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Build A Large Ultrasonic Cleaner

Most readers would know that you can obtain small ultrasonic cleaners for jewellery and similar small items. So why not a much larger version? It would be great for cleaning automotive and other mechanical parts, fabrics which cannot be machine washed, ornate bric-a-brac and a host of other hard-to-clean items.

By John Clarke

Specifications

Power Requirements..............12V at 2.5A

Transducer voltage.............250VAC square wave

Frequency range.............Main mode is 19kHz to 42kHz with irregular variation

Alternative frequency Range.............34 to 44kHz

Timeout Adjustment.............30s to 10m

Anyone who has ever needed to clean the parts for a carburettor, differential, gearbox or any other greasy and intricate parts must have often wished for an easier way.

Generally you dunk the parts in a container of kerosene, dieseline detergent or whatever, to soak for a while and then you return to the task with various brushes and implements to scrape off the grease and other gunk. It is a dirty and tedious task.

Click for larger image
The two "halves" of the project: the controller at left and the ultrasonic transducer, potted in a length of pipe, at right.

But what if you could dispense with all that brushing and scraping? If you could just drop the components in a tank of suitable solvent, press a button and then come back later to remove the parts in sparkling clean condition?

Our ultrasonic cleaner is designed to do exactly that job. It uses a high power piezoelectric transducer and an ultrasonic driver to literally blast away the dirt and grime with ultrasonic energy.

The solvent might be kerosene or hot water and a wetting agent such as a detergent. At low drive levels the solvent conducts the ultrasonic signal throughout the bath. At higher power levels, the ultrasonic wavefront causes cavitation which causes bubbles to form and then collapse. This is shown in Fig.1.

As the wavefront passes, normal pressure is restored and the bubble collapses to produce a shock wave. This shock wave helps to loosen particles from the item being cleaned.

The size of the bubbles is dependent upon the ultrasonic frequency and is smaller with higher frequencies.

Click for larger image
Cleaning a coffee-stained stainless steel tray in our "bath" (actually an old plastic cistern – see page 65). You can't see the bubbles being generated in this photo – they're too small – but they are certainly there.

Industrial ultrasonic cleaners tend to use frequencies between 20kHz and 50kHz while cleaners for small parts typically use frequencies above 50kHz.

Our ultrasonic cleaner sweeps the frequency range from about 19kHz to 42kHz to produce cavitation bubbles of varying sizes. The frequency is varied with an irregular pattern to avoid a constant low frequency sub-harmonic in the cleaning bath or tank. Variation of the sub-harmonic frequency reduces the impact of resonances in small items being cleaned that may otherwise cause them to disintegrate.

This variation in frequency also prevents standing waves in the cleaning bath that can produce cavitation in one area but no cavitation in another area. This can lead to irregular cleaning action of a component.

Actual power delivered is dependent upon the resonant frequency of the piezo transducer. For the Silicon Chip Ultrasonic Cleaner, maximum power delivered by the transducer is at about 40kHz which is the resonant frequency of the specified piezo ultrasonic transducer.

The Ultrasonic Cleaner can be set to run for between 30 seconds and 10 minutes.

Alternative sweep pattern

An alternative sweep pattern is available that sweeps over a frequency range of around 12kHz, centred on the 40kHz resonance. This produces a higher agitation level in the cleaning bath due to the transducer frequently running through its resonance. This alternative sweep pattern should be for intermittent use only. Which sweep pattern is best depends on the component being cleaned and the type of contamination.

The driver for our Ultrasonic Cleaner is housed in a small plastic case. This connects to the piezoelectric ultrasonic transducer itself using a length of sheathed 2-core mains-rated cable. The piezoelectric transducer is housed in a PVC fitting that covers and insulates the terminals from accidental contact. This is necessary because the transducer is driven at a high voltage which could cause a nasty shock if you come into contact with it.

The piezo transducer and housing can be directly immersed in the ultrasonic bath or tank. Alternatively the transducer can be glued to the outside of the bath using epoxy resin for deeper baths.

Circuit details

The circuit of our Ultrasonic Cleaner (fig.2) is relatively simple due to the use of an 8-pin PIC12F675-I/P microcontroller, IC1. This drives the piezoelectric transducer via two Mosfets, Q1 & Q2 and transformer, T1. The microcontroller also provides the timer and the start functions.

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