Forums

Question regarding diac-triac lamp dimmers

Started by Unknown January 7, 2015
Could some electronics guru please hekp ?
Maybe this is a stupid question, but 
here we go.
Simple AC kine based lamp dimmers use a 
diac triac pair with a capacitor that 
gets charged via a resistor chain that 
includes a potentiometer to control the 
charging rate. When the capacitor gets  
charged, it discharges via the diac, and
this in turn triggers the triac. So far 
so good. But the question is -- how exactly 
is the lamp dimming task achieved ? 
Because, when the triac is triggered, 
it conducts the full line voltage between 
its two mains terminals. I understand that
the rate at which the triac switches can 
be controlled, but how is the lamp dimming
achieved ? 
Any hints suggestions would be greatly 
appreciated. Thanks in advance for your 
help.
  
dakupoto@gmail.com Wrote in message:
> Could some electronics guru please hekp ? > Maybe this is a stupid question, but > here we go. > Simple AC kine based lamp dimmers use a > diac triac pair with a capacitor that > gets charged via a resistor chain that > includes a potentiometer to control the > charging rate. When the capacitor gets > charged, it discharges via the diac, and > this in turn triggers the triac. So far > so good. But the question is -- how exactly > is the lamp dimming task achieved ? > Because, when the triac is triggered, > it conducts the full line voltage between > its two mains terminals. I understand that > the rate at which the triac switches can > be controlled, but how is the lamp dimming > achieved ? > Any hints suggestions would be greatly > appreciated. Thanks in advance for your > help. > >
The resistor/capacitor delay causes the triac to only conduct for a portion of the AC cycle, reducing the RMS value of the waveform and thus the RMS current through the lamp -- ----Android NewsGroup Reader---- http://usenet.sinaapp.com/
 daku...@gmail.com wrote:

> Could some electronics guru please hekp ? > Maybe this is a stupid question, but > here we go. > Simple AC kine based lamp dimmers use a > diac triac pair with a capacitor that > gets charged via a resistor chain that > includes a potentiometer to control the > charging rate. When the capacitor gets > charged, it discharges via the diac, and > this in turn triggers the triac. So far > so good. But the question is -- how exactly > is the lamp dimming task achieved ? > Because, when the triac is triggered, > it conducts the full line voltage between > its two mains terminals. I understand that > the rate at which the triac switches can > be controlled, but how is the lamp dimming > achieved ?
** The triac will conduct for the remainder of that half cycle, until the current falls to a low value ( like 50mA ) then go open until it is re-triggered. The circuit you describe will do that at the same point in each half cycle - as set by the potentiometer. If the trigger point is early in the half cycle, the load get near continuous power but if it is triggered late in then half cycle, the load it gets very little power. .... Phil
On Wednesday, January 7, 2015 at 12:57:17 AM UTC-5, Phil Allison wrote:
> daku...@gmail.com wrote: > > > Could some electronics guru please hekp ? > > Maybe this is a stupid question, but > > here we go. > > Simple AC kine based lamp dimmers use a > > diac triac pair with a capacitor that > > gets charged via a resistor chain that > > includes a potentiometer to control the > > charging rate. When the capacitor gets > > charged, it discharges via the diac, and > > this in turn triggers the triac. So far > > so good. But the question is -- how exactly > > is the lamp dimming task achieved ? > > Because, when the triac is triggered, > > it conducts the full line voltage between > > its two mains terminals. I understand that > > the rate at which the triac switches can > > be controlled, but how is the lamp dimming > > achieved ? > > > ** The triac will conduct for the remainder of that half cycle, until the current falls to a low value ( like 50mA ) then go open until it is re-triggered. > > The circuit you describe will do that at the same point in each half cycle - as set by the potentiometer. > > If the trigger point is early in the half cycle, the load get near continuous power but if it is triggered late in then half cycle, the load it gets very little power. > > > .... Phil
Thanks for the clarifications. It appears however that adjusting a potentiometer to hit the correct trigger point would be quite a tricky job. In addition, a typical triac (e.g., BT136) does not have any lower limit on the trigger voltage level and so even a very small (e.g., few 10s of mV) would trigger the triac, rendering the resistor chain(includes the potentiometer) pretty much useless. Wonder why people have dedicated whole Web pages discussing this circuit.
 daku...@gmail.com wrote:

> > > > ** The triac will conduct for the remainder of that half cycle, until the current falls to a low value ( like 50mA ) then go open until it is re-triggered. > > > > The circuit you describe will do that at the same point in each half cycle - as set by the potentiometer. > > > > If the trigger point is early in the half cycle, the load get near continuous power but if it is triggered late in then half cycle, the load it gets very little power. > > > Thanks for the clarifications. It appears however > that adjusting a potentiometer to hit the correct > trigger point would be quite a tricky job.
** Even small children have no trouble adjusting lamp dimmers and fan speed controls. But you might be an exception.
> addition, a typical triac (e.g., BT136) does not > have any lower limit on the trigger voltage level
** Complete bollocks.
> and so even a very small (e.g., few 10s of mV) > would trigger the triac,
** Even worse bollocks. A triac triggers and latches on when a sufficient *current* pulse is applied to the gate - this can range from 10mA to maybe 150mA for a large device. Latching occurs in a few microseconds. The gate voltage when this happens is normally in the range from 0.5V to 0.8V - depending on the chip's temperature and the supply voltage. Where do you get your barking mad ideas from ? ... Phil
>"It appears however >that adjusting a potentiometer to hit the correct >trigger point would be quite a tricky job."
I think he misunderstood the meaning of your question. What they do is use a really high resistance rheostat. yes it is a pot but they only need one side of it for an R/C. The varying trigger currents required by different triacs are taken care of then, ans long as they can assure the thing will be off when the rheostat is at its maximum resistance. The curve of watts to where it is set, they do not care. It is not calibrated. In fact the elcheapo ones that only use two wirse and thus have no reference act as voltage subtractors. (effectively) If you (US) git 120 line voltage and turn it down so the lamp gets 45 volts and then the main voltage goes down to 110, the lamp gets 35. that might not be 100 % accurate because of the dynamic resistance of the filament, but it is damn close. There are other designs for those who spend the money. With the regular ones when you set them low, you will see alot of dimming like when the refrigerator or AC unit starts. It is subtracting a larger portion of the whole voltage.
On 09/01/2015 05:55, dakupoto@gmail.com wrote:
> Thanks for the clarifications. It appears however > that adjusting a potentiometer to hit the correct > trigger point would be quite a tricky job. In > addition, a typical triac (e.g., BT136) does not > have any lower limit on the trigger voltage level > and so even a very small (e.g., few 10s of mV) > would trigger the triac, rendering the resistor ..
It is not really tricky. The triac is triggered by the diac and it is the diac triggering voltage (20-30V ish) that matters. Once the diac fires then the triac gate sees many volts from the capacitor and many milli-amps and gets a strong quick trigger. When the diac is non-conducting the triac gate sees only leakage which is usually negligible. piglet
On Friday, January 9, 2015 at 1:20:07 AM UTC-5, Phil Allison wrote:
> daku...@gmail.com wrote: > > > > > > > ** The triac will conduct for the remainder of that half cycle, until the current falls to a low value ( like 50mA ) then go open until it is re-triggered. > > > > > > The circuit you describe will do that at the same point in each half cycle - as set by the potentiometer. > > > > > > If the trigger point is early in the half cycle, the load get near continuous power but if it is triggered late in then half cycle, the load it gets very little power. > > > > > > Thanks for the clarifications. It appears however > > that adjusting a potentiometer to hit the correct > > trigger point would be quite a tricky job. > > ** Even small children have no trouble adjusting lamp dimmers and fan speed controls. But you might be an exception. > > > > addition, a typical triac (e.g., BT136) does not > > have any lower limit on the trigger voltage level > > ** Complete bollocks. > > > > and so even a very small (e.g., few 10s of mV) > > would trigger the triac, > > ** Even worse bollocks. > > A triac triggers and latches on when a sufficient *current* pulse is applied to the gate - this can range from 10mA to maybe 150mA for a large device. Latching occurs in a few microseconds. > > The gate voltage when this happens is normally in the range from 0.5V to 0.8V - depending on the chip's temperature and the supply voltage. > > Where do you get your barking mad ideas from ? > > > > ... Phil
There are thousands of barking mad people, who for reasons unknown clog the networks and their ISP's Web servers with absolutely nonsensical designs. For example, an imbecile somewhere in Romania or Hungary has put out a design for a switched mode power supply which uses an electrolytic capacitor right on the AC line -- poor capacitor will die in seconds -- but the guy claims his circuit works. I just run into these designs while Web surfing, from time to time. I do have a working single phase AC motor (ones used for table fans and all) speed controller, the design for which I have modified from an design example that a prof discussed in class a few decades ago.
On Saturday, January 10, 2015 at 7:01:47 AM UTC, daku...@gmail.com wrote:
> On Friday, January 9, 2015 at 1:20:07 AM UTC-5, Phil Allison wrote: > > daku...@gmail.com wrote: > > > > > > > > > > ** The triac will conduct for the remainder of that half cycle, until the current falls to a low value ( like 50mA ) then go open until it is re-triggered. > > > > > > > > The circuit you describe will do that at the same point in each half cycle - as set by the potentiometer. > > > > > > > > If the trigger point is early in the half cycle, the load get near continuous power but if it is triggered late in then half cycle, the load it gets very little power. > > > > > > > > > Thanks for the clarifications. It appears however > > > that adjusting a potentiometer to hit the correct > > > trigger point would be quite a tricky job. > > > > ** Even small children have no trouble adjusting lamp dimmers and fan speed controls. But you might be an exception. > > > > > > > addition, a typical triac (e.g., BT136) does not > > > have any lower limit on the trigger voltage level > > > > ** Complete bollocks. > > > > > > > and so even a very small (e.g., few 10s of mV) > > > would trigger the triac, > > > > ** Even worse bollocks. > > > > A triac triggers and latches on when a sufficient *current* pulse is applied to the gate - this can range from 10mA to maybe 150mA for a large device. Latching occurs in a few microseconds. > > > > The gate voltage when this happens is normally in the range from 0.5V to 0.8V - depending on the chip's temperature and the supply voltage. > > > > Where do you get your barking mad ideas from ? > > > > > > > > ... Phil > > There are thousands of barking mad people, > who for reasons unknown clog the networks > and their ISP's Web servers with absolutely > nonsensical designs. For example, an imbecile > somewhere in Romania or Hungary has put out > a design for a switched mode power supply > which uses an electrolytic capacitor right > on the AC line -- poor capacitor will die > in seconds -- but the guy claims his circuit works. I just run into these designs while > Web surfing, from time to time.
I found a 16v cap I'd been running successfully for over a decade at minus 21v once. It was a stuffing mistake and worked fine. So you never know. :) NT