Technical and R&D Dept. - Bulletin No1
Why should a press strip profile use a 1.2kW resistor rather
than a 10kW?
Although terminating the profile with 1.2kW increases the current and power drawn from the supply, there is an advantage gained if the system is used in the real world where interference is present. If the connecting cable between the profile and the sensing electronics is in ducting where other electrical cables are present then the proximity of these can generate interference signals. Cables carrying high frequencies can be troublesome as can cables carrying power at mains frequency when switching transients occur. Radio interference is also another source of interference.
In all cases, magnetic and electric screening will reduce the
interference but, unfortunately, it may not be be possible and certainly will
be prohibitively expensive. So what can be done about it?
The effect of interference can be represented in simple terms by a
voltage source and an impedance representing the coupling between source and
profile. This impedance will usually be quite high compared with the value of
the terminating resistor of the profile e.g. stray capacitance between cable
and profile. The interference appearing across the profile is thus determined
by the ratio of the terminating resistor to this impedance. Thus the worst case
would be with the profile open circuited. But using a 1.2kW resistor rather
than a 10kW resistor will thus give a 1.2/10 reduction in
interference and hence a significant benefit. It should be remembered that it
will not eliminate it and additional protection may be needed in adverse
environments. Hardware and software filtering can be effective in screening
out false triggering from interference
signals.
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