Reverse Phase Dimming for LED or ?

[keithrmatt@aol.com]

I understand forward phase dimmers basically use the peak of the sine wave to come one and gradually use the decline of the wave to power off. Reverse phase dimmers use the the gradual ramp up of the sine wave to come on and trun off at the peak of the wave. I also understand the reverse phase dimmer is easire on the LED driver. My question is about 2-wire dimmers (no neutral) and 3- wire dimmers. As I understand, a 3 wire dimmer uses the neutral to determine "zero crossing" of the sine wave for synchronization purposes even if there is no load. A 2-wire dimmers relys on the load passing through it to determine where "zero crossing" is. Why is the neutral a benefit?

 

[GoldDigger]

The two wire forward phase control dimmer does not use the load connection to "find the zero crossing".
It uses it as a reference for a voltage divider between hot and a lower voltage to set the turn on point.
But since an incandescent bulb is non linear and increases in resistance as it heats there will be a point where the current through the bulb causes the voltage to rise sharply and therefore causes the dimmer to fire even sooner.
This gives rise to the effect where the bulb suddenly jumps to medium or high brightness instead of coming on smoothly from off.
The same effect can cause the bulb to wink off on the way down instead of dimming smoothly.
The latter is not as obvious or annoying.
With an independent neutral reference the three wire dimmer does not have this problem.

 

[Sierrasparky]

interesting

 

[gar]

150429-0913 EDT

See the GE "SCR Manual", 5th edition, 1972, page 253 for a discussion about a phase shift dimmer using an RC delay and an ASBS (asymmetrical silcon bilateral switch) to trigger a Triac.

Fundamentally phase information from the AC line is used in this circuit.

The resistance of the adjustable resistor in the referenced RC circuit is so high that any change in the load resistance (lamp bulb) is of no consequence. There is probably no other trigger circuit where the normal load resistance variation would have an influence on the trigger point.

The 1st edition, 1960, had limited information on dimming. There were no Triacs, basically not invented yet. The 2nd edition, 1961, also had no Triacs. The first GE SCR was made in July 1957. The evolution of the Triac occurred over ther period of 1958 to 1963.

See https://sites.google.com/site/trans...ductor-manufacturers/general-electric-history search for Silicon Controlled Rectifier, and separately Triac.

I have a couple C36Bs with a date code of 934 (34th week of 1959) and various others from the early 60s. The C36B was a nominal 10 A device with a PIV of 200 V.

Shortly after 1963 residential dimmers using a Triac became available. I have two GE units in my home from 1965 and they are still in operation. These are two wire dimmers and their only functional problem is when set at a low level and power is lost thay do not restart on re-application of power.

A simple adjustable voltage divider from the AC line is not an effective method of getting phase shift control of an SCR or Triac.

.

 

[GoldDigger]

Note also that circuits using Triacs or SCRs do not lend themselves to reverse (trailing edge) phase shift control, since it is difficult to turn a Triac off other than at the normal current zero crossing.

 

[Electric-Light]

Note also that circuits using Triacs or SCRs do not lend themselves to reverse (trailing edge) phase shift control, since it is difficult to turn a Triac off other than at the normal current zero crossing.

Reverse phase ones are much more complex and they're based on lGBTs

 

[Sierrasparky]

Reverse phase ones are much more complex and they're based on lGBTs

So what is IGBT?

 

[Electric-Light]

So what is IGBT?

With Triacs and SCRs, all you can do is delay the trigger to determine when it will turn on. You can not turn it off externally. It has to wait it out until zero crossing, then triggering delay restarts on the other half cycle.

Insulated Gate Bipolar Transistor
IGBTs can be turned on or off whenever which is how they can operate as forward phase OR reverse phase.

 

[gar]

150501-0913 EDT

keithrmatt:

I don't believe you have a good understanding of how phase shift dimmers work based on your post #1.

The following two sites may help.
http://www.lutron.com/en-US/Education-Training/Pages/LCE/DimmingBasics.aspx
http://en.wikipedia.org/wiki/Dimmer

Fundamentally phase shift dimmers use a fast switching electronic switch (SCR, Triac, IGBT, transitor, triode, thyratron, ignitron) turned on and off in synchronization with the the AC voltage source to modulated the average output current of voltage.

SCRs, TRIACs, thyratrons, and ignitrons all have the characteristic of being able to be turned on by a control electrode, but indpendent of the control electrode do not turn off until the current thru the electronic switch goes to zero.

Only the TRIAC is bidirectional. All the other devices would need to be in the center of a bridge rectifier, or used as a parallel pair where one is reversed relative to the other.

To get full dimming range the turn on angle has to range from 0 to 180 degrees.

.

 

[gar]

150501-1600 EDT

I did some more searching and the following site is quite good:

http://www.littelfuse.com/~/media/e...rol_using_thyristors_application_note.pdf.pdf

.