0.1uF Adj HT- 32
200V 50k Diac
Dance to the
Q. The capacitance
meter circuit in your
July 2003 article is something I'm very much interested in. However, I don't
understand how to determine the value of the
capacitor being checked.
you use 5% resistors, then expect to
have as much as a 5% error when you
switch ranges. Using 1% resistors
improves the accuracy.
removed, the switch closes, shorting
out the diode, and applies full power
to the soldering iron.
If you're looking for a variable
temperature controller for the soldering iron and want to keep it simple,
check out the circuit in Figure 4. This
is your run-of-the-mill triac dimmer
that's been reduced to the minimum
number of parts. The temperature is
controlled by varying the phase angle
of the sine wave, creating a "dim-ming" effect just like in a light bulb.
The limitation is that you can only
guess at the temperature (or measure
it and calibrate the dial) because
there is no temperature sensor to
control a feedback loop, as would be
the case with a true thermostatic
A. All test equipment, whether
homemade or commercial, has
to be calibrated. In the capacitance
meter described in the July column
("Simple Capacitance Meter"), this is
done by inserting a 1.0 uF capacitor
across Cx, and adjusting the 5 k pot
until the 0-1 mA meter reads full scale
with the selector switch in the S2 position. The scale is linear, which means
that a 0.5 uF cap will read mid-way on
the scale, or 0.5 mA. A 0.1 uF cap will
read 0.1 mA, and so forth.
The accuracy of the meter
changes as you switch from one
range to another and depends on the
accuracy of the timing resistor, R. If
Q. I have a question about the
meaning of parts specifications.
Let's say you buy 36-volt zener diodes
at 5% tolerance. Does it mean that
three-sigma from 36 volts is 5% and
that all the parts bought will fall within that tolerance? I'm looking to see
what the sigma is so as to properly
tune the design.
A. Electronic components are
rated on a fixed deviation of plus
and minus a fixed value, not three-sigma (which is a mathematical deviation curve). For example, a 36-volt
zener diode with a 5% tolerance
means that the voltage of the zener is
guaranteed to be ±1.8 volts of the
standard value; in your example, the
zener voltage is in the range of 34.2 to
37. 8 volts. The actual values tend to
cluster by the batch run, with one
batch having a peak around one
value and another batch centering
around a different value. Temperature
also affects the voltage of a zener,
which is why the tolerance is always
rated at a specific temperature.
Typical temperature ranges are 0° C
to + 70° C (commercial) and - 55° C to
+125° C (military).
Tolerance is not an indication of
poor manufacturing. Closer tolerances can be achieved, but at greater
expense. A zener (or resistor) with a
10% percent tolerance costs less to
produce than one with a 5% tolerance.
NUTS & VOLTS
Power Supply Design
Q. Is there a formula to determine
the size of the output filter capacitor in a DC power supply? I find
many different sizes in different DC
power sources, and would like to