Energy harvesting is the art of extracting useful power from energy sources normally deemed too weak, too dilute, or too impractical to be usable. I have already presented an example of such a “harvester” (“Trick A BJT-Based Converter Into Starting At Only 250 mV DC,” http://electronicdesign.com, ED Online 19152). This time, I will deal with the other extreme—the circuit presented here can convert voltages in the 1- to 30-kV range into “normal” electronic voltages, at power levels ranging from milliwatts to watts.
This presents interesting design challenges regarding the choice of the architecture and active element, and the method to get the circuit started. It’s obvious that none of the traditional solutions can be adapted here. Transistors don’t go much higher than 2 kV, valves require heating power, and multilevel thyristor systems are suitable only for big industrial or railway applications.
The active element selected for this design is a very old classic: a spark-gap. How can this icon of 19th century technology be put to good use in a modern circuit?
Let’s forget any preconception and examine the facts about spark-gaps: • Their voltage can be readily modified by changing the spacing of the electrodes. • They require no standby power, are self-triggering (when the breakdown voltage is exceeded), and self-extinguishing (when the current falls below the holding current). • They have good efficiency—the off-state current is negligible, and the on-state voltage could be anywhere between 100 and 500 V. This may look large, but with a supply voltage 20 or 40 times larger, this is negligible. • They are robust (no electrostatic-discharge precautions required). • They are inexpensive.
The properties of the spark-gap make it an ideal candidate for a relaxation oscillator (Fig. 1). The high-voltage source is symbolized by a current generator. This approximates most practical highvoltage generators, which have a large internal resistance.
The role of R1 is to isolate the oscillator from the external world. It minimizes the egress of interference, and avoids the generation of high peak currents in the cabling that may arise if the power source is capacitive.
The input current charges C1 until the spark-gap reaches its breakdown and discharges it into the primary of the high-voltage transformer. The resulting pulse is transferred to the secondary, and then to the storage capacitor C2, via a Schottky diode.
An additional element, the leakage inductance of the transformer (not visible on the schematic), has a very important role. This inductance spreads the current pulse in time, and reduces its peak value to levels acceptable to the diode and the capacitor. This is excellent news, because it also simplifies the transformer’s design. Even with this inductance, the peak currents are rather hefty for such a low-power circuit, typically between 10 and 100 A. This is why such a large diode is used, together with an organic semiconductor (OSCON) cap. The slightest ohmic losses have a dramatic impact on the efficiency.
Here, only one diode is used, because the input polarity is assumed to be known and constant. If this isn’t the case, a full-wave rectifier must be added, either at the primary or at the secondary, depending on the nature of the power source. If it has a low frequency and a current high enough to charge the oscillator’s capacitor in one cycle, the bridge can be placed at the secondary. If the current has a higher frequency and requires a number of cycles to charge the capacitor, a high-voltage bridge must be used upstream from the converter.
To maximize power and efficiency, the output must be adapted, i.e., the loaded voltage must be half of the open-circuit voltage. In fact, it should be a little less, to account for the losses, imperfect coupling, etc. In this case, the transformer ratio is 1/100 and the spark-gap voltage is adjusted to 4 kV, yielding a theoretical 20-V output. The actual optimum is reached at 17.5 V.
The capacitor must not only be able to withstand the raw input voltage, but also the pulsed mode of operation. General-purpose film or ceramic multilayer caps are unable to withstand repetitive pulse currents, and their metallization will fail rapidly at the junction with the terminals.
Traditional disk capacitors have a much more robust electrode construction, and are thus suitable. They tend to have rather high dielectric losses, due to the nature of their material (typically Y5U or something similar), but considering their low cost, the slight reduction in efficiency is acceptable.
The transformer was based on a RM6 core, made of generalpurpose 3B ferrite material. Good high-voltage practices have to be adhered to for the primary winding. The voltage limits of 1 to 30 kV are only indicative: Below 1 kV, the spark-gap has to be adjusted with micometric accuracy; over 30 kV, the problems caused by corona and similar effects become intractable.
The pulsing part of the converter is a powerful interference generator, which requires careful shielding to avoid disruption to other equipment. The construction must not be airtight, though. The spark-gap generates ozone and corrosive nitrogen compounds, and if they’re allowed to build up inside an enclosure, they will corrode metals and degrade insulating materials. A pure nitrogen environment might be a solution.
The spark-gap must not conform to the stereotype of the two pointed tips. Ideally, the electrodes should be rather massive, smooth, and slightly convex. If a continuous operation is envisioned, their material should be hard and refractory enough to resist erosion, such as tungsten.
On a 1-k load, the prototype delivered 306 mW from an input power of 480 mW. This power was estimated by looking at the frequency of oscillation, which is 153 Hz in this case (P = 0.5 CV2F). This mode of calculation was adopted in order to remove the characteristics of the source from the equation. These figures put the efficiency at 63%, which is quite respectable for such a lowtech circuit.
Possible power sources include primarily piezoelectric or pyroelectric elements, but there are other possibilities. For example, you could build a “Power Thief ” (Fig. 2a). An antenna collects the stray electrostatic field from a high-voltage power line and feeds the high-voltage rectifier. This is similar to the fluorescent-tube trick (http://www.stopgeek.com/wp-content/uploads/2007/05/richard-box.jpg), but gives out usable electrical power.
In theory, it should also be possible to exploit the atmospheric gradient (Fig. 2b). This hasn’t been tested, and if attempted, great care must be exercised to avoid being struck by lightning.
The circuit could also be powered by an electrostatic machine, but only for demonstration purposes. Such machines are rather inefficient when compared to electromagnetic generators, and their output current doesn’t exceed tens of microamperes.
I thought this was "electronics design", not "proposals for a 6th grade science fair project". I see nothing novel in this discussion, and harvesting EMI from power lines is illegal. By the way, an inductive plate would be utilized, not a capacitive plate. Anonymous is right. Let's avoid making broadband noise whenever possible.
JCH -August 10, 2009
Well, at least this topic has "sparked" some reactions... Anonymous seems very busy hammering in his opinions (3 times!), but in 40 years of RF engineering he managed to miss a number of things. Even today, high power spark gaps are still very much in operation: http://www.electroheat-technologies.com/Types%20of%20Induction%20Heating%20Equipment.htm And BTW, while I was making measurements on the prototypes, I was also listening to Radio 4 LW (198KHz), with the receiver 3m away. In operation, a buzz was audible, but not overwhelming, yet the shielding and anti-RFI measures were summary.
Louis Vlemincq -August 05, 2009
WHERE has our broad understanding of RFI gone ???
as an engineer in the RF field for over 40 years i have to say that this idea for design is TERRIFYINGlY BAD!!!
for whatever power may be recovered, the RFI interference potential has to far outweigh any benefit, in our increasingly RF connected world. yet there was NO MENTION of the EXTREMELY high, broadband radio interference that spark gaps cause Don't believe me ? replace the carbon fiber damping cables used for car ignition and try to listen to the radio.
what an AWFUL idea this really is !!!
Anonymous -August 02, 2009
WHERE has our broad understanding of RFI gone ???
as an engineer in the RF field for over 40 years i have to say that this idea for design is TERRIFYINGlY BAD!!!
for whatever power may be recovered, the RFI interference potential has to far outweigh any benefit, in our increasingly RF connected world. yet there was NO MENTION of the EXTREMELY high, broadband radio interference that spark gaps cause Don't believe me ? replace the carbon fiber damping cables used for car ignition and try to listen to the radio.
what an AWFUL idea this really is !!!
Anonymous -August 02, 2009
WHERE has our broad understanding of RFI gone ???
as an engineer in the RF field for over 40 years i have to say that this idea for design is TERRIFYINGlY BAD!!!
for whatever power may be recovered, the RFI interference potential has to far outweigh any benefit, in our increasingly RF connected world. yet there was NO MENTION of the EXTREMELY high, broadband radio interference that spark gaps cause Don't believe me ? replace the carbon fiber damping cables used for car ignition and try to listen to the radio.
what an AWFUL idea this really is !!!
Anonymous -August 02, 2009
Figure 2B makes me think that the author has a death wish or hasn't seen lightening for a while. Maybe he has a source for 50MV isolation transformers. Ever see the damage a lightening bolt can do? Here in Florida I have seen big 100 year old trees blown to bits. I would never consider a lightening rod as a power source.
Anonymous -July 31, 2009
I love schemes like these. Somebody I worked with years ago told me about his friend who wrapped several turns of wire around a chain-link fence parallel to a power transmission line, and used the power to light up fluorescents in his garage. I'd love to see somebody spend some r&d money on a lightning farm--how about setting one up in Florida, with some arrangement to charge a gigantic bank of supercapacitors from captured lightning bolts. Maybe it could never be practical, but it would be exciting to try it! BTW the free power guy was spotted by the utility and made to cease and desist. That would be an interesting court case if somebody declared the right to electromagnetic fields on his property.
Gordon -July 23, 2009
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:
The Find Power Products monthly newsletter brings you the most important new developments within the world of power design. The newsletter includes exerpts from industry leader Sam Davis's exclusive blog, as well as overviews of the latest new products.
Enter Email to Subscribe
Web Seminar
Sponsored By:
Title: Exploring How Good GUIs Drive Adoption in the Digital Power Management Space
Reprints
