Light dimmer switch; can failure just cause lack of bright lights?

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Today I realized that our light dimmer switch, when set at the highest point, does not seem to get the lights as bright as they had previously been. I should point out that the power was also out a bit today in my area. I am suspecting that perhaps a power surge or else maybe just the age of the switch may have caused this problem to "come to light", but I had though that if the switch failed, the lights would simply be at their brightest and that the dimming mechanism would not work.
I am going to replace the switch anyway, but wanted to check out if my suspicions were correct; can the dimmer fail so as to result in a mid- level brightness of the lights?
Thanks for any help! Paul
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On Tue, 06 Nov 2007 23:33:22 +0000, Paul Soderman wrote:

Dimmer switches are no more than rheostats. If carbon were to build up on part of the switch, it may be sufficient to affect voltage. Might cause a voltage drop.
You could test this with a volt/ohm meter.
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franz frippl wrote:

In ye olde days, you were right. However for many years, the triac-based dimmer switch has been the more common. There is a rheostat used in the dimmer, but it it is used to control the triac. For a more detailed explanation:
http://home.howstuffworks.com/dimmer-switch6.htm
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"More common"? Even in ye olde days, rheostat dimmers were extremely rare, and they're now essentially non-existant.
Hint: rheostats have to dissipate a _lot_ of power, and a switch box is not a good place to try doing that.
A rheostat capable of, say, dimming a 100W light bulb probably wouldn't fit into a switch box, and if it did, you could use the wall around it as a cooking surface.
Dimmers tend to fail in one of two ways:
1) Complete failure, no light at any position. 2) triac control fails, get erratic behaviour.
I suspect that the OP is simply seeing a brownout.
--
Chris Lewis,

Age and Treachery will Triumph over Youth and Skill
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Chris Lewis wrote:

Chris, I suspect that your definition of 'ye olde' and 'many years' is more short term than mine. The SCR which enabled solid-state control of dimmers was not invented until the late fifties. Rheostat dimmers and auto transformers (Variacs) were the *only* means of dimming lights until then. So they were hardly rare. Solid state dimmers were not commonly used in homes until the late sixties. But, you are right about the rheostat dimmer's heat output. Most were mounted in a box on the outside surface of a wall, not in a switchbox.
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Probably not. I'm older than I look ;-)
In junior high school we had an electronics lab that a couple of us spent time in. We did some stuff with SCRs. Triacs weren't available then I don't think.
Actually, most of what we did was with vacuum tubes. I built a radio telescope out of a vacuum tube IF strip out of a 50's B&W TV set and used an oscilloscope to display the results. 150' baseline on the antennas allowed me to do interferometry work.
Worked pretty good ;-)

Not quite, but we'll take that as a given.

How many houses had any dimming whatsoever prior to SCR/triac dimmers? How many houses had more than one? How many of these were used in general purpose things rather than very specific (and usually low wattage) fixtures?
I would submit that up until the late 60's, general purpose dimming circuits were almost non-existant in residences. What dimming circuits there were were expensive heavy-duty industrial units, or low wattage (usually considerably less than 50W), often a part of the fixture itself, or the cabinet it was installed in.

And variacs are much larger & heavier. Saw a brand new one a few days ago. 600W IIRC. It weighed about 25 pounds, and was bigger than a regular two slice toaster. >$200 IIRC. Hadn't seen one in a very long time.
I don't think too many people had those in their house to dim the mood lights.
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Chris Lewis wrote:
...

... In 1963 my family moved into a (new) house that had a dimmer switch for the dining room ceiling light. While it was about the same size as today's dimmers, it was not today's 4 component circuit. I remember looking at the schematic. There were a dozen or more components, including a transformer. I didn't know enough then to understand the circuit, so I can't offer more details.
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M Q wrote:

Hi, And big heat sink?
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The classic "basic" dimmer circuit published back in the late 60's and earlier consisted of a SCR (later triac), a diac (looks like an ordinary semiconductor small-signal diode, and my rusty memory seems to indicate it's shown in schematics as two paralleled back-to-back zener diodes), rheostat (variable resistor with the knob ;-) and another resistor or two. The other components you saw (including the transformer) were probably noise filtering which is considered an "optional add-on".
More modern circuits do with less (or no) noise filtering, and use a resistive network instead of a diac to trigger the triac IIRC. SCR-based dimmers need a lot of filtering, and triac-based dimmers need less - since modern circuits don't use SCRs, the need for noise filtering is less.
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Chris Lewis wrote:

Hmmm, Remember first ever popular transistor RCA CK722 PNP Ge junction type? When I got one of them at such a high price and built a small receiver with it, what an excitement. Was back in the '50s. Now I am using florescent lights and dimmers in the house. Waiting for LED lights to become cheap in price. Tony, VE6CGX
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Chris Lewis wrote:

I started in ham radio in the early fifties and most everything was homebrew or converted military. got into solid state design as an EE in early sixties. later ttl, lsi, vlsi, computers, etc. In the fifties, rheostat and switched resistor dimming of lights was common in higher end housing. I installed some myself. Not exactly complictated circuitry.

Yes, they're still available and the Variac brand still exists. However auto transformers are not large if only a couple hundred watts. As I said earlier resistive type dimmers did not mount in inwall switch boxes, but in their own enclosed panel/box on the wall surface. (to accomodate the heat generated).

No, not common, but by no means a rarity. Even nowadays with the cheapness of triac dimmers, think about how small a percentage are used. Especially where they'd be most useful. I'll bet they're used in less than a few percent of homes and apartments.
I use dimmers in many switches in my house. They pay for themselves in lower juice use and longer bulb light. I've got some hard to reach ceiling fixtures with bulbs that have lasted twenty years. I prefer the rotary to the lever type to slow down that filament killing inrush and shock.
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I'd think the contrary - virtually all new housing will have at least one dimmer, and it's a very popular add-on. This is vastly more than the old rheostat stuff. I think I'm justified in calling something that rare "rare" ;-)

Given that many rotaries have "full bright" and "full off" adjacent at one end of the travel, you can't escape abrupt on/offs with them :-(
We're switching mostly to decora-style outlets/switches, and rotary dimmers look stupid in decora. At least slider dimmers "slow start" better than straight switches.
Have you seen slow-start LV fixtures yet? I replaced a burned LV supply in an Ikea track light a few months ago with a new LV supply bought off the Internet. We liked the fixture, and replacing the whole thing would have been difficult - discontinued product. Not only was it higher power and smaller size, it has slow-start - takes a bit less than 2 seconds I think to get to full bright. _Very_ nice feature. Those bulbs should last about forever.
--
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Chris Lewis wrote:

Hmmm, You ought know then vacuum tube version of SCR, the Thyristor! Also include Diac as well on solid state side.
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A thyristor is not a vacuum tube. Thyristors are semiconductors - "four layers of alternating N and P-type material" according to Wikipedia. Some sources consider thyristor to be synonymous with SCR, others have thyristor include GTOs, Triacs, SJTs, SITH and MCTs too.
Didn't have much call to play with vacuum tube switching circuits, power or otherwise - most of my playing around with tubes was RF, with RF mid-power amp tubes and 0D3-type regulator tubes being the more exotic stuff.
That said, we may be getting access to a 60' radio telescope dish to do a little serious radio astronomy. The dish steering system is a vacuum tube and electric motor implementation of an H-bridge, similar to that used to steer things like shipboard radars. The modern semiconductor version is used in the small scale for hobby robots.
This has an example circuit for the small scale: http://tinyurl.com/39xw85 Here's an IC version: http://cache.national.com/ds/LM/LMD18200.pdf
The radio telescope has vacuum tubes controlling an AC motor which in turn generates DC for the actuator DC motors. On the order of at least 5 (and probably 10 or 20HP) apiece... [Times two for the two axis]
[The unit was designed for tracking satellites back in the 60's, so it was designed for rather faster traverse than is necessary for radio astronomy.]
We don't believe the vacuum tube portion will still be working (the dish been locked for ~15-20 years), so we're thinking that we will have to replace everything but the movement motors with modern solid-state very high power H-bridge modules. Eg: a hundred A at 600V or something silly like that.

I've played with those (and mentioned them backthread)
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Chris Lewis wrote:

He meant thyratrons, I think the 2d21's were the smallest ones and I don't remember the number of three huge ones that controlled our Lepel 400KW 250KHz alloy bearing surface welders.
It gets hard to remember all these names when you're old enough to remember real power control. Also, no one mentioned carbon piles, another version of early "dimmers" ;-)
-- larry / dallas
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Yeah, I figgered later he meant thyratron.
The closest I got to those (which isn't very close) is to the particle accelerator in the basement of the University of Toronto that, if I recall correctly, was doing pulses in the megawatts range at 400Mhz with Klystrons.
Aside from that, my exposure to the "big" stuff was multi-kilowatt tube transmitters.

You got that right.
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Bogus wrote:

That information is bogus. Dimmer switches are solid state devices that chop of part of the AC sine wave. I suppose they could fail but I have one in a homemade electric blanket control that has been working for 25 years.
---MIKE---

>> (44 15' N - Elevation 1580')
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They most certainly can fail. Being semiconductor devices, they can fail real fast too. An accidental short on the output side of a dimmer that's too fast even for a breaker to notice can fry a dimmer.
Electric blankets are nice "safe" pure resistive devices that are relatively friendly to semiconductor dimmers. Lightbulbs are worse. There are things that are worse still. Like motors. The motor-rated dimmer on our ceiling fan died a few years ago (installed in 1984).
I've seen a handful of dimmers die within days of being installed, or be dead when the circuit is first turned on. Dimmers also occasionally die mechanically.
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franz frippl wrote:

Hmmm, You must be awake from a long snooze or?...... Now dimmers are all electronic device whih controls conduction angle of sine wave. The failure mode can be anything.
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Paul Soderman wrote:

Yes, The triac (controlling device) has two "sides" that control both the positive and negative cycle that makes up the "AC" that lights the lamp. One side can fail, and at the full level setting, only half the power reaches the lamp. Just what you described. Time for a new one. You might check how warm the dimmer is getting in normal use. You might have more lamp watts than the dimmer is rated for that caused the failure.
-- larry / dallas
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