Image Processing Reference
In-Depth Information
to ensure that the available energy within the device is below the minimum ignition energy of the
potentially explosive atmosphere.
A practical implication of this is that you need to limit the maximum amount of energy supplied by
the power supply, as well as dividing your design into clearly isolated blocks, and limiting the energy
transfer between these blocks. he interfaces between the system blocks thus constitute the majority
of the design work and also have the most dramatic impact on other design aspects.
Additional components are typically required to limit discharge energy or isolate blocks, e.g.,
resistors, fuses, and Zener diodes. These additional components tend to increase the overall power
consumption of the device as you will experience losses when traversing through the different blocks
of your design.
In addition to providing means for limiting the energy, you also need to provide some guarantees
that these protection and power limiting components will not fail. When designing an IS circuit, all
possible scenarios for connections of components need to be considered when calculating the safety
circuit parameters. Defining clear interfaces helps determine worst case failure mode and devise pos-
sible solutions to handle these failures. Typically duplication or triplication of components is required,
as well as using the components below their specified ratings and observing sufficient creepage and
clearance distances. This further lowers the efficiency of the device and can impact the form factor
of the device and require additional redesigns.
Since a component with a high surface temperature could cause an ignition of the gas, the maxi-
mum temperature of any component on a circuit board under fault conditions must be considered.
Limiting the energy alone does not automatically result in a low component surface temperature,
especially when considering small components. This means that you need to take great care when
selecting components and ensure that the heat dissipation is satisfactory to avoid an excessive heat
buildup. Usually, this is solved by choosing sufficiently large components, which again can impact
theoverallformfactoroftheelectronicsandleadtospaceproblems.
Wireless Vibration Monitoring Case Example
A battery with IS approval was selected. he battery is connected to the PCB of the device via an
infallible current limiting resistor. his will ensure that the maximum power loss in any compo-
nent on the PCB is limited. he temperature rise can then be calculated for each component on
the PCB.
In addition, the resistor was carefully chosen with a specified temperature rise that will ensure
that the maximum power dissipation in the resistor upon a short will not result in an unacceptable
temperature rise in the resistor.
Since the battery voltage level will vary with the surrounding temperature, we might not have
enough energy to operate the radio circuit under all conditions (especially during start-up). To
solve this, a capacitor bank was connected across the power supply. he capacitor bank stores just
enough energy required for the radio transmission. he capacitor bank was carefully designed to
comply with relevant safety standards (Figure .).
27.7.3 Other Environmental Considerations
In addition to the IP and EX environments described above, most industrial environments are also
subject to vibration and extreme temperatures.
In an environment where the device is subject to severe continuous or occasional vibrations
(shock), there is a risk that internal components will shake loose due to these vibrations. herefore,
as a protection, a device can be filled with a shock absorbing material. Drawbacks with this solution
are that the filling materials usually has a high cost, and can force a design solution where the battery
is also encased in the material, which make battery changes impossible.
