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