simple hair straightener basic circuit description

Figure 1 circuit shows a simplified application circuit for hair straighteners using the IES5521A with a 60W heating element load. For the complete guide on designing your own circuit, refer to IES5521A datasheet and Application Notes AN001/AN003. R1 and R2 values are set to suit different temperature control ranges from ambient to over 200°C. C3 and R4 set the timeout timer from one minute to over 5 hours. Timeout timer can also be fixed at 1 hour using mains division by connecting only R4 to Vcc. This circuit uses a negative temperature coefficient (NTC) thermistor for temperature sensing. It is important that the design of the circuit needs to take into consideration a suitably sensitive triac (TR1) to achieve optimum performance with minimum external component count while ensuring reliable triggering.

IES5521A control performance graph

Figure 2: The actual performance graph of IES5521A in hair straightener Figure 2 shows the actual temperature control performance of a hair straightener with IES5521A set to operate at high (above 200°C) temperature. The graph shows Rapid heat up to set (specified) temperature Minimum temperature overshoot Constant accurate temperature At set timeout, triac stop switching The graph shows rapid heat up time with minimum temperature overshoot before maintaining a constant accurate temperature. The hair straightener timeout timer is set to switch off after one-hour operation. Heat up time from ambient temperature In this graph; the heating plate of the hair straightener reaches its set temperature within a minute (rapid heat up time). Heat up time is dependent upon the choice of heating element used and the thermal inertia of the total heating system. A faster heat up time can be achieved with low thermal inertia heating and sensing element. Time between switching The blue waveform indicates triac switching. Each cycle of the triac switching time is dependent on the thermal mass of heater plate and the heater load. Timeout Timer (Heater ON time) The hair straightener is preset to timeout (switch off) after 1 hour of operation as indicated on the graph that the triac no longer switches. Other timeout is also possible by setting the RC external component values. Switching the mains off and on again easily reset timer. Refer to OM1654A & IES5521A for more information.