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tion, high resistance values are inherently
less valuable in wirewound construction
due to the fine wire involved. Occasionally, other issues can also evolve, such
as potential circuit interference caused
by inductively wound wire. However,
although there are some difficulties
surrounding high resistance values,
wirewound resistors often offer easier
handling of high current pulse than thick
film on alumina resistors.
High Voltage and
High Current Pulse Handling
High-stress medical devices like defibrillators
and their accompanying monitoring equipment need to withstand both high pulse
and high voltage in the course of operation.
The defibrillator itself endures a great deal
of electrical strain when administering shock
to the patients, with some modern machines
capable of putting out nearly 2,000 V. (While
this capacity is rarely exercised on patients,
it is relevant for designers.) In addition, the
monitoring equipment faces its own voltage
and current challenges. These machines must
absorb any pulse before it travels into the
reading system, or the strong current will
burn up the system.
Pulses in medical equipment can vary
greatly from manufacturer to manufacturer.
For example, defibrillators can experience
current pulses as low as 20 J or as large as
300 J. For this reason, pulse is of varying
concern for different applications. When
designing for demanding circumstances, the
choice between wirewound and alumina
can end up seeming like a simple trade-off.
In general, wirewound is more naturally
suited to handle high current pulse while
thick film is better equipped to endure
high voltage. The question then remains,
“Which is easier to modify?”
Wirewound resistors can handle high
current pulses so well because of the way
they are constructed. The wire wrapped
around the core, though fine and thin,
provides a great deal of surface area to
absorb the pulse. When the current jumps,
the lengths of wrapped wire can handle the
Thick film resistors are not as ideally
suited for current pulse, because they are
smaller, with less overall surface area to
work from. To equip an alumina substrate
resistor for high current pulse, electrical
designers cannot make the resistive deposit
much thicker. Instead, they need to make it
very wide in order to increase surface area.
The more current that must pass through,
the wider the square will get.
In contrast, achieving high voltage handling in wirewound resistors can become
difficult relatively quickly. For voltages of up
to 150 V, it is easy and inexpensive enough
Ohmite’s new TP Series can handle up to 100
joules of energy in a 1% tolerance, feature
higher pulse currents than standard parts, and
were designed with more area for the current
to flow through.