By Ian Darney
Circuit modelling can be used to create designs which ensure the safety of the system under review when it exposed to electromagnetic interference (EMI). The process involved in designing a firing circuit which can withstand high levels of EMI is described in detail.
An electro-explosive device (EED) can be triggered by connecting it to a voltage source via length of twin-conductor cable. A Control Unit containing a battery and a switch provides the necessary power.
The susceptibility of such a system to EMI is analysed using a circuit model. If the interconnecting cable is 15 metres long and exposed to a field of 10 volts/metre, then that cable will pick up more than enough energy to ignite the device even when the switch is open.
By terminating each end of the cable with a resistor equal in value to its characteristic resistance, the power delivered to the cable will be absorbed. By inserting a small value inductor in series with the EED, it can be guaranteed that there is a significant margin of safety between the no-fire-threshold (NFT) and the power delivered to the device.