Current transformer

Current transformer

      A current transformer consists of primary and secondary coils of wire wrapped around a core, usually made of steel or a nickel alloy. Transformer winding are electrically insulated from each other and from the core. The winding connected to the power supply is called primary winding. The transformer winding in which current is induced is called secondary winding.

    As current passes through the primary winding, an electromagnetic field is produced which induces a current in the electrically isolated secondary winding. The number of coils in a winding determine the amount of current and voltage in the circuit. By altering the coil ratio between the primary and secondary winding, a current transformer can alter the current, either stepping it up or down, while conserving power.

For this project, the current transformer is used to step up the voltage at low current. the secondary winding will be connected to the circuit while the primary winding connected to the electrode thus to the patient.

The above two pictures is a transformer with 1.2 VA as for power resulted for this transformer. This transformer had been bought at RS online. As for the picture below, bought from Pasar road. Looks like messy component but it gives 0.9 VA power for the transformer.

Both type of above transformer is not very suit to used in this project because the power needed is only 100 mA to 150 mA but it still can be tested. Thus, the suitable transformer still in finding process.

In this week, I have to look back at circuit diagram before buying it. Below is the circuit diagram for muscle simulator and timer.

                                                          Figure 1: Timer circuit diagram

                                                      Figure 2: Muscle stimulator circuit diagram


Figure 1 shows the timer circuit. It uses IC NE555 wired in monostable mode. Initially, when press switch S2, the monostable triggers and its output goes high for 10 minutes. Thereafter, its output goes low to give a beep sound from the piezobuzzer and lights up the red LED (LED2) indicating that muscle stimulation time is over.

Figure 2 shows the circuit of the muscle stimulator. IC 7555 is wired as an astable multivibrator to generate about 80Hz pulses. Potentiometer VR1 can control the intensity of current sensing at the electrodes. The brightness level of LED1 indicates the amplitude of the pulses. To increase the intensity level, replace the 1.8 kohm resistor with 5.6 kohm or higher value up to 10kohm.

Above is a little bit description for each circuit. I already had a discussion with my supervisor about these circuit and there is a few things need to be considered before buying all the components. As for the output of this circuit, a suitable electrode need to be connected to the output of this device. After do some research, I found that ECG electrodes is suit for the output of this device. Next week will go and buy the components.