TRIAC Light Dimmer
Here is a simple circuit used in most dimmers.
The circuit is shown here on the Neutral but in practice may be in the
LIVE side. It is not really suited to Transformer loads as there is a
risk of DC output, but some makers do use it into transformers!
FILTERING
Filtering must be provided to reduce the high frequency content of Anode
A'' from getting out of the dimmer. The Choke is often 20 turns on a small
ferrite rod or ring.
TRIGGERING
The DIAC is a 20-30v (20v on 110v) device, when the 0.1uF cap charges
up over its' strike voltage, the 0.1uF charge is dumped into the TRIAC
gate.
AT FULL BRILLIANCE
When the pot is set to minimum, the 0.1uF cap charges up through the 1K
almost instantly. The CR provides some false spike mis-trigger protection.
So the TRIAC is triggered when the Live is about 30V, and stays on,
lighting the lamp until the mains is gone to Zero volts at the end of
the 1/2 cycle.
AT HALF POWER
When the pot is about half way around, the 0.1uF capacitor reaches 30v
around 1/2 way along the 1/2 cycle when triggering occurs. This still
produces the full peak voltage, and generally causes the most QRM, but
only half the energy is delivered to the lamp.
AT MAX DIMMING
When the pot is at maximum the 0.1uF cap only just charges up 30V near
the end of the 1/2 cycle. The Traic is then triggered and provides the
very low voltage mains at the end of the 1/2 cycle to the lamp, until
Zero volts releases the triac ready for re-triggering.
DC LOW DIMMING IMPROVEMENT
Due to the uncertainty of triggering the TRIAC when the mains is low etc,
as the 0.1uF may not reach the 30V required, a modification to the circuit
is sometimes incorporated. This applies a small amount of DC to the 0.1uF
cap as the POT resistance increases. This actually puts out unequal portions
of the +- mains phases causing the non triggering effect to be reduced.
But the DC or half frequency (50Hz not 100Hz) powering can cause visible
flicker in small power lamps! If used on Transformer excited lamps, it
will burn out the
transformers!
73 de John, G8MNY @ GB7CIP
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