A Safe and Quick Continuity Tester for Pyrotechnic Circuits
The BRIDGESAFE® continuity tester provides an ultra low test stimulus below 5mV (4mV typ.) with a short circuit current less than 500uA. Despite the exceptionally low output, the tester can distinguish four resistance bands which are particularly suited to the SFX / Fireworks industries (which frequently make use of sensitive igniters with ~1Ω bridgewire and an ‘All-fire’ current ~1A). Circuit resistances above 20Ω are rarely encountered; low voltage firing systems capable of suplying 24V would have little margin to drive the ~1A sufficient to fire. Although a multitude of system voltages, igniter sensitivities, and wiring configurations exist, leading to no perfect choice of indication ranges to suit all users, the following five ranges have been selected for geatest overall usability.
- < 1Ω Suspiciously low: further investigation needed, or there may be a short circuit.
- 1Ω to 10Ω: the typical range for a single igniter circuit, or up to 3 igniters in series.
- 10Ω to 20Ω: a plausable resitance range for a series circuit, up to the maximum resistance for 24V firing.
- 20Ω to 200Ω Suspiciously high: further investigation needed to confirm if the circuit can be fired.
- > 200Ω shows no continuity indication.
Testing pyrotechnic circuits for continuity demands careful engineering to produce reliable results with a sufficiently low voltage and current. It is an unfortunate fact that eventually all equipment will at some point develop a fault. In order to design tools such as BRIDGESAFE® to ‘fail-safe’, the design must fulfil two criteria. The failure of any given component in any of it’s possible failure modes must not result in a hazarderous state. To confirm this a ‘Failure Mode and Effects Analysis (FMEA)’ is performed to systematically verify the behaviour under a failure. Furthermore and most crucially the failure when it occurs must be evident to the user or be caught by some diagnostic action (either automatic or by the user). IF a failure is not evident, the user will continue to use the equipent until another failure occurs.
A classic exampe of this is seen in some testers currently on the market which use a pair of resistors in series to limit current in a simple cell +