Ultrasonic Transducer driving by Chopper Based Transistor CircuitBy Youssef Edward
May 2, 2019
Driving transducers is an easy task if the basic principles are well understood. The basic idea is to build very fast switching circuit in the frequency range specified for the ultrasonic transducer. It can be in the range of 22-50 KHZ, 100-300 KHZ, or 1MHZ to 3 MHZ.
The basic switching element is the transistor. A pulse generator is used to drive the transistor either directly or via a coupling technique. The pulse generator could be crystal based or a simple RC based generator. For this application, RC based oscillator is a poor idea. This is because the stability of the circuit will be low and the power of the ultrasonic transducer will vary with time. Crystal based oscillator is the best choice here.
There is two main techniques to generate clock. The first will be via PLL. The second is with the aid of PIC microcontrollers. PLL is little complicated and needs big explanation. The simplest one is to use PIC microcontroller so the clock pulses will be generated programmatically.
A simple code to generate clock pulses using 12F675 PIC microcontroller is as follow
Trisio.f5=0;//make the pin an output
Asm :nop ;
Asm :nop ;
If the crystal frequency in the above example is 20 MHZ, the resulting frequency will be 833 KHz. The high pulse will be 0.6 us, and the low pulse will be 0.6 us. The timing could be changed by varying the crystal and the code.
Once we have the clock pulses, it is ready to drive the switching element which could be BJT or MOSFET. Here we use MOSFET as shown below.
The MOSFET driver is used to turn on the transistor and supply the needed current during turn on. The MOSFET, unlike the BJT withdraw huge current during turn on especially if the drain sinks very high current. If the microcontroller provides the clock pulses directly to the MOSFET, it will blow at least for the Pin used.
Between the drain and the power supply, there must be a chopping element to limit the current when the transistor is ON. The chopping element is made of coil as shown below:
When the oscillator outputs logic one, the driver will switch on the transistor. Then the current will rise in a ramp fashion until the driver switch off the transistor. If the ON time is high compared to the time constant of the circuit, high current will flow in the output circuit and the transistor will blow down.
The coil value is the key for the operation of the circuit. It must be chosen based on the frequency of the circuit and the power of the transducer. If the L is too high, lower current will flow in the transistor and the transducer for the same frequency