Certain irreversible operations (like releasing a parachute, cutting a rope, or starting a solid combustible missile engine) may be performed by pyrotechnic igniters. These devices generally consist of an electrically controlled igniter, some chemical (such as gunpowder) to produce high-pressure gas after firing, and some mechanical assembly to perform the desired operation.
Igniters contain a filament in an explosive mixture. When a current flows through the filament, its temperature increases until reaching the ignition temperature of the flammable chemical, thereby igniting the device.
Igniter characteristics are generally described in terms of filament resistance (e.g., 1O) and energy (some level of millijoules) to supply in a maximum time period (i.e., milliseconds) to start the device. Another parameter usually specified is the maximum current you can safely supply to the device for measurement purposes without igniting it. Operating such a device with an MCU port requires some important considerations:
Avoid unwanted operations (i.e., some unpredictable port status during the startup that may trigger the device).
You should be able to check the device status using the same port.
Implement the requirements in terms of current (high), energy, and device timing without disturbing the electronics.
Figure 1 shows the block diagram of the fire-starter interface. The current through the filament is controlled by a power MOSFET (Q1). A charge pump connected to the MCU port through C1 operates Q1's gate. This will avoid:
An unwanted port level operating Q1 (dc is decoupled by C1).
A single spike operating Q1 (several transitions are required, depending on the charge-pump features, to increase the gate level).
The strong current through the filament affecting the power supply (C2 is chosen to supply the right amount of energy to the filament, and it's slowly charged by R2).
Figure 2 shows the schematic of a working circuit that implements the requirements in the following manner. A charge pump drives the power MOSFET (M1). Several transitions are needed on the MCU port to cause M1 to saturate. As a result, the system is insensitive to spikes and unwanted level changes.
By looking at the filament resistance, you can detect the igniter status. R4 and R5 accomplish this by bringing the MCU port (used as an input) near to +5 V with an intact filament and to 0 V with a burnt filament.
M1 discharges capacitor C6 through the filament, supplying the required amount of energy. The igniters used (Daveyfire N28BR) require a minimum of 2 ms at 1 A to ignite (or 1.1 mJ/O). The microprocessor port is supposed to be an input when it's not sending oscillations. A high-level signal is returned before the burst (igniter new) and a low-level signal after the igniter is burnt. The circuit well exceeds the minimum requirements in terms of current and energy/resistance needed to ignite the Daveyfire igniter.
Dear editors: I dislike the design, "Drive Pyrotechnic Igniters…" A circuit that keeps one side of a pyrotechnic squib permanently connected to an energy source -- capacitor or battery -- provides a recipe for disaster. An accidental grounding of the squib's other contact will cause ignition! This type of accident could occur during connection of the squib, during an accidental severing of a cable, and so on. An ignition circuit should provide a high-side switch that supplies energy to the squib to fire it. In addition, the circuits shown lack any sort of safety interlock -- such as a temporary shorting shunt across the squib -- a must in this type of a system.
Jon Titus
<b>The author's reply:</b>
When writing about a section of a system you must neglect some parts, like safety jumpers nearby the igniters, which I did not consider as a part of the circuit, or the power switch. I intended to communicate the principle of operation to an experienced readership, so anyone could modify the circuit to match actual requirements (i.e., adding a transistor to switch the high side of the igniter). This circuit has been designed to release ballast on small, disposable, stratospheric balloons, and a board containing eight drivers exactly like the one in the magazine worked successfully for more than 30 days during the prototype experimental flight last summer. This design was meant to be light and simple. After checking the system with an igniter simulator, the system is shut off, igniters are connected, and the system is turned on again. Accidental shorts are highly improbable, but just in case, the current flowing through R6 after the power-on (C6 is not charged) is below the maximum non-ignition current, and can not operate the igniter. This condition is detected by the controller, which reads the erroneous information, "igniter operated," and stops the launch procedure.
Giovanni Romeo
<b>Editor's note:</b>
Both the correspondent and the author are correct. Obviously, there is more to a full pyrotechnic igniter system than just the circuit shown in the Design Brief. Anyone who would like to use this circuit should make sure he or she knows and follows all recognized safety procedures for such devices.
Jon Titus -December 21, 2004
Your Comments:
Enter the text from the image below
Please refresh the page if you have trouble reading this text.
Search Electronic Design
Email Newsletter
Sponsored By:
Electronic Design UPDATE provides readers with late-breaking news, opinions from industry experts, and timely technology stories. It's a unique opportunity to get your product message in front of engineers, engineering managers, and corporate managers while they're reading about critical information online.
Reprints
