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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?

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    Care to tell us what kind of dimmers you have? – Speedy Petey Sep 8 '15 at 10:58
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    It doesn't have a dial. There is just one light switch which you press once to turn on/off and press and hold to adjust dimming level. Switch connected to a unit behind the plate which then powers some transformers for a bunch of 12v 10w halogens. Mains is 240v here. That's my lay understanding of my setup. – Louis Sep 9 '15 at 11:22
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    What kind of bulbs do you have in the sockets and what kind of dimmer switch. FYI many non rated dimmers do this with LED lights. And the LED manufacturer matters in that you could buy bulb A from ABC and it works and GHF and XYZ don't work but same specs. This is a major issue in the LED industry right now - there is no conformity. – DMoore Nov 9 '15 at 17:50
  • I have lost a little interest as I'm switching this whole business to LED. This happens on two dimmers I own. One drives 20 10w 12v halogens. The other drives 50 10w 12v halogens. What is the appropriate action for me on this question? Shall I leave it open until I find switch code? – Louis Jan 12 '16 at 12:39
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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.

Triac trigger angle.

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.

  • I should have read those comments. I missed the part about 12v halogens. You're definitely on to something, but I highly doubt the OP has capacitance dimmers (I haven't seen those since the 80's), I'd assume they're Maestro's or the equivalent. – Mazura Jan 9 '16 at 1:03
  • @Mazura: yes, posters should add critical info like that back into the question. You're probably right about the capacitive touch. I've just updated because OP mentions a push-button. The Maestros seem to be 120 V and OP is on 240 V (Britain?) so probably a different brand. Thanks. – Transistor Jan 9 '16 at 1:14
  • Thanks for this detailed reply. I have lost a little interest as I'm switching this whole business to LED. This happens on two dimmers I own. One drives 20 10w 12v halogens. The other drives 50 10w 12v halogens. Switching these to LED will save me power. I can check the dimmer once it comes out as it will be replaced. Finding LEDs for this is a business though. – Louis Jan 12 '16 at 12:35
  • OK. Have fun! If you think this answer addressed your question I'd appreciate the "accepted". – Transistor Jan 12 '16 at 18:22
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If the dimmer controls any recessed lights it may be that the dimmers temperature limit switch is turning the power off to the light. This happens when the bulb is of a higher wattage than the fixture is rated for.

  • This happens for me even if the light is light is turned on for the first time (i.e. from cool). Also it also happens on a unit which doesn't control recessed lights. – Louis Sep 9 '15 at 11:33
  • @ojait, that is not what the OP describes. It's not a temperature problem. He can (by fiddling) adjust the setpoint and the lamps will run until interrupted. See my answer. – Transistor Jan 9 '16 at 13:51
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Sounds like the dimmers are burnt out. You may have 'dirty' power and that has shortened the life of your digital dimmers. I'd suggest swapping them out for the type that have a slider switch. I'd only install digitals where I'm willing to replace them every few years.

If you continue to have problems you might want to go with a resistance dimmer (the oldschool type with a knob) instead of triac dimmers. Then again, if your power keeps burning them out you might want to find out why; power spikes, high/low voltage, transient current, a poor ground, etc.

Trying a standard switch first will tell you if the rest of the install is in working order.

  • They're not 'burnt out'. He says they're working but not at all control points. Resistance dimmers are not a good solution. At half power they will dissipate the same power as the lamps. – Transistor Jan 8 '16 at 23:50
  • @transistor - Perhaps 'failing' would be a better word, but half working is burnt out IMO. I mention resistance dimmers only as a stopgap solution. – Mazura Jan 9 '16 at 0:06
  • I doubt one can buy resistance dimmers and I certainly wouldn't put one in my wall as it would present a fire risk. I've posted an answer explaining a likely cause of the problem. – Transistor Jan 9 '16 at 0:44
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I just ran into this same problem with a dimmer switch connected to a ceiling fan. Turns out that the problem was being caused by current limiting devices now required in ceiling fans in the US. https://www.americanlightingassoc.com/Downloads/Technology/CeilingFanInfo_9-08-1-.aspx

These fans are required to limit power going to the lighting to be less than 190 watts, so for a 4 bulb unit each bulb must be less than 32 watts. This same limiting device can also cause high power (eg incandescent) bulbs to blink on/off several times when you first turn them on.

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