 4, Q3 and Q4 first buffer
and amplify the input signal to two
volts peak-to-peak. Secondly, Q1, Q2,
C2, and R13 clamp the negative-going
sync tips to 1.29 VDC (determined
by R2 and R8). This is called "DC-restoration." Thirdly, U1 performs a
voltage comparison against a video
black reference input on pin 2. U1
pin 7 goes high as video falls to black,
cutting relay coil current.
LM311 comparators are capable
of driving sensitive relays directly. You
must use a relay with at least 100
ohms of coil resistance. D1 protects
U1 from inductive kickback. R11 and
C3 filter out sync and average DC-restored video into a slowly moving
DC voltage, but also cause up to 300
mS of delay in recognizing the black
level. R6 provides a little hysteresis to
prevent relay chatter near the
detection threshold. Adjust R6 if
necessary. Only a few millivolts of
hysteresis should be sufficient.
Tentative values for R4 and R14 are
given because you will need to
experiment with the black level
reference. You should aim for about
1.9 VDC at the junction, but it's
likely that you will need to optimize
this voltage for the best detection
accuracy. Use a pot if desired.
Remove R6 temporarily if it interferes
with this adjustment. The specified 1%
tolerance resistors aren't mandatory
