UM1559

Circuit description

Doc ID 023542 Rev 1

9/13

example control software provides a series of “picket fence” pulses to the gate, to improve

the reliability of the gating, always finishing with the pins back high, ready for the next gating.

Current sensing

Shunt resistor R7 is used to sense the motor current during the Triac conduction angle.

Using a value of 0.01

Ω generates a signal that is 10 mV/A. This signal is amplified by U1 to

get a signal of 220 mV/A that is fed into an ADC input of the STM8. With a 5 V full scale

value on the ACD, the circuit can read peak currents up to 22.7 A.

Although the current through R7 is both positive and negative, the inverting configuration

circuit amplifies only the negative current since the amplifier output cannot go below ground.

An alternate configuration would be to add a bias to the amplifier circuit so that at zero

current the output would be half of the full scale voltage, or 2.5 V. In this way both positive

and negative current may be measured but the peak current capability would be reduced by

½ if the gain stayed the same.

The current sensing is included primarily for a sensorless speed control algorithm. The

algorithm senses current during the negative half cycle and performs the speed regulation

calculations during the positive half cycle.

The current sensing is not needed for the open loop or closed loop speed control with

tachometer feedback.

Motor voltage sensing

Figure 4.

Voltage sensing circuit

The measured motor voltage is also required for a sensorless speed estimation algorithm of

the software. Since neither side of the motor terminal voltage is at ground potential, a

conventional differential amplifier stage, built around U1A, is used to interface the signal to

the ground referenced STM8 ADC input, as shown in Figure 4. The differential amplifier also

provides attenuation, biasing and resistive isolation. The gain of this circuit is 1 k

Ω/78 kΩ.

Since the circuit is biased at 2.5 V out for 0 V in, the input voltage range is +/- 195 V peak

over the full scale range of 0 to 5 V on the ADC input.

Two 1206 case resistors are used in series for the input resistors to provide sufficient voltage

rating. For operation on 230 V/50 Hz mains, the values of input resistors R2, R3, R5 and R6

are changed to 82 k

Ω.