Light dimmer

AN392

6/11

Doc ID 1863 Rev 2

2.2

Operation with a transformer

Low power halogen spots use low voltage lamps (12 V typ.) usually supplied through a low

voltage transformer. For good application performance, the MCU program should ensure the

following:

●

At start-up, the delay time between the first gate pulse and the synchronization instant

is greater than 5 ms. This limits transformer coil induction and the risk of saturation with

associated high peak current.

●

The circuit starts on a positive line half cycle and stops on a negative one. Thus it starts

with positive induction and stops after negative induction has been applied. This helps

to minimize the size of the magnetic core material, and the current rating of the TRIAC.

●

The timer is precisely tuned in order to obtain 8.3 ms (for 60 Hz) or 10 ms (for 50 Hz)

delay between two gate pulses. As a result, the TRIAC is driven symmetrically in both

half cycles so that DC voltage content is avoided across the transformer terminals.

Saturation risk is then also reduced here. Otherwise, the voltage across the TRIAC is

monitored to detect a spurious open load condition at the secondary of the transformer.

●

The inrush current at lamp switch-on (halogen or incandescent) is also reduced due to

the soft start feature of the circuit (Figure 6).

2.3

TRIAC drive

The TRIAC is directly driven by the MCU. The pulse driving the TRIAC lasts 50 µs. The logic

level TRIAC is driven in quadrants QII and QIII with a gate current of 20 mA provided by two

I/O lines of the ST6210 in parallel. The logic level TRIAC has a maximum specified gate

triggering current of 10 mA at 25 °C.

The TRIAC is multi-pulse driven. Therefore, inductive loads can be driven without the use of

long pulse drives. As a result, the consumption on the +5 V supply can be reduced and the

supply circuit components are downsized. Before supplying the first drive pulse, the TRIAC

voltage is tested. If no voltage is detected, a spurious open load or a supply disconnection is

assumed to have occurred and the circuit is stopped. After the first driving pulse, the TRIAC

voltage is monitored. If the TRIAC is not on, another pulse is sent. The same process can be

repeated up to four times. Then, if the TRIAC is still not on, the circuit is switched off.

2.4

Circuit components

The light dimmer board (Figure 4) is almost the same as the motor drive board (Figure 2).

The major differences concern the point where the voltage is measured and the TRIAC

choice. When the board is dimming a resistive load, an RFI filter should be added to limit the

conducted noise.

In a dimmer, because of the resistive load, dynamic constraints are lower than in a motor

control, so a logic level TRIAC (BTA08-600SW) can be used. It is a sensitive TRIAC

switching capabilities ((dI/dt)c > 2.98 A/ms, (dV/dt)c > 10 V/ms). Thus it can also operate

without any snubber across it.

The MCU board in Figure 4 is supplied only when the TRIAC is off. A minimum off-time of