What we want is a nice smooth controllable sine wave to allow us to resonate the cavity. The amplitude of the wave needs to be high enough to drive the ultrasonic transducer with sufficient intensity. A couple of ways to achieve this:
a) Circuit resonance – careful design of the driver circuit to achieve maximum electrical resonance of the LC circuit (the transducer having a capacitance) for a given driver voltage. This is nicely shown in the techmind.org article.
b) Brute force – using a powerful audio amplifier and signal source and a transformer to generate the necessary power.
It was probably best to decouple the signal using a simple transformer, with this method we do need to protect expensive equipment against electrical surge and current as the transients can be damaging. The components for my driver circuit are:
4 x TVS Diodes
1 x Gas Discharge Tube
1 x Resistor
1 x Transformer 6VA 0-6v 0-240v
At this point I am not sure if the 6VA transformer will be up to the job, I am still waiting for the transducer to arrive so I can’t test it; if it burns out I will upgrade it with something better.
The TVS diodes are transient voltage supressors. They break down when the voltage exceeds a certain amount and conduct, shorting out the spike and will afford some protection to connected instrumentation from transients generated from the transformer coils.
The Gas Discharge Tube does the same thing for high voltage spikes and is probably complete overkill – but I really don’t want to blow up my signal generator and scope, maybe I should have stuck one of these across the output.
Finally the resistor reduces any current surge that might occur – again probably overkill. The images below show the bits and the assembled circuit board, which actually IS a bit of board with the components glued on. The terminals are just little nuts and bolts you can pick up from the £($ US) store.
WARNING: when driving this circuit – the voltage on the primary coil of the transformer is likely to be in 100s of volts = unpleasant shock hazard – don’t touch.
Rules of thumb, don’t touch any bare wires, hook up equipment before switching things on, make sure no clutter around that might get in the way, make sure cats, gerbils, young children are not in the vicinity and cannot interrupt, (cats love jumping up on workbenches at critical times, young children fiddle with everything despite your best attempts to tell them not to, can’t speak for gerbils), just be sensible.
Next we’ll hook up the signal generator directly, get some measurements and see how this little circuit performs on its own into no particular load attached – since the load ( the ultrasonic transducer) is in the mail somewhere.
The circuit was connected to the frequency generator and scope with a x10 attenuation probe, and the amplitude of the driving frequency dialled up to it’s maximum. The frequency was set between 15 and 25Khz.
Here is the output with around 25kHz driving the circuit using a x10 attenuation probe. We have 5.0v x 4 squares on the scope x 10, so we’re achieving a good 200V peak to peak output at this frequency.
Here is the output with around 15kHz – the scale is set to 10.0V squares on the scope with x10 attenuator probe – now we’re achieving around 10.0v x 4 x 10 400V peak to peak.
As we decrease the frequency down from 25khz the voltage goes up, if we increase the frequency from 25khz the output voltage drops down, except around 26khz there is a brief increase in amplitude over a few Hz – this must be the electrical resonance point of the circuit.
There is not much else to be done with this circuit now until the transducer shows up, and we can guage what kind of power it will deliver – the resonance point is going to be different with the transducer attached.