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Micropower and Intrinsic Safety

The design of micropower electronic circuits has become much easier in recent years due to the introduction of new integrated circuits targeted at this market. There is now a wide range of analogue and digital ICs that can be powered from a pair of alkaline cells or a lithium ion rechargeable battery. The Sevensoft parametric opamp chooser is a useful resource when selecting micropower opamps. When a system draw less than 100uA, battery life can be measured in years . Micropower circuits also make it possible to develop products that run on a small solar cell or some other off-grid power source. There are a lot of other techniques that reduce system power consumption, such as the use of sleep modes and underclocking, which means that a system wide approach needs to be adopted.

Intrinsic safety is a solution to the problem of using electronic systems in a hazardous environment, where flammable or explosive gas or dust may be present. The fundamental principle of the intrinsic safety (IS) concept is to limit power, stored energy and maximum temperatures to levels that are known to be less than those that can ignite the gas hazard. The limits for a product that is certified to be IS are set in the relevant standard, and vary according to the hazard. Different levels of protection are applicable according to the probability of exposure to the hazard and the consequences of a failure. Much of the task of IS design is good engineering practice, with attention paid to thermal management and using components within their design limits. Intrinsically Safe products usually need approval by a certification body, and Sevensoft has experience in this field.

The IS design challenge is tougher than micropower design because the limits on power and stored energy have no discretion at all. Many limits in engineering have some amount of flexibility, but a circuit that asks for more power than there is available will invariably fall over. Other difficulties that can catch the unwary are the unexpected voltage sources. Sounders for example will generate an EMF or a piezovoltage when exposed to load noises or knocks. Switching regulators are hard to certify as it is difficult to guarantee the maximum stored energy in the inductor and capacitor.

Portable IS equipment will usually be battery powered, which may be from alkaline, lithium ion or even lead acid batteries. The maximum cell voltage for each type of cell is set in the relevant standards, and equipment is generally not designed to be recharged in a hazardous area. Due to the previously mentioned difficulties with switching regulators, they are rarely used and this makes it difficult to extract the maximum charge from the batteries.

Cable powered IS devices may require careful power management to ensure that they do not try to draw more power than the energy limiting barrier permits. This is not a typical micropower problem, as the power levels are much higher, but the discipline is much the same, and micropower ICs are a popular choice. The widespread use of the 4-20mA interface between sensor and control panel means that there is a large section of products that are powered from 10V to 30V with a current limit of 2mA. Squeezing the current under this limit is attractive to customers who can then use two wire cables as opposed to three wire cables. With cable lengths sometimes running to kilometers, the cost saving can be significant.

An energy limiting barrier is placed at the boundary between safe and hazardous areas, with segregation between the two sides. Fuses are used to limit current and zener diodes or crowbar circuits are used to clamp voltages. If there is a substantial length of cable on the hazardous side of the barrier, then its inductance and capacitance must be taken into account as they store energy