Dimmer turn on phase angle

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gar

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131026-0757 EDT

If you have a delay to turn on type of dimmer or similar device, a DVM, a diode, and a sine wave voltage input, then how can you measure the phase angle of turn on?

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gar said:
131026-0757 EDT

If you have a delay to turn on type of dimmer or similar device, a DVM, a diode, and a sine wave voltage input, then how can you measure the phase angle of turn on?

.
Click to expand...
If you mean without signal info, then you can't. The voltage must be sampled and a reference established. I'm sure I did not follow your question since you know this already.
 
131026-0906 EDT

mivey:

No scope, no trigger, or timing reference is required. Just a thought process of how it can be done with simple tools.

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Last edited:
Dimmer Turn-On Angle

Dimmer Turn-On Angle

Gar...

With math! Assuming sine-wave supply, measure Vrms across load! Compare it to rated! Then back-calculate to find turn-on angle!

Easier to do if Arms is known!

Regards, Phil Corso
 
Phil Corso said:
Gar...

With math! Assuming sine-wave supply, measure Vrms across load! Compare it to rated! Then back-calculate to find turn-on angle!

Easier to do if Arms is known!

Regards, Phil Corso
Click to expand...
With the available measurement tools, that's the only way, but will add that the result will only be an approximation.

But I have to ask how knowing ARMS makes it easier?
 
gar said:
131026-0757 EDT

If you have a delay to turn on type of dimmer or similar device, a DVM, a diode, and a sine wave voltage input, then how can you measure the phase angle of turn on?
Click to expand...
If your DVM is true RMS you can calculate it from the RMS voltage.
I've produced spreadsheets where the delay angle can be programmed in and it gives the RMS. If you know the RMS, you can work back to the angle. Or use the solver facility.
Or use an oscilloscope.
 
131026-1745 EDT

There may be no single answer to my original post question, but with the suggested items it is likely there is only a single answer.

This was not intended to be any sort of trick question, but just one to stimulate thought, and work from basics.

My problem was I did a calculation on the RMS current or voltage of a sine wave turned on at the 90 degree point, and I wanted a way to verify that my calculation was probably correct.

The method I used was to connect a diode between the chopped sine wave and a DC voltmeter. Thus, measure the average voltage of a sine wave over a full cycle period.

The average value of a full wave rectified sine wave is 0.636 of the peak value. The average of the waveform of interest is 0.5*0.5*0.636*Peak. If the input sine wave is 120 V RMS, then the average voltage of the half wave rectified 90 degree chopped sine wave is 27.0 V DC. The RMS value of the waveform is the square root of ( (1/2)-(1/4) )*0.707)*Peak = 0.707*RMS = 0.707*120 = 84.8 V RMS.

Experimentally I got results sufficiently close to check my calculation.

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gar said:
131026-0906 EDT

mivey:

No scope, no trigger, or timing reference is required. Just a thought process of how it can be done with simple tools.

.
Click to expand...
Without a reference, you simply can't; not to any real degree of accuracy (no pun intended). Assume you have a good waveform and that we can deduce the half wave or full wave, and assume a frequency, and assume the DVM is accurate enough (it probably isn't). Without a reference you are not sure what you have.

For example: If you assume 120 Vrms but really have 125 Vrms, the turn-on phase angle could be around 42.67 degrees but you will measure 120 Vrms and think the angle is zero.

How about a very small voltage difference? Let's use 1/10th of a percent: If you assume 120 Vrms but really have 120.12 Vrms, the turn-on phase angle could be around 12 degrees but you will measure 120 Vrms and think the angle is zero.
 
Besoeker said:
Or use an oscilloscope.
Click to expand...
Which would be the easiest way. Same result could be obtained from a trigger and timer circuit but the o-scope is readily available. Perhaps DVMs with a reference reading could be used to compare the un-chopped waveform rms value with the chopped waveform rms value but I would not think the DVMs are accurate enough.
 
Speaking of accuracy, since the gradient is changing, using rms values to find the angle will mean the accuracy of the result is not consistent.
 
131026-2318 EDT

mivey:

My goal was not to find an optimum high accuracy means to make the measurement. For that I would probably use a zero-crossing detector and a counter timer. Rather I wanted a quick and dirty means to verify that I had not made a calculation error in the calculated determination of an RMS value. With my scope presently dead it could not be used. Input source voltage can be controlled to 0.1 V at 120 V. Then the average DC voltage for the experiment can be calculated. Phase shift adjusted to get this value, 27 V, can be done to 0.1 V. Now read the RMS voltage of the waveform. Accuracy good enough to verify no major error in the RMS calculation.

Some error points are:
Non-simultaneous readings and some line voltage fluctuations.
Voltage drop across the dimmer Triac.
Voltage drop across the DC measuring diode.
Waveform distortion.
Different phase angles will result in different accuracies.

The intent of the post was simply to initiate a discussion, which it has.

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gar said:
131026-2318 EDT

mivey:

My goal was not to find an optimum high accuracy means to make the measurement. For that I would probably use a zero-crossing detector and a counter timer. Rather I wanted a quick and dirty means to verify that I had not made a calculation error in the calculated determination of an RMS value. With my scope presently dead it could not be used. Input source voltage can be controlled to 0.1 V at 120 V. Then the average DC voltage for the experiment can be calculated. Phase shift adjusted to get this value, 27 V, can be done to 0.1 V. Now read the RMS voltage of the waveform. Accuracy good enough to verify no major error in the RMS calculation.

Some error points are:
Non-simultaneous readings and some line voltage fluctuations.
Voltage drop across the dimmer Triac.
Voltage drop across the DC measuring diode.
Waveform distortion.
Different phase angles will result in different accuracies.

The intent of the post was simply to initiate a discussion, which it has.

.
Click to expand...
While we are identifying error sources, I would add the accuracy of the true RMS measurement with a sharp edged, high harmonic, waveform.
Probably a small source of error.
If your meter can give you a straight average value for the DC you could calculate from that too.
 
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