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Current Fuses and Fusing Resistors |
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Current Fuses and Fusing Resistors |
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Current Fuses are one of the simplest devices
to protect a circuit. The resistors with fusing function protect the equipment
or parts from burning by breaking the abnormal current in an accident such
as short-circuiting.
TF ,
CCP , and CCF are
lined up.
Fusing Resistors also have the fusing function. It works to fuse when more than
10 times of rated power is dissipated. It is non-combustible with UL94(V-0) approved
coating. It is suitable where a failure of any other device might cause smoke
or fire. In general, blow speed is slower than that of current fuse, it will
not be recommended for instantaneous fusing requirement. RF73 RF
RF26 and
WF are lined up. |
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Current Fuses |
Fusing Resistors |
| Normal function |
Zero  current path |
Specified value resistor |
| Overload |
Blow |
Blow |
| Cause of blow |
Over current |
Over power dissipation |
| Fusing Characteristics |
Instantaneous fusing
Precise fusing |
Slow blow
Wide scatter in fusing |
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Current Fuses, where to use |
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One main fuse is enough only when any local
failure of power circuit can blow that fuse. But it is often the case for today's
equipment that the power supply is divided into several independent blocks.
A failure, severe enough to cause a danger in the local block, might not always
be detected at the main fuse. Circuits shall be tested to confirm that neither
emission of smoke nor catching fire is caused by short-circuiting the local
power supply, for instance. Otherwise, fuses or some protective devices should
be included in each branch or block. |
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Diagram of CCP structure |
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Diagram of CCF structure |
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Fusing Resistors, where to use |
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In the IC circuit recommended, a resistor
is inserted in to the power supply Vcc in series, it works as a current limiter,
and at the same time as a decoupling device with noise together with the capacitor.
But in case of failure, when the IC or the capacitor is short circuited, depending
on the supply voltage and resistance value, the resistor might emit smoke or
catch fire without operating any protector circuit. Fusing Resistors RF
RF73 are suited to break the current
without burning. |
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Resistors to suppress parasitic oscillation of
emitter follower circuit |
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Emitter follower circuits are often used to
realize low output impedance amplifiers. But they can oscillate parasitically
unless proper attention is paid. When this phenomenon is overlooked, unexpected
trouble may be caused, e.g. EMI. Moreover, it is difficult to check oscillation
because the condition is so delicate as to be changed by touching with a probe
of an oscilloscope. Still more, oscillation can be stopped even with changing
temperatures. An effective solution for this kind of oscillation will be to
decouple the collector from the Vcc and to put a resistor Rb, ranging from
several 10 to several 100, to the base circuit as illustrated. When the emitter
load is capacitive, inserting a low resistance resistor into the load in series
also has an effect.
For power amplifiers, this series resistance becomes very small. And it might
bring about unexpected heating or going up in smoke depending on the transistor
failure mode. In such situations, Fusing Resistors RF
or RF73
which has a fuse function is recommended. Rush current should be taken into
consideration
together when choosing Fusing Resistors. |
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Resistors to stabilize FET switching operation |
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MOSFET is popular high frequency switching
device. A gate resistor of a low value has an effect of stabilizing switching
operation. If resistance is too low, it may give instability due to chattering
at transistor "on" and "off" transient. On the other hand,
if gate resistance is too high, it makes the waveform dull due to Miller effect.
Therefore the suitable value should be determined by observing the waveform.
This consideration is especially important for power MOS devices, because the
gate resistance may have correlation to surge voltage and switching energy
loss.
For power MOSFET, this gate resistance becomes very small. And it brings unexpected
heat and emission of smoke due to the transistor failure mode. In such situations,
Fusing Resistors RF or RF73
which has a fuse function is recommended. Rush current should be taken into
account
together when choosing Fusing Resistors. |
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Application Notes
Brush up your circuit
Basic Knowledge
