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#126924 - 03/10/01 01:31 AM Dimmer Switch
T, T, Ness Offline
Junior Member

Registered: 03/10/01
Posts: 3
1.) What is a full-wave-form type dimmer switch?

2.) What do the following dimmer output specifications mean:

Rise Time: >300 microseconds
Overshoot: <400 Volts (peak)?

3.) What is a "ghost" load, as it relates to insuring proper, full-range dimmer operation?

Thanks, in advance, to anyone who can field any of these questions for me.

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#126925 - 03/10/01 03:55 AM Re: Dimmer Switch
Scott35 Offline

Broom Pusher and
Member

Registered: 10/19/00
Posts: 2724
Loc: Anaheim, CA. USA
I'll take a shot at this

1: A full wave dimmer would use a Triac, as opposed to a single SCR. This would work on both waves of a circuit [the waves of both flow directions].
A full wave bridge device would work on both waves [directions] and on each wave above and below the center line [similar to a full wave bridge rectifier]. It too, would use Triacs if the lamp was to continue receiving AC, or 4 separate SCRs, if the lamp could use DC.
A Half wave device [dimmer] would use an SCR and only control one wave, or flow direction, and only the part above the zero line.

2: Time rise would be the time it takes for the "triggor" circuitry to switch on the Triac, or the SCR. Could also relate to the minimum amount of dimming [the smallest wave that can pass the Thyristor {Thyristor is an SCR, Triac or Diac}].
Overshoot is spike level, or maximum voltage that can be seen at either the triggor circuit, or the thyristor. Peak is the level on either the wave above, or below the zero line, to the fullest amplitude of the wave [the peak part of the wave].
Typical voltage meaurements are done using the RMS value of the wave, which is 0.707 of the peak value. When you say your house has a 120 Volt circuit, you are referring to the RMS voltage.
Diodes [LEDs, SCRs, Triacs, etc] see the entire wave at all points, so they are rated for peak values [typically peak inverse voltage].
Rectifiers use the average value, which is 0.63 of the peak value.

3: Ghost load is either the minimum, or maximum Resistance connected to the output of the dimmer. This would be the Incandescent lamps that are being dimmed.
Ghost load is a value between a low and high limit that will determine how much current will flow with the dimmer set for the lowest dimming setting available before the thyristor ceases to conduct.
If you have seen dimmers with LED indicators on it, these use the "Ghost Load" of the connected lamps to connect the AC power to them. There cannot be too little of Resistance [like using more than a total of 300 watts worth of lamps], nor can there be too high of Resistance [like using only one 5 watt lamp].
That determines the level of current that can flow outside the dimmer [in a round about way ].

If you have absolutely no idea of what I am talking about here , please reply and maybe with help from others in the group, I can explain these things!

Scott SET
_________________________
Scott " 35 " Thompson
Just Say NO To Green Eggs And Ham!

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#126926 - 03/10/01 04:10 AM Re: Dimmer Switch
sparky Offline
Member

Registered: 10/18/00
Posts: 5545
Scott,
this is a good Q in the respect that many field electricians are always having to correct homeowners that confuse dimmers with fan speed controllers.

One is often used for the other application, which we can usually quickly pick out and correct.

It's the WHY part that is hard to explain

T,T, Ness, i realize this is a diverting a little here, sorry


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#126927 - 03/10/01 06:18 AM Re: Dimmer Switch
Scott35 Offline

Broom Pusher and
Member

Registered: 10/19/00
Posts: 2724
Loc: Anaheim, CA. USA
Ya, and explaining the Why constitutes in a "crash course" in advanced theory, plus physics, along with semiconductor devices, for the person that wants to know "Why"

In some cases, where the loads are very light, the mixmatch of the two different types will not create major problems or hazards - favoring more towards the motor speed controller being used for Incandescent lamps.

The Primary Thyristor [The actual Triac that the load current flows through and is controlled through] is similar in both applications on low end and small power devices, such as the ones used in the typical house and purchased at Do-It-Yourself type homecenters.
There triggering methods and control techniques are the main difference between the two.

Large power devices are specifically made and designed to that particular application. Mixmatching of these would cause odd operation and possibly quick failure to the control device [???].

What sucks about an Incandescent lamp dimmer [Resistive Only] used for a motor, is it was designed to trigger the True Power parts of a wave, rather than the Volt-Amps [Apparent Power], so the Lag of either current or voltage messes with the control circuit and the Triac - most likely creating rude harmonics.
Another thing is that type of control can set a speed that is at a multiple of the synch frequency. That makes the Induction Motor slow down to below that speed, at which point it can produce torque, which makes it speed up again to once again stall at the synch Hz - over and over and over! Might even jump above the synch speed, then down, then up - due to unstable voltages.

I kind of shy away from explaining the exact Why to someone that mismatched a control device, mainly because you almost certainly have to carry 20 tech books to explain stuff, along with the fore mentioned crash course! But after all is said and done, everything gets shot to heck with the infamous phrase:
"The Guy at The Home Center Said It's OK",
or:
"My nextdoor neighbor said it's OK! He's an Electrical Engineer! I know this because he can run a computer, has a real cool website and rewired his car"
I mean no insult to anyone that is reading this message, whom can use a computer, has a website - cool or lame, or has rewired their own car. This is just something that I hear alot from people and it's kind of funny!

I'll go back to my room now

Scott SET
_________________________
Scott " 35 " Thompson
Just Say NO To Green Eggs And Ham!

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#126928 - 03/10/01 01:20 PM Re: Dimmer Switch
T, T, Ness Offline
Junior Member

Registered: 03/10/01
Posts: 3
Thanks, guys! It's all good.

Is the device about which I've inquired, namely, a full-wave-form type dimmer switch, more commonly, or properly, referred to as a fan speed controller? (I'm aware of the incompatibility of "conventional" dimmer switches with ceiling-fan motor loads.)

It might be of some help to you to know that the device about which I ask is specified for use with a dimmable, lighting-effects, flicker module.

[This message has been edited by T, T, Ness (edited 03-10-2001).]

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#126929 - 07/27/04 09:36 AM Re: Dimmer Switch
james S Offline
Member

Registered: 05/13/03
Posts: 102
Loc: West England
on the half wave dimmers why would you not get a visible flashing output due to the wave being cut on only one side of the zero line?

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#126930 - 07/27/04 05:20 PM Re: Dimmer Switch
ElectricAL Offline
Member

Registered: 10/10/01
Posts: 615
Loc: Minneapolis, MN USA
James,

There are a pair of SCRs, one for the positive half cycle, and one for the negative half cycle.

I think of the basic graph from my electricity textbook:

This shows what a standard $8.00 600 Watt incandescent only dimmer waveform looks like. . .kind of. It's actually cleaned up a bit with a nice square turn on "rise of voltage" that smoothly connects to the sine wave.

T, T, Ness, your #2 question goes to the heart of what actually happens when the dimmer switches ON each half cycle. The voltage doesn't go instantly on, rather it ramps up. The straight up and down line actually leans towards the sine wave, so the voltage that was at zero increases at a quick, but predictable rate. When the voltage gets up to the sine wave, the voltage should stop there, but because this is a real world circuit, not a theoretical circuit the voltage overshoots, falls back to the sine wave, undershoots, returns to the sine wave, overshoots. . .well, it settles to the sine wave, but it takes extra circuitry to get it to do it with very little oscillation.

Compared to the sinewave, this over - undershoot oscillation dies out quickly, but it does it every time the dimmer switches on each half cycle.

Rise Time is the amount of time that the voltages ramps up to the sinewave.

For Question #1, the effect of the full wave dimmer is to raise and lower the voltage of the entire sinewave keeping the sinewave intact, rather that keeping it off for a part of each cycle. Noticably absent is the sudden switch on of the rapidly rising ramp of voltage.

Now, that rapid rise time turn on is a curious animal, electrically. It is, in effect, a collection of high frequency sinewaves, much higher than the 60 Hz power sinewave. Even though the rise time of the "turn on" is short, it can have destructive effects on magnetic loads or electronic loads.
_________________________
Al Hildenbrand

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#126931 - 07/30/04 03:38 PM Re: Dimmer Switch
Trumpy Offline

Member

Registered: 07/05/02
Posts: 8540
Loc: SI,New Zealand
Sparky,
 Quote:
this is a good Q in the respect that many field electricians are always having to correct homeowners that confuse dimmers with fan speed controllers.

I hear you there!.
I've had a rash of these over the last few months, cooked light dimmers all over the place!.
One house I went to had a big black explosion mark on the wall around where the dimmer plate was, after the HO tried to dim a twin 5ft T8 Fluorescent fitting, with a 100VA incandescent dimmer.
Silly boy!.
_________________________
Let's face it, these days if you're not young, you're old - Red Green grin

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#126932 - 08/03/04 12:50 PM Re: Dimmer Switch
uksparky Offline
Member

Registered: 07/06/04
Posts: 199
Loc: UK
 Quote:
One house I went to had a big black explosion mark on the wall around where the dimmer plate was, after the HO tried to dim a twin 5ft T8 Fluorescent fitting, with a 100VA incandescent dimmer.
Silly boy!.


LOL

Heeh...makes a hell of a mess that doesn't it!!

Thanks for posting your graphic/explanation! Clears things up nicely.
_________________________
If hindsight were foresight, we'd all be millionaires!

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#126933 - 08/03/04 01:07 PM Re: Dimmer Switch
pauluk Offline
Member

Registered: 08/11/01
Posts: 7693
Loc: Norfolk, England
 Quote:
One house I went to had a big black explosion mark on the wall around where the dimmer plate was, after the HO tried to dim a twin 5ft T8 Fluorescent fitting, with a 100VA incandescent dimmer.

Hey, we can get the same effect here when running a normal 100W incandescent bulb.

Just use a 100VA Powerlektric brand dimmer!

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