3-23
ohmmeter should be over 500,000 ohms. (A rough rule of thumb for high-voltage capacitors is at least
1000 ohms per volt.) Low-voltage electrolytic capacitors (below 100 volts rating) should indicate
approximately 100,000 ohms.
If the ohmmeter does not deflect when you make the resistance check explained above, you have
found an open-circuit capacitor.
A steady full-scale deflection of the pointer at zero ohms indicates that the capacitor being tested is
shorted.
An indication of a leaky capacitor is a steady reading on the scale somewhere between zero and the
minimum acceptable value. (Be certain this reading is not caused by an in-circuit shunting part.) To be
valid, these capacitor checks should be made with the capacitor completely disconnected from the circuit
in which it operates.
In high-voltage filter capacitor applications, paper and oil-filled capacitors are used in addition to
mica and ceramic capacitors (for low-capacitance values). In this case, polarity is of no importance unless
the capacitor terminals are marked plus or minus. It is, however, good maintenance practice to use the
output polarity of the circuit as a guide, connecting positive to positive, and negative to negative. Thus,
any adverse effects of polarity on circuit tests are minimized and the possibility of damage to components
or to test equipment is eliminated.
The LC Choke-Input Filter
The LC choke-input filter is used primarily in power supplies where good voltage regulation is
important and where the output current is relatively high and subject to varying load conditions. This
filter is used in high-power applications such as those found in radar and communication transmitter
power supplies.
In figure 3-26 you can see that this filter consists of an input inductor or filter-choke (L1) and an
output filter capacitor (C1).
Figure 3-26.—Full-wave rectifier LC choke-input filter.
Inductor L1 is placed at the input to the filter and is in series with the output of the rectifier circuit.
Since the action of an inductor is to oppose any change in current flow, the inductor tends to keep a
constant current flowing to the load throughout the complete cycle of the applied voltage. As a result, the
output voltage never reaches the peak value of the applied voltage; instead, the output voltage
approximates the average value of the rectified input to the filter, as shown in figure 3-27.
