Sunday, 2 December 2012

The advantage of using 1.2kW resistor on Affray/ Panic Strips

Greetings, 'Cronapress Ken' here presenting -  

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.

 Look out for more Technical Bulletins on the both the Cronapress Blogspot and Cronapress Website.

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