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Usage of Resistors |
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Resistors to limit the rush current |
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A typical power supply circuit is shown below.
Rush current flows to charge the capacitor having a large value, when the power
is turned on. A resistor is used to eliminate the rush current. This resistor
should withstand the rush current of several ten's of Amperes and neither emit
smoke nor catch fire even when the switching transistor breaks down to be subjected
to a larger current. From these standpoints, wire-wound ceramic cement resistors BGR and
BWR
or metal-plate ceramic cement resistors BPR are
recommended. Wire-wound with a built-in fuse ceramic resistors WF are
also recommended to protect from continuous over-current. |
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Power type Resistors (Wire-wound type and Metal-oxide
Film type) |
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Power type Resistors include wire-wound: CW,
and metal-oxide film: MOS MOSX
MO and MOX. Wire-wound
types are stable and have superior pulse resistance characteristic. The high-temperature
capability of resistance wire alloys provides a practical advantage except for
high frequency applications. The wire-wound structure has considerable inductance
and parasitic capacitance to cause resonance, to add to the large physical size.
Usage of wire-wound type resistors is more frequent in heavy-duty equipment than
electronics. A good deal of thought should be given to use this type of resistors
for high frequency applications. Metal-oxide type resistors are physically small
sized, excellent in frequency characteristics, and the resistance value can be
high. But for pulse resistance characteristic, when compared to wire-wound type
resistors, they are inferior. To compare the possible resistance range, wire-wound
types are rather low between several ten's of  m and several k while that of metal-oxide
types covers from several ten's of to several hundred of k. In the line-up of
ceramic cement type resistors, both wire-wound resistors BGR BWR
and metal-oxide resistors BSR are lined
up. |
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Resistors to absorb the surge current -1 (snubber
circuit) |
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A typical switching power supply circuit is
shown below. The transformer's secondary circuit has diodes to rectify. But
these diodes often produce surge due to steep switching current waveforms.
This surge may give stress to IC, so that CR circuits are used as a means of
absorbing surge energy. Note that there exists a trade-off between surge absorption
effect and energy loss, and therefore it is important to find a resistor with
suitable resistance value. Furthermore, be careful that the resistor should
neither emit smoke nor catch fire in an accident when the capacitor is short-circuited.
Metal-oxide film resistors MO and MOS
are recommended for this purpose, as they are noncombustible and compact sized. |
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Discharge Path Resistors for safety standard |
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Audio and/or video apparatus, having an easily
accessible antenna or input/output terminals, are under obligation in the US
to add a static charge drain. (UL1410: UL Standard for Safety for Television
Receivers and High Voltage Video Products, UL1492: UL Standard for Safety for
Audio - video Products and Accessories). The discharge path resistor should
be a conductive element connected to the power supply circuit. Generally, for
equipment utilizing insulating transformers, the ground of the primary circuit
and that of the secondary circuit are connected to a discharge path resistor
whose resistance should not exceed 12 M. Such a resistor should be filed in
UL1676. (The standard for Conductive-Path and Discharge-Path Resistors for
Use in Radio-, Video-, or Television-type Appliances). Another requirement
to regulate the lower limit of the discharge path resistor is a leakage current,
which should not exceed 0.5mA. Therefore, resistance value 1 - 4M is generally
used. Furthermore, this resistor shall withstand the voltage demanded for primary
to secondary insulation (1,000V, 1 minute). The resistor type RCR50+
is recommended for this purpose.
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Discharge Path Resistors for antenna circuit |
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A tuner of CATV or STB(set top box) that has
multiple cable inputs, has a control switch to change the input. As DC voltage
is supplied for this switch, static electricity of cable side should be drained
to the secondary circuit common through a resistor. The resistor type RCR
is recommended. |
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Resistors to absorb the surge current -2 ( CRT
monitor) |
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Discharge may take place inside the CRT used
for TV and monitor. Video amplifier IC connected to the cathode of the CRT
may sometimes break down due to such a discharge. The resistor, added between
the cathode and the IC, has an effect to prevent failures by absorbing the
high voltage discharge energy. Surge absorbing resistor, such as RCR is
recommended. Also, besides the usage in the cathode electrode circuit, it can
be used to prevent destruction of circuit connected to G1 and G2 electrodes.
It is desirable to make the resistance value of a resistor big enough to decrease
the influences of surge. But large resistance value influences the frequency
characteristic of the CRT to worsen the picture quality. If the Surge Absorber SA10 is
placed between cathode and ground, it is good for such an occasion. |
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Application Notes
Brush up your circuit
Basic Knowledge
