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Notes & Errata
Use Your old PC
Power Supply For
High Current Outputs, December 1998:
the circuit diagram on
page 75 incorrectly
shows the nega
tive
terminal of the bridge
rectifier as being connected to earth. It
should go to negative
side of the bottom 220µF electro
lytic capacitor instead. Fig.1 (above
right) shows the correct circuit
arrangement.
Thermocouple Adaptor for DMMs,
December 1998: the 4.7kΩ resistor
from ZD1 to the 2kΩ trimpot VR2
is incorrectly shown as 47kΩ on the
wiring diagram of page 34. A 15kΩ
resistor has been omitted from the
parts list.
The current limiting and current
limit LED don’t agree too well. Pulling
4A say and winding the current limit
back would result in the current dropping appreciably (say to 2A) before
the current limit LED would light.
The power supply would growl even
worse during current limiting. Aren’t
the main filter caps a bit underrated
voltage wise? My supply measures
52V on the caps. I’m fearful of a bang
– or five! (A. W., Grange, SA).
• An audible squealing from the
power supply is normal when in current overload or when current draw
exceeds the output setting. This was
mentioned on page 57 of the April
1998 issue.
The 470Ω resistor at the input to
The accuracy of the current limit
LED can be adjusted by altering the
bias voltage on pin 10 of IC5b. You
may wish to use a trimpot (100kΩ) in
place of the 220kΩ and 100Ω resistors.
Adjust the trimpot so that the Overcurrent LED lights when the supply
just begins to current limit.
The 50V rating for the filter capacitors is satisfactory.
Troubleshooting
an amplifier
Fig.1: the corrected power supply circuit.
Improvements To AM Broadcast
Band Reception (Vintage Radio),
December 1998: the diagram on
page 67 shows the two twin flex
leads as being joined where they
connect to the antenna loop. This
is incorrect – there should be no
connection between the leads at
this point.
REG1 should be sufficient to reduce
its input voltage to below 35V. This
is because the combined regulator
and IC standby currents will total
more than 35mA to produce a 16.5V
drop from the 50V supply. However,
it will do no harm to add the extra
470Ω resistor in series to reduce the
voltage even further.
You will be able to reduce the output ripple from the power supply by
adding a small amount of capacitance
between pin 1 of IC1 and ground. This
will filter the feedback voltage from
the output of the supply before it is
applied to the pulse width modulation circuitry. Try 100pF or a larger
value until the squealing noise and
excess ripple disappears.
I have built an audio amplifier and
it was working OK but now isn’t. It
uses two MJL21194 and MJL21193
Mosfet transistors and I am afraid
that these might have blown but I
am not sure.
Do you have any extra information
you could send me about these transistors, such as how to test them? I
have checked the voltages that were
given in the instruction manual and
the negative side is fine, but the positive side reads as 0.2V instead of
55.8V! The positive (NPN) transistors
get extremely hot and the PNP transistors stay cold. (S. E., St. Ives, NSW).
• Just a small point, the two transistors you mention are bipolar types,
not Mosfets. You also did not mention
when the circuit was published so we
can’t be too specific in suggestions.
From your description, it appears that
you might have blown the positive
rail fuse.
If you are lucky, this might be all
you have damaged. If the transistors
are damaged they will usually have
a direct short between collector and
emitter and you can check this with
your multimeter (switch to a low
ohms range).
You should check the other transistors in the circuit to make sure that
SC
they have not blown too.
WARNING!
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be considered dangerous or even lethal if not used safely. Readers are warned that high voltage wiring should be carried out according to
the instructions in the articles. When working on these projects use extreme care to ensure that you do not accidentally come into contact
with mains AC voltages or high voltage DC. If you are not confident about working with projects employing mains voltages or other high
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92 Silicon Chip
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