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Improved circuit drives one or two transducers If you have a boat and keep it in a berth or on a mooring in salt or fresh water, it will be inevitably plagued with marine growth on the hull. Left unchecked, this slows down the boat considerably and leads to a huge increase in fuel consumption. It’s the same story for a yacht; marine growth slows it down and makes it less manoeuvrable. So your boat has to be hauled out of the water at least once a year so the hull can be water-blasted and coated in fresh anti-fouling paint. Unless, that is, you have ultrasonic anti-fouling fitted – it keeps the barnacles at bay much longer! A nti-fouling paint is the tried-and-tested method for preventing marine growth on the hulls of boats but it only works if you use the boat on a regular basis. Anti-fouling paint works by ablation. As the boat moves through the water (the faster, the better) the surface of the anti-fouling paint is worn away to expose fresh coating, which then continues to do its job of inhibiting marine growth. So anti-fouling is a sacrificial coating – it is meant to be worn away. If you don’t use your boat regularly, the anti-fouling quickly becomes ineffective and marine growth can become rampant. So what’s the answer? Ultrasonic anti-fouling! This may not entirely replace the need for anti-fouling paint but it can greatly increase the interval at which the boat must be pulled out of the water to have this essential maintenance. Furthermore, the closer you live to the equator (ie, warmer water), the more cost-effective ultrasonic anti-fouling be74  Silicon Chip comes. On the Queensland or northern New South Wales coast, you will need to have anti-fouling done far more frequently than if you live in the colder climes of Victoria and Tasmania. The worst situation for marine growth involves boats moored in canal developments, such as on the Gold and Sunshine Coasts, where the water is warm and has poor tidal flow. What sort of marine growth are we talking about? Everything from algal slime to marine plants and shellfish of all types . . . and coral. Coral on boat hulls? Isn’t coral a threatened marine life-form? Certainly not on seldom-used boats moored in relatively warm water! Salt or fresh water We originally envisaged that this project would be for boats which remained in salt water. While this is certainly true, one thing we hadn’t counted on was that boats which are permanently in fresh water also suffer from the problem. siliconchip.com.au Features By Leo Simpson & John Clarke • Suitable for boats up to 14m (up to 8m with on e transducer). • Ideal for boats with sin gle-skin glass-reinforce    or fibreglass, steel d plastic (GRP) or aluminium hulls. • Powered by the boat’ s 12V battery. • Adjustable low-battery shut-down. • Very low current drain during shut-down. • Soft-start feature red uces surge current. • LED indicators for powe r, low battery or fault. • Neon indicators for ult rasonic drive operation. Maybe it isn’t quite as bad as salt but Jaycar Electronics have told us that they sold significant numbers of the original Ultrasonic Anti-fouling kit, and their built-up version, apparently with great success to boat owners who kept their craft on the freshwater lakes of Canada. So there goes our theory of warm, salt water! OK, we know that it’s still true but Jaycar’s experience is that Ultrasonic Anti-fouling also works in cold, fresh water. You’ll still need to clean her bottom! We must emphasise that fitting an ultrasonic anti-fouling system to your boat will not eliminate the need to pull the boat out of water from time to time to clean it, but also to inspect and replace sacrificial anodes and to generally inspect the hull and running gear for any damage. Nor can ultrasonic anti-fouling provide complete inhibition of growth on propellers, rudders, trim tabs and in bow and stern thrusters. But compared with conventional anti-fouling measures, ultrasonic anti-fouling is far more effective on boats that are used infrequently. And Ultrasonic Anti-fouling has a very big advantage in that it does not pollute waterways. This new version of our popular ultrasonic anti-fouling system has an improved circuit which drives one or two ultrasonic transducers which are mounted inside the hull of the boat. It is suitable for boat hulls made of single-skin glass-re- Excessive fouling after a boat had been in the water for two years with minimal usage. There was no Ultrasonic Anti-Fouling fitted. This amount of growth would severely impact speed, handling and fuel use. siliconchip.com.au inforced plastic (GRP or fibreglass), aluminium or steel/ stainless steel. These materials provide good transmission of ultrasonic vibration throughout the hull. It vibrates the hull at frequencies around 20-40kHz, which makes marine creatures less likely to adhere to the hull. This is explained in more detail below. Ultrasonic anti-fouling does not work well on boats with timber hulls due to their poor transmission of ultrasonic vibration. Similarly, hulls that use a composite sandwich construction comprising a foam core with an outer skin (usually a styrene core and fibreglass skin) are generally not suitable. That’s because the foam core dampens the ultrasonic wave propagation throughout the hull. How ultrasonic anti-fouling works Ultrasonic vibration of the hull disrupts the cell structure of algae and this reduces algal growth on the hull. And because there is less algae on the hull, larger marine organisms have a lesser incentive to attach themselves to it. The principles of ultrasonic anti-fouling have been known for a long time. The effect was discovered a century ago by French scientist Paul Langevin, who was developing sonar for submarines. He found that ultrasonic energy from his sonar tests killed algae. Since he was working with high power transducers, it was assumed that cavitation was causing algal death. In recent times, though, it has been found that high Same boat, eighteen months after cleaning AND having the original SILICON CHIP ultrasonic anti-fouling unit fitted. This illustrates that boats still need to be taken out of the water periodically but it’s a whole lot better than the shot at left! May 2017  75 3A S1 CON3 0V +12V F1 ATO BLADE FUSE POWER SWITCH 76  Silicon Chip SC 20 1 7 TP1 GND TP1 HYSTERESIS 12k 100nF 22pF 3 2 8 1 16 15 100nF 100nF X1 20MHz TP2 GND TP2 47k 22pF GND OUT BATTERY MONITOR 16V 470 F IN 4 10 F +5V AN4/RA4 OSC1 OSC2 14 5 Vss 13 12 RB4 RB5 RB3 RB1 RA1 10 11 9 7 17 RA0 18 RB7 RB6/AN5 6 100nF +5V RB0/PWM Vdd IC1 PIC16F88 PIC1 6F8 8 –I/P AN3/RA3 RB2 MCLR/ RA5 AN2/RA2 10k REG1 LP2950AC Z -5.0 D7 1N4004 22 130k K A D Q5 D4 1N5819 +5V D3 1N5819 +5V D2 1N5819 +5V A K A K A K 100k D9 BAT46 1nF A K G S ULTRASONIC ANTIFOULING DRIVER MK2 LOW 5k BATTERY THRESHOLD VR1 1k VR2 5k 4.7k 20k WARNING! This circuit produces an output voltage of up to 800V peak-peak to drive the ultrasonic transducer(s) and is capable of delivering a severe electric shock. DO NOT touch any of the components or tracks on the board within the pink area shown on the PCB overlay when power is applied. All exposed leads must be covered with insulating tubing. To further ensure safety, the PCB must be installed in the recommended plastic case and the transducer(s) correctly housed and fully encapsulated in resin, ie, as supplied in the kit. siliconchip.com.au K A A K 10k 10k +5V 10k 1W ZD4 5.1V 10 1W ZD3 5.1V 10 1W ZD2 5.1V 10 1W A A A ZD1 5.1V 10 D1 1N5819 10k A K 470 470 470  A K A K A K A K LED3  LED2  LED1 K K K 100nF D10 1N5819 OUTPUT VOLTAGE MONITOR D8 BAT46 16V 10 F G G S D S D FAULT G G LOW BATTERY POWER L1 470 H/5A Q3 Q1 S D S D 47k 130k Q4 Q2 K A K 25V LOW ESR S3 T2 ETD29 F3 A 1.6kV 220k S3 F3 1.6kV 220k T1 ETD29 2200 F F1 F2 S1 S2 F1 F2 S1 S2 2200 F 25V LOW ESR V+ A A 130k 130k TO ULTRASONIC TRANSDUCER 2 CON2 NEON2 DRIVER 2 INDICATOR TO ULTRASONIC TRANSDUCER 1 CON1 DRIVER 1 INDICATOR NEON1 D OUT LP2950 COMPONENTS IN THIS SHADED AREA ARE ONLY REQUIRED FOR SECOND ULTRASONIC TRANSDUCER D6 UF4007 2kV 1nF D5 UF4007 2kV 1nF IN K GND K ZD1–ZD4 Q1–Q5: STP60NF06L OR HUF76423P3 G D S K A LEDS K 1N5819, BAT46 A 1N4004, UF4007 Fig.2; the yellow and green waveforms in each of these four scope grabs show the alternating gate signals to Mosfets Q1 & Q2, while the lower (blue) trace shows the the resulting high voltage waveform from the secondary of the transformer T1. This waveform is applied to the piezoelectric ultrasonic transducer. ultrasonic power and cavitation is not required to kill algae. Instead, ultrasonic vibrations cause resonance effects within algal cell structures and relatively low powers are still enough to cause cell death. So if the boat’s hull can be vibrated over a range of ultrasonic frequencies, algae will not be able to attach to it and so other more menacing marine growth will similarly be discouraged. Our first Ultrasonic Anti-fouling project for boats was published in the September & November 2010 issues and this has proved to be very popular with boat owners. We have also had lots of good feedback from boat users not only in Australia and New Zealand but from all over the world. Its popularity is partly due to the fact that the build-ityourself kit, exclusive to Jaycar stores, is much cheaper than any commercial unit and has proved to be effective in minimising marine growth. But feedback from boat owners has also indicated that improvements could be made to our original design and the first of these is the ability to use it on larger boats. Our recommendation for our first design was that it was suitable for boats up to 10 metres, with larger boats up to 14 metres or catamarans requiring two transducers and two drive units. Our experience is that one transducer is not quite enough for a 10-metre power boat. Used on a 10-metre fly-bridge cruiser with twin shaft drive, the prototype has performed well in inhibiting marine growth and considerably increasing the intervals at which the boat must be pulled out of the water for service. But a 2-transducer unit would do a much better job. So our MkII version can drive one or two ultrasonic transducers. With two transducers, it is ideal for larger boats and catamarans, up to about 14 metres. Fig.1 (facing): the PIC16F88 microprocessor provides alternating gate signals to Mosfet pairs Q1, Q2 & Q3,4. Each pair of Mosfets drives a step-up transformer (T1 & T2) and these drive separate ultrasonic transducers. The micro also monitors the battery voltage and shuts down operation if the battery drops below a threshold set by trimpot VR1. Neon indicators show the presence of high voltage at the secondary windings of the two transformers. siliconchip.com.au May 2017  77 Fig.3: taken at a low sweep speed of 200ms/div, this scope grab shows that the transducer is driven in two frequency blocks, as described in the text. Fig.4: taken at an even lower sweep speed of 500ms/div, this shows the gate drive for Mosfets Q1 & Q4, in the separate channels, and this demonstrates how each transducer is alternately driven with its bursts of frequencies. The single transducer version would be suitable for boats up to eight metres or perhaps a little larger. This latest version is also much easier to build, with the Jaycar kit utilising pre-wound transformers and alreadypotted ultrasonic transducers. Jaycar has funded the development of both the original and latest version of this project and so the kit is exclusive to that company. Other changes made to the MkII version include LEDs for power, low battery and fault indication while each ultrasonic driver output has a neon indicator which shows when a transducer is being actively driven. As well, the low-battery shut-down voltage is now adjustable. We have also reduced current consumption during lowbattery shut-down from 6.7mA down to 170A. That’s a worthwhile saving and this low current drain prevents any further significant discharge of the battery after low-battery voltage shut-down. The circuit also includes a soft-start feature, where the high-value supply decoupling capacitors are charged slowly when power is first applied. This prevents a high surge current that could cause the fuse to blow. Lights, (ultra)sound, action Our new Ultrasonic Anti-fouling project provides far more visual indication that something is happening while it is operating. When power is first applied, the green LED comes on and stays on for 30 seconds which is the initial power on delay and soft-start feature. Then it flashes very brightly, in unison with the alternating flashing of the two neon indicators which show that high voltage is being delivered to the ultrasonic transducers. If the micro shuts down operation because of low battery voltage, the red low battery LED will flash very briefly at full brightness – helping to conserve the low battery. And of course there is the fault LED which comes on (when there is fault!). Specifications • • • • • • • • • • • • • Operating supply voltage: 11-16V DC Average current drain: typically 320mA for one transducer, 640mA for two transducers Peak current: 2A Output frequency range: 19.08kHz to 41.66kHz in 14 bands Frequency steps: 12 steps in each band; 80Hz steps at 20kHz increasing to 344Hz steps at 40kHz Signal burst period: 1000 cycle bursts, ~600ms at 20kHz and ~300ms at 40kHz Burst interval period: between 300ms and 600ms Dual transducer drive: alternate Transducer drive voltage: 250VAC (about 700V peak-to-peak) Low-battery cut-out threshold: adjustable from 0-15V Low-battery cut-in threshold: 0-2.5V above cut-out threshold Low-battery shut-down quiescent current: 170 A Power-up delay: 30 seconds 78  Silicon Chip siliconchip.com.au The component parts of our new Ultrasonic Anti-Fouling project: centre is the driver, as described in the text. Plugging into this are one or two ultrasonic transducers, which are attached to the boat hull. The Jaycar kit will have these transducers already potted, as shown here. You can also listen to the unit operating with an AM radio. If you bring the radio near the driver unit or the transducers, you will hear it tweeting and buzzing away, giving you a clear indication that something is happening. And if you have very keen ears and very quiet surrounds (no water lapping on the hull) you might hear faint clicks from the ultrasonic transducers, in concert with the neon indicators. sulated in high-pressure plastic plumbing fittings. On the lid, there is an on/off switch, while the LED and neon indicators can be seen through the lid. The circuitry for the Ultrasonic Anti-fouling MkII is based on a PlC16F88-I/P microcontroller, power Mosfets and step-up transformers. It can be powered from a 12V battery or a 12V DC 3A (or greater) power supply if shore power is available. Operating principle Ultrasonic bursts Our Ultrasonic Anti-fouling system works in a similar manner to commercial systems – at a fraction of the cost. It uses high-power piezoelectric transducers which are attached inside the hull, driven with bursts of ultrasonic signal ranging between about 20kHz and 40kHz. The reason for using a range of frequencies is two-fold. First, so that various resonance modes of the hull are excited and secondly, a range of frequencies is required to kill the various types of algae. While a high-power transducer is used and we do drive it with very high voltages, the actual power level is not very great. So typical average current consumption from a 12V battery is around 320mA per transducer, with peak currents of around 2A. The Ultrasonic Anti-fouling system should be run continuously while ever the boat is moored. In fact, there is no reason to turn it off while the boat is in use, unless you have divers underneath – we have had reports that divers can find the ultrasonic energy immediately underneath the hull causes unpleasant sensations in the ears. You will need to make sure that the boat’s 12V battery is always kept charged. This is no problem for boats in berths which have shore power (ie, 230VAC mains). For boats on swing moorings, a solar panel and battery charge controller will be required. The Ultrasonic Anti-fouling MkII driver is housed in a sealed plastic IP65 case with a transparent lid. There is one cable gland on one side of the case for the power supply and one or two 2-pin IP67-rated sockets for connection of the transducers. The piezoelectric transducers are encap- Each piezoelectric transducer is driven with bursts of high-frequency signal ranging from 19.08kHz through to 41.66kHz. This is done over 14 bands, with each band sweeping over a small frequency range. The first band is 19.08-20.0kHz and comprises 12 frequencies with approximate 83Hz steps between each frequency. The other bands also contain 12 frequencies but with larger frequency steps. For example, in the middle band of 24.75-26.31kHz, the steps are about 141Hz. For the top band between 37.87-41.66kHz, the steps are 344Hz. Each band overlaps the following band by a few hundred hertz. This overlap ensures that the whole range of frequencies is covered from 19.08kHz to 41.66kHz. Each burst of signal comprises two separate frequency signals each for 500 cycles. The burst period for the total 1000 cycles depends on the actual frequencies that are being produced and ranges from 300-600ms. Each transducer is driven alternately to reduce peak current draw. The two frequency bands within each burst are varied in a pseudo-random way so that the entire range of frequencies is covered every 16 seconds. This sequence is repeated after about 64 seconds. Note that there is a concentration of signal about the resonant frequency of the transducer(s), between 35.21kHz and 41.66kHz. siliconchip.com.au Circuit description The complete circuit is shown in Fig.1. PIC microcontroller lC1 drives step-up transformer T1 in push-pull mode via N-channel Mosfets Q1 and Q2. If the circuit is built to drive two transducers, IC1 also drives transformer T2 via May 2017  79 With the obvious exception of the transducer/s (which mount on the boat hull) all components mount on one double-sided PCB, as shown here. Full construction details, along with information on mounting on the boat, will be presented next month. Mosfets Q3 and Q4 in the same manner. The microcontroller runs at 20MHz (using crystal X1) and this allows it to provide the small ultrasonic frequency shifts required. Mosfets Q1 and Q2 are driven from the RB1 and RB3 outputs of IC1, while Q3 and Q4 (if fitted) are driven from RB5 and RB4. Since these outputs only swing from 0V to 5V, we are using logic-level Mosfets, type STP60NF06L or CSD18534KCS. Their on-resistance (between the drain and source) is typically 10-14mΩ for a gate voltage of 5V. The current rating is 60A/73A continuous at 25°C. There are several other logic level Mosfets that are suitable, including the HUF76423P3. Mosfets Q1 and Q2 are driven alternately and in turn drive separate halves of transformer T1’s primary winding. The centre tap connection is from the battery via the fuse (F1) and soft start Mosfet Q5. When Q1 is switched on, current flows through its section of the primary winding for less than 50µs, depending on the frequency, after which Q1 is switched off. After 5µs, Q2 is then switched on for less than 50µs. Then, when Q2 switches off, there is another gap of about 5µs before Q1 is switched on again and so on. Dead-time The 5µs period during which both Mosfets are off is the “dead time” and it allows one Mosfet to fully switch off before the other is switched on. The alternate switching of the Mosfets generates an AC waveform in the primary of T1 and this is stepped up in the secondary winding to provide a voltage of about 250VAC, depending on the particular frequency being switched and the piezoelectric transducer impedance at that frequency. Mosfets Q1 and Q2 are rated at 60V. Should the drain voltage exceed this substantially, they will enter “avalanche breakdown”, acting a bit like zener diodes and clamp the voltage to around 80V. This is safe as long as the shunted current and conduction time are within the device’s ratings, which is the case for all recommended Mosfets. This is important since a highvoltage transient is generated each time the Mosfets switch off, due to the transformer’s magnetic field collapsing. Protection for the gates of the Mosfets is provided by 5.1V zener diodes ZD1 & ZD2 (and ZD3/ZD4 for Q3/Q4). This might seem unnecessary since the Mosfets are only driven from a 5V signal but the high transient voltages at the drains can be capacitively coupled to the gate. These 5.1V zener diodes also help prevent damage to the RB1 and RB3 outputs of IC1 due to coupled voltage spikes (RB5/ RB4 are similarly protected by ZD3 and ZD4). Further protection is provided for the outputs of IC1 by schottky diodes D1-D4. These clamp the voltages at these pins to about +5.3V. They are in parallel with the internal protection diodes of IC1. The SILICON CHIP READY RECKONER Gives you instant calculation of Inductance - Reactance - Capacitance - Frequency It’s ESSENTIAL For ANYONE in ELECTRONICS You’ll find this wall chart as handy as your multimeter – and just as useful! Whether you’re a raw beginner or a PhD rocket scientist . . . if you’re building, repairing, checking or designing electronics circuits, this is what you’ve been waiting for! Why try to remember formulas when this chart will give you the answers you seek in seconds . . . easily! Read the feature in the Januar y 2016 issue of SILICON CHIP (you can view it online) to see just how much simpler it will make your life! All you do is follow the lines for the known values . . . and read the unknown value off the intersecting axis. It really is that easy – and fast (much faster than reaching for your calculator! HU 420x59G4Em m Printed on heavy (200gsm) photo paper Mailed flat (rolled in tube) or folded Limited quantity available Mailed Folded: Mailed Rolled: $20.00 inc P&P & GST ORDER NOW AT www.siliconchi p.com.au/shop $10.00 inc P&P & GST on heavy 80  Silicon Chip siliconchip.com.au photo pa per Parts list – Ultrasonic Anti-Fouling for Boats (Mk2) 1 double-sided PCB coded 04104171, 158.5 x 110.5mm 1 panel label, 123 x 89mm 1 IP56-rated sealed polycarbonate enclosure with clear lid, 171 x 121 x 55mm (Jaycar HB-6248) 1 50W 40kHz ultrasonic transducer potted and wired (Soanar YS-5605) (2 for 2 transducers [T2]) 1 50mm BSP flanged backnut (2 for 2 transducers) 1 IP67-rated 2-pin panel mount socket (Jaycar PP-0542) (2 for 2 transducers) 1 IP68-rated cable gland for 4-8mm diameter wiring (Jaycar HP-0724) 1 pre-wound transformer using ETD29 3C85 bobbin and cores (Jaycar EM2791) (T1) (2 for 2 transducers) 1 IP65-rated 10A SPST push-on/push-off switch (S1) 1 470µH 5A toroidal inductor (L1) (Jaycar LF-1278) 1 PCB-mount ATO blade fuse holder 1 3A ATO standard blade fuse (F1) 1 3-way PCB mount screw terminals, 5.08mm pitch (CON1) (2 for 2 transducers [CON2]) 2 2-way PCB mount screw terminals, 5.08mm pitch (CON3) 1 18-pin DIL IC socket 1 20MHz crystal (X1) 1 NE2 pigtail neon indicator lamp (blue [Jaycar SL-2695] or orange [Jaycar SL-2690]) (NEON1) (2 for 2 transducers [NEON2]) 2 5kΩ top-adjust multi-turn trimpots (VR1,VR2) 4 M3 x 6mm pan-head machine screws 3 M3 x 10mm pan-head machine screws (5 for 2 transducers) 3 M3 star washers (5 for 2 transducers) 3 M3 nuts (5 for 2 transducers) 4 PC stakes (optional) 1 100mm cable tie 1 120mm length of 3mm diameter heatshrink tubing 1 20mm length of 6mm diameter heatshrink tubing 1 200mm length of 5A or greater rated wire (for S1) 1 200mm length of mains-rated wire (for transducer(s)) Semiconductors 1 PIC16F88-I/P microcontroller programmed with 0410417A.HEX (IC1) 1 LP2950ACZ-5.0 5V low dropout regulator (REG1) 3 STP60NF06L or HUF76423P3 60V N-channel logic-level Mosfets or equivalent (Q1,Q2,Q5) (5 for 2 transducers [Q3, Q4]) 1 high-brightness 5mm green LED (LED1) 2 high-brightness 5mm red LEDs (LED2,LED3) 2 5.1V 1W zener diodes (ZD1,ZD2) (4 for 2 transducers [ZD3,ZD4]) 3 1N5819 40V 1A schottky diodes (D1,D2,D10) (5 for 2 transducers [D3,D4]) 1 UF4007 1000V 1A ultrafast diode (D5) (2 for 2 transducers [D6]) 1 1N4004 400V 1A diode (D7) 2 BAT46 100V 150mA schottky diodes (D8,D9) Capacitors 1 2200µF 25V low-ESR PC electrolytic (2 for 2 transducers) 1 470µF 16V PC electrolytic 2 10µF 16V PC electrolytic 5 100nF 63V/100V MKT polyester 1 1nF 63V/100V MKT polyester 1 1nF 2000V ceramic (2 for 2 transducers) 2 22pF 50V ceramic Resistors (0.25W, 1%) 1 220kΩ 1600V (eg, Vishay VR25 1.5%) (2 for 2 transducers) 3 130kΩ (4 for 2) 1 100kΩ 2 47kΩ 1 20kΩ 1 4.7kΩ 1 1kΩ 3 470Ω 1 22Ω Additional parts for installation 1 long marine-rated 12V 2A+ twin core cable, to reach battery 1 pack J-B Weld 2-part epoxy (Jaycar NA-1518) 1 pack “Fix-A-Tap” waterproof lubricant 1 small jar petroleum jelly or vaseline 4 long M4 stainless steel machine screws, shakeproof washers and nuts various cable ties, etc. siliconchip.com.au 1 12kΩ 2 10Ω (4 for 2) 3 10kΩ (5 for 2) Jaycar Electronics will have available a complete kit for the Ultrasonic Anti-Fouling Unit within a few weeks. With one transducer, the kit will retail for $249 (Cat KC-5535). The add-on second transducer kit (with the parts shown in red above) will retail for $169 (Cat KC5536). Visit www.jaycar.com.au/ultrasonic for more info. May 2017  81 (Left): here’s the “business end” of the system, the Ultrasonic Transducer, which sets up the vibration pattern in the boat hull which marine vegetation doesn’t particularly enjoy! Because these operate at high voltage (~700-800V peakto-peak) they must be fully enclosed (“potted”) in a suitable enclosure, as shown above. (The Jaycar kit will have potted transducers). Neon relaxation oscillators The output from transformers T1 and T2 is a high-voltage 250VAC waveform; up to 700V peak-to-peak. We use neon indicators to show whenever the transformer is delivering its voltage. Note that the NE2 neon lamps are not fast enough by themselves for this job. They can flash at a maximum rate of 20kHz, while the transformer output frequency can be above 40kHz. So the neons are driven via a circuit comprising high voltage fast diode D5 (or D6), a high voltage 220kΩ resistor, a high voltage 1nF capacitor and 130kΩ current-limiting resistor. The diode and 220kΩ resistor charge the 1nF capacitor up over several cycles of ultrasonic signal until the voltage across the capacitor reaches the breakdown voltage of the neon lamp. The 1nF capacitor can charge because the neon draws very little current until breakover, at around 70V. When this voltage is reached, the neon conducts by a gas discharge between its electrodes and the voltage across it drops to around 50V. The series 130kΩ resistance limits the current, which must be kept under 300µA to prevent electrode erosion. Once the 1nF capacitor has discharged, it starts recharging on the next cycle. Hence, the neon and its associated components form a classic relaxation oscillator. Battery voltage monitoring In addition to driving Mosfets Q1-Q4, microcontroller IC1 monitors the battery voltage and if necessary, shuts down the drive signals to prevent the battery from discharging below a set threshold. This is done to prevent long-term damage to the battery and also to avoid discharging a boat’s main battery if it is also used to power automatic bilge pumps or to start the motor. Of course, larger boats will have multiple batteries but the circuit still needs low battery protection. The incoming 12V supply is monitored via a voltage divider consisting of 130kΩ and 47kΩ resistors and the resulting voltage is filtered with a 100nF capacitor and monitored by lC1 at pin 1, the AN2 analog input. The resistors reduce the battery voltage to a 0-5V range, suitable for feeding to IC1. So for example, if the battery voltage is 11.5V, pin 1 will be at 3.054V. IC1 converts this voltage into a digital value using its internal analog-to-digital converter (ADC) and this is compared against a reference voltage set by trimpot VR1. Trimpots VR1 and VR2 are fed with 5V from IC1’s RB2 82  Silicon Chip output at pin 8 which is held at 5V during normal operation. VR1 connects to pin 8 via a 1kΩ resistor and VR2 connects via a 4.7kΩ resistor, both of which limit their adjustment ranges. RB2 drops to 0V during low battery shut-down, to eliminate the current drawn through VR1 and VR2. VR1 is used to set the lower voltage threshold, below which the Anti-fouling Unit switches off. VR1 is adjusted so that the voltage at TP1 is 1/10th the desired cut-out voltage. TP1 is connected to VR1’s wiper via 20kΩ/12kΩ resistive divider. So say you set the low battery shut-down to 11.5V, by adjusting VR1 until TP1 reads 1.15V. Given that the division ratio is 0.375 [12kΩ÷(20kΩ + 12kΩ)], we can infer that the voltage at the wiper of VR1 (and thus IC1’s AN4 analog input) is 3.067V [1.15V÷0.375], which is very close to the 3.054V quoted above for the voltage at pin 1 with a battery at 11.5V, as you would expect. The 5V supply rail for IC1 comes from REG1, an LP2950ACZ-5.0 low quiescent current regulator. This has a factory-trimmed output that is typically within 25mV of 5V (ie, 4.975-5.025V). Quiescent current is typically 75µA and this is part of the reason that during low battery shutdown, the current drawn by the Ultrasonic Anti-fouling circuitry remains so low. When low-battery shut-down occurs, LED1 is switched off and the Low Battery indicator, LED2 flashes briefly about once every two seconds. Mosfets Q1-Q5 are all switched off and the 5V supply to VR1 and VR2 from output RB2 goes low, as the microcontroller goes into sleep mode, with the 20MHz oscillator also stopped. An internal watchdog timer then wakes the microcontroller up every two seconds to re-measure the battery voltage and flash LED2. One problem with this is that as soon as the unit goes into shut-down, the battery voltage is likely to rebound and then the circuit will restart normal operation, the battery voltage drops again, shut-down is reinstated and so on; not ideal. To prevent this, we have incorporated hysteresis into the shut-down function and this is set with trimpot VR2. It sets the increment of voltage by which the battery voltage must rise above the low battery threshold, for normal operation to be restored. The increment or difference between these two thresholds is known as the hysteresis. Typically, you might decide that the battery voltage must rise by 1.5V above the low battery threshold, ie, the battery should rise to 13V. To do this, you would set VR2 to 1.5V, measured at test TP2. So if the unit has shut down and the battery is subsequently charged to 13V, normal operation will resume, with siliconchip.com.au LED1 flashing in unison with the neon indicators. Soft start facility N-channel Mosfet Q5 provides soft starting, whereby the 2200µF bulk bypass capacitors are slowly charged at power-up to prevent high surge currents. If the capacitors were directly connected to the 12V supply, a high surge current of many amps is liable to blow the fuse. The high capacitor charging current will also momentarily exceed the current rating of the capacitor. The gate of Q5 is driven by a switched-capacitor charge pump comprising diode D8 and D9 together with 1nF and 10µF capacitors. The 1nF capacitor is connected to the pulse width modulated (PWM) output pin of IC1, pin 6. Initially, this pin is at 0V but shortly after power-up, it is set to produce a 4.88kHz square wave. Each time pin 6 goes high, the 1nF capacitor couples this voltage to the anode of D9 and thus current flows into the positive end of the 10µF capacitor, charging it slightly. Because the 10µF capacitor is 10,000 times the value of the 1nF capacitor, the increase in voltage across the 10µF capacitor is very small. When the PWM output is low, at 0V, any voltage across the 1nF capacitor is discharged via schottky diode D8. D8 is connected to the Mosfet source and so voltage developed across the 1nF capacitor is with respect to this source terminal, which is connected to the V+ rail powering transformers T1 and T2. The 10µF capacitor charges to a few volts above the source terminal after about 10,000 cycles, which at 4.88KHz is just over two seconds. It never quite reaches 5V though, in part because of the forward voltages of diodes D8 and D9 but also because the 10µF capacitor has a 100kΩ discharge resistor across it. In combination with the capacitor value, this gives a one-second discharge time constant. So there is a constant battle between the 1nF capacitor trying to charge the 10µF capacitor while the 100kΩ resistor is discharging at the same time. With a 4.88kHz PWM frequency, this tug-of-war results in a gate-source voltage of about 1.6V, insufficient for Q5 to reach full conduction. Higher PWM frequencies give a higher gate voltage, as there are more charge cycles per second to counter the slow discharge of the 10µF capacitor. For example, at 19.53kHz we get a 3.2V gate-source voltage. At this point, the Mosfet should be conducting sufficiently to charge the 2200µF capacitors. So the soft start feature is provided by increasing the PWM frequency from pin 6 to increase Q5’s conduction over the first few seconds of operation. Once Q5 is in at least partial conduction, the voltage across the 2200µF capacitors can be measured via the 130kΩ and 47kΩ voltage divider resistors at the AN5 analog input of IC1, pin 12. If there is a short circuit (eg, due to a faulty capacitor or Mosfet), the capacitor voltage will still be near zero. The gate drive can then be switched off and a fault indicated by Fault LED3 flashing. If there is no short circuit, the PWM is also switched off and pin 6 goes to 0V. The 10µF capacitor will start to discharge via its parallel resistor, switching Q5 off. However, there is no current draw as Mosfets Q1-Q4 remain off so the V+ voltage rail should remain at 12V, held up by the 2200µF capacitors. siliconchip.com.au If any of the 2200µF capacitors are leaky, the V+ rail will drop. IC1 can detect this by re-measuring the voltage at input AN5 and comparing it to the voltage while Q5 was switched on. If V+ has dropped by more than 2V, there is a problem and so the unit switches off and flashes the Fault LED. The slow charging of the 2200µF capacitors during power-up and the testing described above should prevent the fuse from blowing unless a fault occurs while the unit is running. In that, case the fuse will blow to protect the rest of the circuit. Once the checks have completed, Q5 is switched on fully by producing a 156kHz square wave at pin 6, giving a gatesource voltage of around 4.6V for Q5, giving a very low onresistance in order to feed the ultrasonic drive circuitry. Inductor L1 is included in series with Q5 to reduce high transient current flow through Q5 and the fuse from the 12V supply. Instead, any high current transients are drawn from the 2200µF capacitors. It also limits the peak current drawn from the input supply. This helps to prevent any nuisance blowing of the fuse and it also reduces the amount of hash radiated from the supply wiring. Reverse polarity protection for the circuit is provided by diode D7, which protects regulator REG1, its associated capacitors and microcontroller IC1. However, if the unit is hooked up with reverse supply polarity, current can still flow through the body diodes of Mosfets Q1-Q4, via the primaries of transformers T1 and/or T2, through Q5’s body diode and through fuse F1. The fuse will then rapidly blow, isolating the circuit and preventing further damage. That’s it for this month. In our June issue we will give SC the full assembly, set-up and installation details. LOOKING FOR PROJECT PCBS? PCBs for most* recent (>2010) SILICON CHIP projects are available from the SILICON CHIP On-Line Shop – see the On-Line Shop pages in each issue or log onto siliconchip.com.au/shop You’ll also find some of the hard-to-get components to complete your SILICON CHIP project, plus back issues, software, panels, binders, books, DVDs and much more! Please note: the SILICON CHIP OnLine Shop does not sell complete kits; for these, please refer to kit suppliers’ adverts in each issue. * PCBs for some contributed projects or those where copyright has been retained by the designer may not be available from the SILICON CHIP On-Line Shop May 2017  83