Generally speaking, circuit protection is often concerned with ensuring excessive energy sources cannot damage or impede the operation of a circuit. But with low-energy solutions becoming highly popular, could such extra sources of energy be redirected for alternative purposes?
Low energy solutions
With the ever-increasing influence of technology coupled with the falling cost of electronics, it would come as no surprise that by the end of the decade we could very well see every aspect of life being processed by some circuit. The rise of the internet of things (IoT) is seeing the gathering of untold amounts of sensory data, while the reduction of transistor sizes is seeing incredibly powerful processors being fabricated on silicon dies no bigger than a grain of rice.
Indeed, this abundance of technology has designers wanting to push the boundaries of where electronics can operate, which has led to electronics being implemented in harsh remote environments such as the oceans, deserts, and space. When designing for such environments, designers must understand that not only does the circuit have to be able to operate in those environments (such as chemical damage, heat, and radiation), it also requires a power source.
Power sources can vary greatly, with each having their own advantages and disadvantages, including reliability, storage capability, and practicality. The reliance of a power source can be minimized by reducing power consumption with the use of sleep cycles but even then, a power source is still required.
One technology that looks promising is energy harvesting. This solution tasks a circuit with absorbing energy from its environment, storing this energy, and then converting the energy into a more useable format. Such sources include solar, wind, and vibration. But could there be an alternative energy source that is actually being overlooked here?
Circuit protection purpose
As stated earlier, the fundamental purpose of circuit protection is to divert harmful energy sources away from sensitive circuits that are easily damaged. This diversion can be achieved with a wide range of techniques, including clamping diodes to prevent voltages exceeding the circuit’s threshold or PTC resettable fuses that prevent large amounts of current flowing through a circuit.
Most modern circuit protection techniques involve diverting or dissipating the excess energy, whether it be from a static source (such as people) or from unexpected surges (like that which would come from an electrical distribution network). If the purpose of circuit protection is to prevent harmful energies from damaging components, then is it possible for this energy to be stored instead of dissipated? How could this be realized in a design and what applications would benefit from such a system?
Energy harvesting and circuit protection
Typical energy harvesting techniques involve an energy storage element (such as a capacitor) directly connected to their energy source with minimal circuitry in between. For example, a solar cell may be connected to a capacitor which, in turn, is connected to a DC/DC converter. When the voltage across the capacitor reaches a specific threshold, the DC/DC converter can step the voltage up and then power the main circuitry. This could be anything from a simple light beacon to an IoT sensor.
Converting energy from harmful sources can be problematic, though, as the source of the energy has to be diverted away from an electrical path as opposed to coming from a dedicated electrical path. To better understand this design problem, let’s take a look at two different scenarios; an ESD source and an inductive element.
ESD sources
Sources of electrostatic discharge are often high voltages for very brief amounts of time. Common examples of everyday ESD sources include shopping carts on laminate floors and clothing rubbing against skin. In both of these examples, the voltages produced can be as high as 10 KV, and they can cause a person to jump.
Since the length of the static shock is often in the millisecond range, the total energy transferred is very small, which is why these sources are not harmful to people. What’s more, many electrical circuits are now based on CMOS technology, which involves incredibly thin gates. These gates are highly susceptible to dielectric breakdown, which is why static shocks can very easily damage them (hence the need for anti-static packaging etc.). Protecting from such sources often involves the use of zener diodes, which clamp voltages that go beyond a specified range (such as 5.1 V for 5 V logic circuits).
Diverting energy from an ESD source would be difficult as the circuit would have to be able to respond quickly using minimal amounts power in doing so. Therefore, such a method would need to rely on analog circuitry (i.e. no active microcontrollers or digital logic) with one potential arrangement being the use of thyristors. Voltages above the desired level (such as 5.1 V) could cause a diode configuration to electrically isolate the main circuit from the ESD source and then electrically connect a power storage element (such as a super capacitor). Components that may be able to achieve this would be those based on PN junctions such as diodes, SCRs, and thyristors.
The challenge in this circuit design would be to ensure that the circuit is electrically isolated from the ESD source at the same time it is transferring the energy into a storage element instead of dissipating it as heat across a diode. This method for energy harvesting would be highly beneficial in wearable applications, whereby movement which generates vibration and mechanical energy also generates static electricity. ESD energy storage would not be practical in remote environments that are rarely handled (e.g. monitoring stations).
Inductive sources
Inductive elements can be highly problematic for circuit designers when it comes to circuit protection due to their back EMF behavior. Inductors are essentially a form of electromagnet, wherein as current flows through them, a magnetic field is produced. If the current through an inductor remains constant then the magnetic field generated is also constant. On the other hand, if the current flow changes then the resultant magnitude of the magnetic field changes. This changing magnetic field, if you will, induces a voltage in the inductor whose polarity is opposite to the changing current. This resistance to change is useful in filter circuitry whereby sudden changes in current (such as a surge) can be resisted and therefore prevent damage to circuitry down the path. It is worth noting, though, that inductors can be a potential source of circuit damage especially in situations involving switching circuitry. Relay coils are a common example of where back EMF can damage circuitry which could otherwise be protected against with a reverse polarity diode. Turning relay coils on does not result in a voltage spike due to the only source of voltage being the power supply (when a current spike is observed). When a relay coil is de-energized, the collapsing magnetic field results in a very large back EMF, often measured in many hundreds of volts. This issue is resolved with the use of a flyback diode, which essentially shorts the relay coil and prevents the large back EMF from getting to sensitive switching circuitry such as transistors.
The back EMF from an inductor could be stored into an energy harvesting circuit but doing so would have similar challenges to storing ESD energy. The brief pulse of energy would have to be handled by circuitry that is not reliant on an external power source or processing system. This could be achieved with a zener diode arrangement which, when activated, electrically isolates the sensitive control circuitry from the inductor. A capacitor bank would then be able to store the back EMF and the energy repurposed later. This method for energy harvesting while protecting circuits simultaneously could be implemented in low power home automation devices such as door locks, which are required to control solenoids but are only used briefly. The resulting pulse of energy from the solenoid can then be used to drive a wireless module for submitting information about the house entry to the cloud.
Conclusion
For the longest time, electronics have been built into products that have some kind of reliable power source, whether it be a battery or a mains supply. With the desire to mount electronics into all kinds of locations and the need for more energy efficient systems, energy harvesting is becoming an ever more popular sector of the industry.
As the energy requirements for electronics continues to reduce, the useable energy from small sources such as ESD and induced voltage increases. Will the next smart health sensors be ESD powered? Can door locks be made to last years on batteries? Will circuit protection move into energy storage? Time will tell.
