This is only a preview of the March 1999 issue of Silicon Chip. You can view 34 of the 96 pages in the full issue, including the advertisments. For full access, purchase the issue for $10.00 or subscribe for access to the latest issues. Articles in this series:
Items relevant to "Simple DIY PIC Programmer":
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Items relevant to "Easy-To-Build Audio Compressor":
Items relevant to "Low Distortion Audio Signal Generator; Pt.2":
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pickup on a kart)? Any help would
be highly appreciated. (W. M., Newcastle, NSW).
• The voltage output produced by
the LM2917 is directly proportional
to the product of the resistance and
capacitance at pin 3 so if you want
to obtain 1V at 167Hz, you need to
reduce the resistance or capacitance
by a factor of 75/167. For example,
you could use 47kΩ instead of 100kΩ
and then use a trimpot on the output
for final calibration.
Current drain for
interface card
I bought the “Flexible Interface
Card For PCs” kit from Jaycar electronics and have a question about
it. Could you tell me what current
should it draw on the 5V line? It
seems to be drawing an average of half
an amp and is burning out the power
supply we have. (J. A., via email).
• The current drain from the 5V
rail should be quite modest; no more
than 50 to 100mA at a guess; nothing
like 0.5A. You have a fault there
somewhere.
Current sharing in
amplifier output stage
I have constructed several of
the 125W amplifiers based on the
MJL21193/4 output transistors (April
1996) to use in my home theatre system. All went find for a while but I
am currently having a problem with
two of the channels blowing pairs of
output transistors. I have checked for
dry joints shorts etc and can‘t find
anything obvious. When I power the
amplifier up all goes well until it
blows the fuses, which could be half
an hour later or even a couple of days.
After replacing the transistors
one time I ran the amplifier again,
resetting the bias as described in the
instructions and let it run. A while
later the fuses popped again and
on removing the amplifier from the
heatsink I noticed one each of the
MJL21193 and MJL21194 transistors
were extremely hot (and blown) while
the other pair was quite cool. Is this
thermal runaway?
Could it be the gain of the output
transistors are mismatched? I don‘t
think I am overloading the amplifier
as I have run it very hard into low
impedances with no problems before
with the heatsink getting very hot.
Unfortunately I have no oscilloscope
or any other test equipment other than
a DMM. Can you please help me? (G.
W., Auckland, NZ).
• If one pair of transistors is getting
hot while the other pair is cool, it
suggests that the second pair are not
connected at all. You can verify this
by checking the voltage drops across
the 0.47Ω resistors. They should all
be roughly the same. It sounds to us
as though one pair of transistors is
doing all the work and yes, they are
ultimately suffering from thermal
runaway or just straight-out overload.
Check that the bases of all the transistors are connected to the relevant
points on the circuit. You could possibly have open circuits on the copper
tracks of the PC board.
Confusion with Low
Ohms Tester
I am currently building the Low
Ohms Tester as described in the June
1996 issue. I have checked your Notes
and Errata file, but have not come
across this problem. On the last page
of the above article, under Test &
Calibration, there is an incorrect statement under paragraph two. It states
that pin 2 of IC1 should be at the same
voltage as pin 3. I can understand
how you can draw this conclusion,
if this is an op amp voltage follower
circuit. However it is not, because the
feedback loop is also in parallel with
the RANGE switch S2b.
Only with the S2b disconnected,
can you get identical vol
tages appearing at pins 2 & 3 (the BE emitter
junction of Q1 was bypassed under
test. Otherwise, there is a voltage
differential of 1.2 volts if switch S2b
is left in, let’s say on position 4 (see
schematic diagram). Therefore does
this outcome in anyway affect the
calibration procedure? (P. B., Canterbury, NZ).
• IC1 simply buffers the reference
voltage which is applied to its input
at pin 3. The pin 6 output drives
transistor Q1 so that its emitter, which
is connected to pin 2, is at the same
voltage as pin 3. Thus the statement
in our article that the pin 2 voltage
will be equal to the pin 3 voltage is
correct.
If you are measuring a 1.2V difference between pins 2 & 3, then this
will be due to the lack of a collector
load for Q1. Connect up a low value of
resistance across the Rx terminals and
then check the voltages at pin 2 and
pin 3. Assuming that IC1 is operating
correctly, there will be no problems
with the calibration procedure.
Notes & Errata
Command Control Decoder, May
1998: the circuit on page 62 shows a
100kΩ resistor connected to pin 1 of
IC3 whereas the component overlay on
page 65 shows it as 3.3kΩ. It should
be 100kΩ.
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92 Silicon Chip
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