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High-power Ultrasonic Cleaner Part 1 By John Clarke This large, High-power Ultrasonic Cleaner is ideal for cleaning bulky items such as mechanical parts and delicate fabrics. It’s also quite easy to build and is packed with features. Y ou’ve probably seen the small, low-cost ultrasonic cleaners available online. They’re great for cleaning items like jewellery or glasses, but what if you want something a bit bigger and more powerful to suit a wider variety of cleaning jobs? Cleaning fuel injectors, an old carburettor or any other intricate part is a messy and time-consuming task, requiring soaking in harsh solvents such as petrol, kerosene or a degreaser and then scrubbing with various brushes to clean up the parts. It is a difficult and tedious task, and often does not reach the small apertures that are usually the essential areas that need cleaning. 28 Our Ultrasonic Cleaner makes this task so much easier. Just place the components in a solvent bath, press a button and then come back later to remove the parts in sparkling clean condition. It will even clean internal areas! It uses a high-power piezoelectric transducer and an ultrasonic driver to release the dirt and grime with ultrasonic energy. For more delicate parts, the power can be reduced to prevent damage to the items being cleaned. How does it work? A metal container is filled with a solvent, deionised water, or normal hot water and a detergent or wetting agent. The ultrasonic transducer agitates the contents of the bath; at higher power levels, the ultrasonic wavefront causes cavitation, creating bubbles which then collapse. This is shown in Fig.1. As the wavefront passes, normal pressure is restored, and the bubble collapses to produce a shockwave. This shockwave helps to loosen particles from the item being cleaned (Fig.2). The size of the bubbles is dependent upon the ultrasonic frequency – the higher the frequency the smaller the bubble. We are using the commonly available bolt-clamped Langevin ultrasonic transducer, depicted in Fig.3. Practical Electronics | September | 2021 Features  Drives a nominal 40kHz, 50W or 60W-rated transducer  Adjustable power level  Power level display  Stop and Start buttons with run operation indication  Auto-off timer from 20 seconds to 90 minutes  Soft start  Over-current and startup error shutdown and indication  Power level diagnostics  Automatic or manual transducer calibration  Standing wave minimisation  Supports a resonance frequency of 34.88Hz to 45.45kHz It comprises piezoelectric discs sandwiched between metal electrodes. The centre bolt not only holds the assembly together, but is critical in ensuring the piezo elements are not damaged when being driven. The bolt is torqued to a pre-determined tension and locked (glued) in place to prevent it loosening. The bolt tension ensures the piezo discs always remain in compression, even while it is operating, preventing the discs from breaking apart. When a voltage is applied to the piezoelectric discs, forces are generated by the piezo elements that move the two metal ends closer together and then further apart at the ultrasonic drive rate. Our Ultrasonic Cleaner drives the piezo transducer at close to its nominal 40kHz resonant frequency. Fig.4 shows the power applied versus frequency for the particular ultrasonic transducer we are using. It claims to have a resonant frequency of 40kHz with a 1kHz tolerance either side of this frequency. We found that the transducer resonates at 38.8kHz under load. The transducer drive frequency needs to be controlled to within a fine tolerance to maintain a consistent power level. A small change in frequency from the resonant point will reduce the power quite markedly. Additionally, their impedance varies depending on load. So when operating in free air, the impedance is much lower compared to when the transducer is driving a bath full of cleaning fluid. Circuit details The circuit of the Ultrasonic Cleaner is shown in Fig.5. It is based around a PIC16F1459 microcontroller (IC1). This controls the two MOSFETs (Q1 and Q2) that drive the primary windings of transformer T1 in an alternating Practical Electronics | September | 2021 The ‘works’ of our Ultrasonic Cleaner before the transducer is attached to the cleaning bath. Operation is pretty simple: turn on, set the timer and push the ‘start’ button! fashion. T1 produces a stepped-up voltage of 100V AC (RMS) to drive the ultrasonic transducer. IC1 also drives the power LED (LED1) and level LEDs (LED2-LED6); plus it monitors the timer potentiometer (VR1) and switches S2 and S3, used for starting and manually stopping the cleaner operation. IC1 also monitors the current flowing through MOSFETs Q1 and Q2 at its AN11 analogue input, at pin 12. And it controls the soft-start charging of the main bypass capacitor using transistor Q5 and MOSFET Q6. Transformer drive A complementary waveform generator within IC1 is used to drive MOSFETs Q1 and Q2 in push-pull mode. The transformer is centre-tapped to allow this type of drive. IC1’s PWM generator includes an adjustable dead time, so that there is time for one MOSFET to switch off before the other MOSFET is switched on (Scope1). This prevents ‘shoot-through’, which would otherwise cause the MOSFETs to overheat. Fig.1 and Fig.2: the sound waves produced by the Ultrasonic Cleaner rapidly create and destroy bubbles in the liquid. When the bubbles collapse, they generate localised shockwaves. This ‘cavitation’ stirs up the solvent layer that’s in contact with the dirt, grease and grime, helping to break it up and more rapidly dissolve it away. You can do this by hand – it’s called scrubbing – but it’s a tedious job, and it’s hard to get into nooks, crannies and internal spaces in the parts being cleaned! 29 Scope1: the gate drive to Q1 (top trace, yellow) and Q2 (bottom trace, cyan) measured at pins 5 and 6 of IC1. The vertical cursors show the dead time when both MOSFETs are not driven as 2µs. That is for when Q1 switches off and Q2 switches on; the dead time is the same between Q2 switching off and Q1 switching on. IC1’s RC5 and RC4 digital outputs provide the complementary gate drive signals for MOSFETs Q1 and Q2. Since these outputs only swing from 0V to 5V, we are using logic-level MOSFETs. Standard MOSFETs require gate signals of at least 10V for full conduction, but logic-level MOSFETs will typically conduct fully at 4.5V, or sometimes even lower voltages. With the STP60NF06L MOSFETs we are using, the on-resistance (between drain and source) is 14mΩ at 30A with a 5V gate voltage. They are rated at 60A continuous and include over-voltage transient protection that clamps the drain-to-source voltage at 60V. Q1 and Q2 are driven alternately and these, in turn, drive the separate halves of the transformer primary of T1, which has its centre tap connected to the +12V supply. When MOSFET Q1 is switched on, current flows in its section of the transformer primary winding. Q1 remains on for less than 25µs (assuming a 40kHz operating frequency) and is then switched off. Both MOSFETs are off for two microseconds before Q2 is switched on. Q2 then draws current through its section of the T1 primary winding and remains on for the same duration as for Q1. Both MOSFETs remain off again for two microseconds before Q1 is switched on again. The gap when both MOSFETs are off is the ‘dead time’ and accounts for the fact that the MOSFET switch-off takes some time. Without dead time, the two MOSFETs would both be switched on together for a short duration. This would cause massive short-circuit current spikes, not only resulting in overheating of the MOSFETs but also drawing large current spikes from the supply filter capacitor and DC power supply. The alternate switching action of the MOSFETs generates an AC Fig.3: this shows the construction of the ultrasonic transducer that we’re using. Two piezoelectric (ceramic) discs are sandwiched between the two halves of the body, with electrodes to allow a voltage to be applied across the piezo elements. The compression of the piezoceramics due to the tension from the bolt holding the whole thing together is critical to preventing early failure from the ultrasonic vibrations. 30 Scope2: the lower trace (cyan) shows the transformer output voltage when driving the ultrasonic transducer at 39.26kHz. The top trace shows the current measurement voltage at the AN11 input of IC1 (TP1). 4.18V represents a 2.98A current driving the transformer primary with a 12V supply. This equates to approximately 35.8W delivered to the transducer. square wave in the secondary winding of transformer T1. With a turns ratio of 8.14:1 (57-turn secondary and 7-turn primary), and 12V AC at the primary, the secondary winding delivers about 98V AC to the piezoelectric transducer. Standing waves Running the Ultrasonic Cleaner at a constant frequency near resonance is efficient, since the impedance of the transducer is almost purely resistive under those conditions. However, this is not ideal for minimising standing waves within the cleaning bath. Standing waves can build up in strength while the frequency remains constant. These waves are caused by reflections from the parts being cleaned and the tank walls being in-phase. This can damage delicate parts. Our Ultrasonic Cleaner has the option of reducing the power for use with Fig.4: the frequency vs power curve for the transducer in our prototype. Most transducers with a nominal 40kHz resonance should be similar, but the exact frequency of the peak will vary, as will the steepness of the slopes. Hence, our Ultrasonic Cleaner has an automatic calibration procedure to find this peak; the 100% power setting runs it at a frequency close to the peak, while lower power settings are at higher frequencies. Practical Electronics | September | 2021                                SC HIGH POWER High-power Ultrasonic Cleaner ULTRASONIC CLEANER  Fig.5: the complete Ultrasonic Cleaner circuit. IC1 produces complementary drive signals to the gates of MOSFETs Q1 and Q2, which in turn drive the primary of transformer T1 in a push-pull manner. This results in around 100V AC at CON3. Current is monitored via two 0.1Ω shunt resistors at the sources of Q1 and Q2, via amplifier IC2b into analogue input AN11 of IC1; the power is computed from this and a voltage measurement at analogue input AN8. delicate parts, but even larger parts can have delicate sections within them, especially in thin-walled cavities. To avoid standing waves, the frequency can change over time to prevent the constant phase of the waveform, which would cause constructive interference at various locations in the bath. As the power versus frequency graph shows, changing the frequency even by a small amount will drastically alter the power. So it is not ideal if the frequency is varied continuously, as it reduces the cleaning power. Instead, we operate the transducer at a fixed frequency for 10 seconds at Practical Electronics | September | 2021 a time, then run it over a range of different frequencies for a short time before returning to the maximum power frequency for another 10-second burst. In the intervening time, the frequency varies in small 37.5Hz steps over a 2.4kHz range for around 400ms. That means that power is reduced only about 4% of the time. The cycling in frequency alters the phase of the ultrasonic vibrations in the bath, giving time for standing waves that occur during the fixed frequency period to die down, thus preventing them from building up to a damaging level. Over-current protection The over-current protection for the MOSFETs is provided in two ways. Both of the methods rely on current detection via the voltage across the 0.1 between the sources of Q1 and Q2 and ground. The first method uses NPN transistors Q3 and Q4. These have their baseemitter junctions connected across those 0.1 current-sense resistors. Over-current starts when the voltage across the 0.1 resistor exceeds about 0.5V; ie, with more than 5A through either Q1 or Q2. The associated transistor Q3 or Q4 then begins to conduct. 31 The current flowing from its collector to its emitter reduces the gate voltage to the associated MOSFET. This has the effect of increasing the MOSFET on-resistance, which then reduces the current. This protection is a fast-acting, cycle-by-cycle protection measure. At the same time, the voltages across the two 0.1 current-sense resistors are averaged by a pair of 10k resistors and filtered by a 100nF capacitor. This averaged voltage is then applied to non-inverting input pin 5 of op amp IC2, which amplifies the signal 28 times ((27k ÷ 1k + 1). The averaging effectively halves the sensed voltage, since only one of Q1 or Q2 is on at any given time. So this results in an overall amplification of 14. The output from pin 7 of IC2b is measured by the AN11 analogue input of IC1 (pin 12) – see Scope2. This voltage is converted to a digital value and processed by IC1. Should this voltage stay at 4.9V or more over a 160ms period, the drive to the transducer is switched off. This voltage represents an average of 350mV measured across each 0.1 resistor, or a 3.5A average current flow. That’s calculated as (4.9V÷14) ÷ 0.1. An over-current error is indicated by flashing LED2, LED4 and LED6 on the front-panel level display. When this happens, the power will need to be switched off and restarted to resume cleaning. If the problem persists, the cause will need to be found. Power control The current measured at the AN11 input is also used for controlling the power applied to the ultrasonic transducer. The maximum power rating of the transducer is 50W, but this is not a continuous rating. The recommended continuous power is 43W. We limit power to a more conservative 36W. For a 12V supply, the current required for this level of power is 3A. During operation, the current is monitored via AN11 and the drive voltage is also sampled, via a resistive divider, at analogue input AN8 (pin 8). This allows the micro to calculate the power flowing into the transformer as the frequency is adjusted, so that it can maintain the power at the required level. IC1’s instruction clock is derived from its internal oscillator, and thus the PWM output frequencies are derived from this as well. The internal oscillator can be adjusted in small steps using the OSCTUNE register. This can vary the internal oscillator frequency over a 12% range in 128 steps. For the 40kHz drive to the ultrasonic transducer, this allows a 4.8kHz control range in steps of 37.5Hz. 32 levels use a frequency above resonance that has the transducer producing a lower power. Nine power levels are available, ranging from 100% (36W) down to 10% (about 3.6W). Depending on the transducer characteristics, the lowest power level may not be available. The 40kHz transducer is available online. Remember that if you do buy online you need to make sure you get a 40kHz type – there are other frequencies available and they look pretty much identical. (See the NOTES in the Parts list opposite). The 37.5Hz-step resolution is sufficiently small to drive the ultrasonic transducer at the desired power level. However, the OSCTUNE register does not have sufficient frequency range to ensure we can drive an ultrasonic transducer that is resonant outside the range of 37.6kHz to 42.4kHz. To widen the operating range, the unit calibrates itself automatically (it can also be initiated manually). This finds the approximate resonant frequency of the transducer using a coarser adjustment. Fine-tuning is then done via OSCTUNE; this allows a variety of different transducers to be used. This coarser calibration is performed using the PR2 register, which sets the period and thus the frequency of the PWM drive waveform. For our circuit, this provides steps of approximately 540Hz. We restrict the coarse adjustment range to be from 34.88kHz to 45.45kHz. This range caters for all transducers that have a nominal 40kHz resonance. So the transducer’s resonance is found to within 540Hz by adjusting PR2, and this value is stored in nonvolatile Flash memory. OSCTUNE can then vary the frequency at least 1.8kHz above and 1.8kHz below the value initially set by the PR2 register (1.8kHz 2.4kHz − 540Hz). Different power levels are available by adjusting the drive frequency. The highest power is at the frequency closest to resonance, while lower power LED indicators LEDs 2-6 indicate which of the nine power levels is selected, with LED2 lit to indicate the lowest power level. The next step up is with LED2 and LED3 lit, then LED3 and so on until LED6 only is on, showing the highest power level. The power level is adjusted by holding down the Start switch. It will then cycle up through the nine possible levels to the maximum, then down again. The switch can then be released at the desired level setting. The transducer is not driven during power level adjustments. The On/Run LED (LED1) shows when power is applied to the circuit. This LED also acts as an operation indicator. The LED goes out during transducer calibration and then lights when the required value for PR2 is found. This takes a few seconds, unless there is something wrong, such as when there is no transducer connected. Once running, LED1 only lights when the transducer is being driven at the required power setting; it acts as an ‘in lock’ indicator. When the Stop switch is pressed, the drive to the transducer ceases, the level LEDs go off and the power LED turns on. LED1 then goes out when the main power source is switched off via S1, or if the supply itself is disconnected or switched off. Cleaning timer VR1 is the timer control. The voltage from its wiper is applied to the AN9 analogue input of IC1 (pin 9), and it varies between 0V and 5V. This corresponds to a timer range from 20 seconds through to 90 minutes. The timer starts when the Start switch is pressed. After the selected period, the transducer drive stops. Switches S2 and S3 connect to the RA0 and RA1 inputs of IC1 respectively. The inputs are held high (at 5V) by 10k pull-up resistors. A closed switch is detected when it is pressed as the input is pulled to 0V. Note that we are using pushbutton changeover switches that have common (C), normally closed (NC) and normally open (NO) contacts. The pins on the switch are in a line, with the common pin at one end, NO in the middle and NC at the other end. Usually, that means that you would Practical Electronics | September | 2021 Parts list – High-power Ultrasonic Cleaner 1 double-sided PCB coded 04105201, 103.5 x 79mm 1 double-sided PCB coded 04105202, 65 x 47mm Both PCBs available from the PE PCB Service 1 panel label, 115 x 90mm (see text) 1 diecast aluminium box, 115 x 90 x 55mm (Jaycar HB5042) 1 50/60W 40kHz ultrasonic horn transducer (resonance impedance 10-20) [see NOTES below] 1 12V DC 60W switchmode supply or similar [Jaycar GH1379, Altronics MB8939B] OR 1 12V battery (10Ah or greater) with 5A+ rated twin lead 1 EPCOS ETD29 13-pin transformer coil former, B66359W1013T001 (T1) [RS Components 125-3669, element14 1422746] 2 EPCOS ETD29 N97 ferrite cores, B66358G0000X197 (T1) [RS components125-3664, element14 1422745] 2 EPCOS ETD29 clips, B66359S2000X000 or equivalent (T1) [RS components 125-3668, element14 178507] 1 6A SPST mini rocker switch (S1) [Altronics S3210, Jaycar SK0984] 2 SPDT momentary push button switches (S2,S3) [Altronics S1393] 2 switch caps for S2 and S3 [Altronics S1403] 1 5A PCB-mount barrel socket, 2.5mm ID (CON1) [Jaycar PS0520, Altronics P0621A] 1 5A barrel plug, 5.5mm OD x 2.5mm ID [Jaycar PP0511, Altronics P0165] (optional) 1 vertical 2-pin pluggable header socket with screw terminals (CON2) [Jaycar HM3112+HM3122] 1 2-way PCB mount screw terminal with 5.08 spacing (CON3) [Jaycar HM3130, Altronics P2040A] 1 14 pin box header (CON4) [Altronics P5014] 1 14 pin IDC plug (for CON4) [Altronics P5314] 1 14-pin IDC transition plug (CON5) [Altronics P5162A] 2 3AG PCB-mounting fuse clips (F1) 1 4A 3AG fuse (F1) 1 10k 16mm linear potentiometer (VR1) 1 knob to suit potentiometer 1 20-pin DIL IC socket (for IC1) 1 8-pin DIL IC socket (for IC2) 3 TO-220 silicone washers and bushes 4 stick-on rubber feet Transducer housing parts 1 50mm length PVC DWV (Drain, Waste and Vent) fittings; end cap and adaptor or 1 40mm length of 50mm ID pipe 1 cable gland for 3-6.5mm cable Neutral cure silicone sealant (eg, roof and gutter) Epoxy resin (eg, JB Weld) Parts for testing 1 100mm length of 0.7mm tinned copper wire 4 9mm-long M3 tapped spacers 4 M3 x 6mm machine screws extra length of 0.63mm diameter enamelled copper wire need to orient the switch correctly on the PCB for correct operation. However, we have designed the PCB pattern so that either orientation will work by wiring the C and NC connections together on the PCB. Power supply 12V DC power for the circuit is fed in via CON1. It needs 4A minimum. If using a 12V battery, it should be rated at 10Ah or more. Power is switched Practical Electronics | September | 2021 Cables, wiring and hardware 1 M3 x 6mm machine screw (for REG1) 3 M3 x 9mm machine screws (for Q1, Q2 and Q6) 4 M3 hex nuts 1 cable gland for 3-6.5mm diameter cable 1 800mm length of 1mm diameter enamelled copper wire (T1 primary) 1 3.6m length of 0.63mm diameter enamelled copper wire (T1 secondary) 1 1m length of 0.75mm square area dual sheathed cable or figure-eight wire (for transducer connection) 1 160mm length of 5A (1mm2) hookup wire 1 200mm length of 14-way ribbon cable 8 PC stakes 1 30mm length of 5mm heatshrink tubing (for S1 connections) 1 roll of electrical insulating tape Semiconductors 1 PIC16F1459-I/P microcontroller programmed with 0410520A.hex (IC1) 1 LMC6482AIN CMOS dual op amp (IC2) 1 7805 5V 1A linear regulator (REG1) 2 STP60NF06L logic level N-Channel MOSFETs (Q1,Q2) 3 BC547 NPN transistors (Q3-Q5) 1 SUP53P06-20 P-channel MOSFET (Q6) 1 13V 1W zener diode (ZD1) 1 1N5404 3A diode (D1) 1 1N4004 1A diode (D2) 6 3mm LEDs (red or green) (LED1-LED6) Capacitors 1 4700µF 16V low-ESR PC electrolytic 2 100µF 16V PC electrolytic 2 10µF 16V PC electrolytic 1 470nF MKT polyester 4 100nF MKT polyester Resistors (0.25W, 1% unless specified) 1 1M 2 100k 1 27k 1 20k 8 10k 7 1k 2 47 2 0.1 1W (SMD 6432/2512-size; Panasonic ERJL1WKF10CU or similar) [RS Components 566-989] NOTES: The transducer is rated at 50W and designed for 40kHz operation. At the time of publication, eBay.co.uk part number 283977349993 is suitable (ensure you choose the 50W/40kHz option). Otherwise, a search on line for ‘50/60W 40kHz ultrasonic horn transducer, resonance impedance 10-20’ will yield further options. You can get the remaining electronic parts for this project from the usual suspects (for purchasers outside Aus/NZ, the Jaycar/ Altronics references provide sufficient information for choosing parts); use element14, Digi-Key or RS Components for the more specialised parts. The PVC components for the transducer housing are readily available from hardware and DIY stores. by S1, which is wired back to the PCB using a plug-in screw connector and socket (CON2). Power then passes to the 5V regulator (REG1) via reverse-polarity protection diode D2. Linear regulator REG1 provides the 5V required by IC1 and IC2. 12V DC also goes to MOSFET Q6 via fuse F1. This MOSFET is used as a soft-start switch to charge the large 4700µF low-ESR bypass capacitor slowly. Without soft starting, charging the 4700µF capacitor would cause a substantial surge current. This can blow the fuse or cause a 12V switchmode supply to shut down. When power is first applied, Q6 is off and the 4700µF capacitor is not charged. When the Start switch is pressed, the RC3 output of IC1 goes to 5V and this switches on transistor Q5. The gate voltage of P-channel MOSFET Q6 then begins to drop towards 0V as the 10µF capacitor 33 Reproduced by arrangement with SILICON CHIP magazine 2021. www.siliconchip.com.au (Above) If I knew you were comin’ I’d’ve baked a cake... these are some of the stainless steel containers we found at a kitchen supply shop which would be ideal for this project. Choose the size and depth which best suits your application. (Left) This shows what the completed Ultrasonic Cleaner will look like when we cover the construction and testing side next month. We’ll also show you how to set up your ultrasonic cleaning bath using cheap ‘cooking’ containers. charges via the 100k resistor to the collector of Q5). As the MOSFET begins to conduct, it slowly charges the 4700µF capacitor. After half a second, the gate charging is stopped by switching off Q5 and after a 250ms delay. The voltage across the 4700µF capacitor is then measured using the AN8 analogue input of IC1. If the voltage across the capacitor is under 9V (3V at AN8), all the level LEDs flash twice per second. This indicates that either the 4700µF capacitor is leaky, or there is a short circuit causing the capacitor to discharge. Power can then be switched off, and the fault investigated. If there is no error, Q5 is switched back on, to continue charging the gate of Q6. It takes one second for the gate to drop 7.5V below the source, at which time Q6 is almost fully on. After a few more seconds, the gate voltage will be very close to 0V, leaving the full 12V between the gate and source. Zener diode ZD1 protects the gate from over-voltage by limiting the gate-source voltage to −13V. Reverse polarity protection for the power section of the circuit is via a 4A fuse F1, diode D1 and the integral reverse diodes within MOSFETs Q1 and Q2. These diodes conduct current, effectively clamping the supply voltage at −0.7V and protecting the 4700µF electrolytic capacitor from excessive reverse voltage. This current will quickly blow the fuse and cut power. The bath The ultrasonic transducer needs to be attached to the outside of a suitable container. This can be made from stainless steel, aluminium or plastic 34 so that the ultrasonic vibration is efficiently coupled to the fluid. Stiffer materials couple the ultrasonic waves with fewer losses. Ideally, the bath should have a flat side or base where the transducer can be attached. The bath material also needs to be compatible with the epoxy resin used to glue the transducer to the bath. For our transducer, metals are the most compatible material. We found a series of ‘gastronorms’ (kitchenware tray/container) at a kitchen supply shop that are ideal. These are the types of food containers you often see at buffets. They slot into steam tables that keep the food warm, and they are available in various shapes and sizes, with several good options at or near the ideal 4L (four-litre-volume) capacity. You can get them made from stainless steel, polycarbonate or polypropylene with the first two options being the best. Just do a quick search in Amazon or eBay for ‘gastronorm container stainless steel, 4 Litre,’ or similar. We recommend either a 150mmdeep ¼ gastronorm tray (capacity 4L), a 100mm-deep 1/3 gastronorm tray (capacity 3.7L) or a 100mm-deep ¼ gastronorm tray (capacity 2.5L). The 150mm-deep ¼ gastronorm tray is tall and rectangular while the 100mm deep 1/3 tray is more square and shallow. The other tray is in-between the other two. You can also get stainless steel or clear or black polycarbonate lids to suit all these, which would be a good idea if you’re cleaning with a strongsmelling solvent (especially if you plan to leave the solvent in the bath when you aren’t using it). Larger-sized baths with more liquid will have a reduced cleaning effect compared with smaller containers with less fluid. The fluid used in the bath can be tap water with a few drops of detergent as a wetting agent. Other fluids that can be used include deionised water, alcohol (methylated spirits, isopropyl alcohol), acetone or similar solvents. Cleaning effectiveness is greatly enhanced when the fluid is warmed. Filling with around four litres is ideal for the power available from the ultrasonic transducer. With deeper containers, it might be possible to fill them with less liquid for cleaning smaller items. However, you would need to recalibrate the unit after each fluid level change, and you might find that it would shut down with less liquid in the tank due to the transducer impedance dropping, and the power delivery going above 40W. This approach would require some experimentation for successful use. The recalibration procedure will be described later. Note also that you would need to mount the transducer quite low on the container (or on the base) to allow different fluid levels to be used. Conclusion Next month, we will present the construction details, including how to wind transformer T1, the PCB assembly steps, wiring it up, encapsulating the transducer, case preparation and final assembly. We’ll also describe the testing and calibration procedures, plus give some hints on how to use the Ultrasonic Cleaner most effectively. Practical Electronics | September | 2021