■ FIGURE 6
current, say 10 amps into the device
being measured and then measuring
the voltage drop across the device in
question. Is this possible and if so,
can you come up with any circuit
suggestions to accomplish this?
— Jeff
AIn the circuit (Figure 5), I
chose a supply voltage of
five volts to keep the power
dissipation in Q1 low and
still allow measurement of up to 0.1
ohms. To measure up to one ohm,
reduce the current to one amp by
increasing R1, to one ohm. There is a
small error due to the base current of
Q1 flowing through R1, but this is less
than 1% in the worst case and less
than 0.1% typically. I would glue the
diodes to the transistor case so they
are at the same temperature; wait
for thermal stability before taking a
measurement.
The diodes will not exactly match
the Vbe of the transistor, so you will
need to adjust R4 for exactly one volt
across R1. The transistor is in a TO- 3
case and will need a good heatsink.
Wakefield part number 423-A would
be a good choice but I don't know
who stocks it. Mouser has 567-423-K
which doesn’t have holes, so you will
have to drill your own.
FM SHUTDOWN
PROBLEM
QFM translators consisting of
a receiver and a transmitter
on another frequency still
in the FM band require a
transmitter shutdown if the signal
from the receiver fails.
28 January 2009
The problem is that when the
signal fades or drops out below the
receiver capability, the discriminator
output turns to random noise. It
seems that a variation of the night
light control (Oct 08; page 23) would
do the job if I can get a signal from
the AGC or some other signal from
the FM receiver. The transmitter load
is less than 2.0 amps at 120 VAC.
— Bob Ziller
AI don't see how the night
light control circuit applies,
and since an FM receiver
does not have AGC, we
have to look elsewhere. If you have a
decent signal-to-noise ratio ( 6 dB or
better), you can tap off some signal
from an intermediate IF stage,
probably just before the limiter. The
circuit of (Figure 6) shows how
this could work: The capacitor, C1,
"remembers" the signal level, so when
the signal drops out the switch is
flipped. Since FM can still work when
the signal is down at the noise level,
the only alternative is to insert a pilot
tone that can be detected and used
to turn the transmitter off if the tone
is not present.
The LM567 phase lock loop was
made for this application. (Figure 7)
is similar to a circuit I built some years
ago to replace a tuning fork filter that
was used in a business band radio for
squelch control. The signal was 7 Hz
and low level so it was not audible,
but it was enough to be detected.
The oscillator would be placed in the
originating transmitter and added to
the voice at a low level.
HYDRO PROJECT
QI contemplate installing a
low voltage, mini hydro
generator in a small river
in my backyard. The unit
■ FIGURE 7