I have various dimmer switches around the house. Some of them have an issue that when the lights are at their brightest the lights switch off. When switching the lights on you have to then hold it in to make the lights go just below most bright and it will then not switch off. Sometimes this can be tricky…
It's espcially tricky after a power cut when all dimmers default to full brightness again.
What is the cause of this?
Best Answer
Your question is lacking details but reading between the lines it seems that you have something similar to the 'touch dimmer' switches which became available in the 1980s. The 'switch' appeared usually as a brushed aluminium plate covering the whole switch box opening (but not touching the mounting screws because that would earth the plate). Touching the plate introduces mains hum into the circuit via body capacitance. This signal is used to trigger adjustment of the lighting level. In your case you have a push-button in place of the capacitive touch plate and electronics.
Electro Schematics have a sample circuit which uses the Siemens SLB0586A chip. The OP's dimmers probably use something similar. These chips are analogue electronics. (There are some logic elements in the chip but no CPU, etc., so their not digital in the modern sense.)
The dimmer uses a triac to control the power level. These work by delaying the switch-on of the lamp on each half-cycle of the mains. Triggering close to 0° will give full power. Delaying triggering close to 180° will give low power.
Dimming by phase angle control using a triac. 100% power is achieved by turning on at 0°.
The data sheet for the SLB0586A says that the maximum brightness it puts out is 152° of 180°. i.e., the earliest it will turn on at maximum brightness is 180 - 152 = 28° into the cycle. This isn't as bad as it may sound because the voltage is low at that point of the cycle and doesn't contribute much power.
The chip needs to monitor the zero-cross of the mains to get a reference for its timing circuit.
The possible clue to the OP's problem is in his comment that he has 12 V halogens fed by transformers. Transformers are inductive and shift the phase of the current relative to the voltage. This may be enough to advance the trigger pulse (which in normal operation shouldn't start until 28°, remember?) to before the zero-cross resulting in so little power that the lights appear off.
If the OP is still interested in resolving this and can identify the controller chip in one of the dimmers we can have a look at the data sheet and see if there's a fix.