Triacs and Optocouplers

[plasmadrive]

Oh BTW, I had a few minutes today to give this some thought. As you mentioned the voltage goes from a positive 110v to a negative 110v one one hot wire and mean time the other hot wire goes from a negative 110v to a positive 110v. So between the two hot leads you have 220v. But we are working with voltages that would be found in a home. And in a home you will also have a Neutral wire. This wire is a reference point, it does not swing from positive to negative, but the hot lead does. Thus, I will not step over to the dark side....

Max,

I don't think you posted this in response to my other postings but I can shed some light on it for you.

 

If you put a scope (isolated of course) from N to L1 you will see that the AC waveform does indeed swing in a sinusoidal pattern where the peaks switch directions and are 180 deg out of phase with each other.  Same as if you were to look between L1 and L2 just half the voltage.  Remember when doing this you are using one of the phases as your reference point.

 

If however you used the N as your ref point, (which is at the center tap of the power transformer), L1 and L2 would be each doing the same thing but in opposite polarities to each other (180 deg out of phase) at any given time.  A dual trace scope would show you those results. Now you are using the N or center tap as your reference point. 

 

Does that help?

 

[-klb-]

.637 x Vpk = avg value of the full wave pulsating unfiltered DC waveform.  Remember, pulsating DC is measured in terms of Avg and not RMS.  So you are correct about it not connecting to RMS. 

Much of the misconception you may be having Max might have to do with terminology.  For example, you don't measure capacitance values in terms of nanometers.. you also don't measure pulsating DC in terms of RMS. 

 

Actually, what value is useful will depend heavily on what you are trying to do with it. 

 

I suppose average voltage of pulsing DC might be a decent representation of expected motor speed. 

 

If it is square wave, it should still be a good representation of power which will heat a resistor or light bulb.  But if power into a linear load is what you are interested in, and the waveform is not square, RMS is still relevant.

 

But even with a square wave, if you have a non linear load, average is only useful if it is known relative to peak, and I would rather have it as peak and duty cycle personally.  At some level, this will tie back to how the meter is expected to behave, so you have some idea what its readings are really telling you, given the system you are trying to validate.

[plasmadrive]

KLB, I disagree.. to a point.

 

Leaving square waves out of this since we are not using them....

 

The RMS value is useful only for AC as the avg value of and AC sign wave is 0.  The RMS value does give you the usable power equal to the Avg of the pulsating DC.  That I agree on.  But Avg is used for DC and RMS for AC.  Terminology counts.

[Max-Paul]

Plasma,

 

I suppose in order to remain friends with you, I am just going to say that we will have to agree to disagree.

1st point, is that Why is it that if you check the voltage across a full wave bridge is 110v but not across a half wave? it does not matter if you have one or two peaks, they are both the same voltage. And I agree with what KLB was saying. With a square wave and it is running 50% duty cycle. The avg voltage will be one half. But with a sine wave you have to use RMS, even if it is pulsed D.C. the wave is not square but sine. Ok so you have a 50% duty cycle, but there is a rise time to full on and a fall time to full off and you have to take this into account, you can not dismiss this issue. Thus you can not apply your avg numbers. You have to apply my RMS numbers.

 

Seriously guys you need a refund on your EE degrees.

[Max-Paul]

Max,

I don't think you posted this in response to my other postings but I can shed some light on it for you.

 

If you put a scope (isolated of course) from N to L1 you will see that the AC waveform does indeed swing in a sinusoidal pattern where the peaks switch directions and are 180 deg out of phase with each other.  Same as if you were to look between L1 and L2 just half the voltage.  Remember when doing this you are using one of the phases as your reference point.

 

If however you used the N as your ref point, (which is at the center tap of the power transformer), L1 and L2 would be each doing the same thing but in opposite polarities to each other (180 deg out of phase) at any given time.  A dual trace scope would show you those results. Now you are using the N or center tap as your reference point. 

 

Does that help?

 

Even you in your reply say about the same thing I have been saying all along. the voltage from L1 to L2 = 220V and L1 or L2 to N will be half of L1 to L2, which just so happens to be 110 V. So, why in the Lords green earth do you and A31ford have problems seeing that just one half of the sine wave is still 110v RMS or abt 155V peak. Just cause you are only conducting one half of the sine wave does not change the voltage. Just the power available over the time that would normally be two conduction periods.

[a31ford]

Max... I really hate to do this (Sorry), 

 

the AC cycle... (forget about transformers)

 

Lets look at one simple sinewave, 0 to the top of the peak (60 volts RMS), back down to 0 again, NOW we swing under 0 and down to the peak facing down (valley if you will) NEGATIVE 60 volts RMS.

 

NOW, throw a diode or an SCR in the mix (in-line) it will block 1/2 of the waveform (depending on which way the band or cathode is oriented.

 

"TA-DA" only have the wave, therefore only half the voltage. (60, thereabouts) 

 

Greg

[Max-Paul]

Grasshopper,

 

You still have a misconception about what the voltage is... It appears you can not apply logic and thus come up with the correct answer. Quick apply for that refund. You and plasma can holler 60 volts from the tops of your roofs. I will not be mislead by those who can not understand that there volt meters will not read properly with dirty D.C. voltage. You still have not looked at the waveform on an OScope as I have suggested, have you?

[a31ford]

54-60-66, somewhere thereabouts. we are talking RMS (what we call "nominal AC voltage in North America") one half of the wave 30 cycles of the AC is in a positive state with reference to neutral, the other 30 cycles are at a negitive value when referenced to the neutral.

 

IN other words, all the meters (and scopes) that I have worked with do NOT deal in VARS, Nor do they deal in IPP.

 

RMS or DC, that's it Max, call it what you will.

 

Oh, BTW, do you know WHY the voltage goes up when you place a capacitor after a full wave  (or even just a diode) rectifier ? I do.... It's called "Summing effect" it has NOTHING to do with "Dirty DC voltages"

 

IPP converted to DC is the correct way to think IF (and only IF) you are attempting to run a DC appliance (LED's for example) on an AC circuit.

 

There IS a forward voltage drop in EVERY type of DIODE, (some are lower than others) Most of the lighting I've dealt with takes this into account.

 

If you want to build your own LED strings, go ahead, no one is going to stop you, HOWEVER, why must you "hijack" MY thread about Triacs and Optos ? to speak about your own stuff ??

 

60 volts is still 60 volts in my books.

 

Greg

[Max-Paul]

See Grasshopper,

 

You really dont understand what you are talking about. A cycle is both positive and negative. A whole sine wave is a cycle.So in one second you have both 60 positive and 60 negative pulses. It was not a hijacking of your thread to speak about my experience with working with diodes. It was to give you an example of why I am right about your statements concerning triacs.

 

I would not have said a word if you had not said that output of a triac is 30 hz and 60 volts. But when you became confused on these items I came in to clarify the actual values. I have lead you to water. Not my fault if you are going thirsty. I have done my part and done with this.

[a31ford]

I give up, you can have the thread for what ever you want it for, 

 

I had corrected myself in post #6, I had meant that 1/2 of the waveform was positive going, and half was negative going (and being real tired and not really thinking about it, I had said 30 and 30).....

 

For the second time, SORRY for the mistake.

 

Max, you are still wrong about the waveform thing and the voltage, BUT like I said,,,, I give up.

 

Greg

[plasmadrive]

Plasma,

 

I suppose in order to remain friends with you, I am just going to say that we will have to agree to disagree.

1st point, is that Why is it that if you check the voltage across a full wave bridge is 110v but not across a half wave? it does not matter if you have one or two peaks, they are both the same voltage. And I agree with what KLB was saying. With a square wave and it is running 50% duty cycle. The avg voltage will be one half. But with a sine wave you have to use RMS, even if it is pulsed D.C. the wave is not square but sine. Ok so you have a 50% duty cycle, but there is a rise time to full on and a fall time to full off and you have to take this into account, you can not dismiss this issue. Thus you can not apply your avg numbers. You have to apply my RMS numbers.

 

Seriously guys you need a refund on your EE degrees.

Max, you can disagree all day long and it will not upset me.   As long as we are have a good discussion and not a battle I'm game. 

 

To your 1st point, it matters because of how the meter reads it and displays it.  The Vpk it the same, but the Vavg is not. The Vavg is really what allows the "work" to be done, not the Vpk.

 

Let's take your square wave, (though we are not using them except for PWM but that is another thread).  If the square wave is on from 0v to some value either pos or neg but not both, then it would be a Vavg of 50% of Vpk.  If however your square wave goes alternating (pos and neg) with a 50% duty cycle and equal voltages in both directions, your Vavg is ZERO!  That is where you would use RMS.  Again, lots of this has to do with correct terminology and math.   I haven't "done the math" in a very long time.. but it still remains the same. 

[plasmadrive]

Even you in your reply say about the same thing I have been saying all along. the voltage from L1 to L2 = 220V and L1 or L2 to N will be half of L1 to L2, which just so happens to be 110 V. So, why in the Lords green earth do you and A31ford have problems seeing that just one half of the sine wave is still 110v RMS or abt 155V peak. Just cause you are only conducting one half of the sine wave does not change the voltage. Just the power available over the time that would normally be two conduction periods.

Because Max, it is NOT 110v RMS.  By your same logic, if I had one pk in say 20 seconds, the voltage would still be 110v RMS.  Doesn't work that way. 

 

You say "Just cause you are only conducting one half of a sine wave does not change the voltage"... actually it does.  I does not change the Vpk, but it does change the RMS voltage. 

Max, go to this web site and do a bit of reading.  It has a lot of math (hell, I don't even remember how to do some of that math), but it will explain better what we have been unable to do. Hopefully that you help clear it up in your mind. 

http://en.wikipedia.org/wiki/Root_mean_square. 

[a31ford]

Plasma, that link truncated, however, It did lead me here http://en.wikipedia.org/wiki/Alternating_current#Power_and_root_mean_square

 

And I thank you for that....

 

Gawd, my math has, well, been forgotten somewhere in my last 54 years. (J/K) But that is intensive)

 

If you only new about one of my other hobbies..... (It includes playing with REAL Plasma (the hot stuff, NOT the stuff in blood).

 

Greg

[plasmadrive]

That one is good too! 

 

This one I was trying to post.  (hope it works this time)

 

http://en.wikipedia.org/wiki/Root_mean_square 

[-klb-]

And going back to RMS, and the importance of terminology, even on positive only waveforms, there is usually a difference between RMS and average value.

 

Take a sawtooth waveform, 220V at peak, declining to zero, and immediately repeating.  This has an average voltage of 110V.  But it has an RMS voltage of 126 V RMS

 

If I place this across a 126 ohm resistor, do I get 1A of current, or do I get 0.873 A?   Am I dissipating 126 W, or 96.03W?    Or are you doing more adjustments to finally get back to 126W, or do you think it is  some other level of power dissipated?

[Ken Benedict]

+1

[plasmadrive]

And going back to RMS, and the importance of terminology, even on positive only waveforms, there is usually a difference between RMS and average value.

 

Take a sawtooth waveform, 220V at peak, declining to zero, and immediately repeating.  This has an average voltage of 110V.  But it has an RMS voltage of 126 V RMS

 

If I place this across a 126 ohm resistor, do I get 1A of current, or do I get 0.873 A?   Am I dissipating 126 W, or 96.03W?    Or are you doing more adjustments to finally get back to 126W, or do you think it is  some other level of power dissipated?

klb, 

I believe that in this case given your values for RMS and AVG,  Pavg = Vrms x Irms  If I got that correct, then in this case, 126W is the AVG power.   You get 1 amp of RMS current....

 

But, to be honest about this, I don't remember for sure... been a very long time since I had to do any of these calcs. 

 

You have hit my ceiling for brain power... We now return you to your regularly scheduled program.

 

[-klb-]

But I thought in response to me talking about RMS of rectified sine waves a few days ago that you said that using RMS for dc waveforms was incorrect, and that only average applied for DC waveforms?

[plasmadrive]

But I thought in response to me talking about RMS of rectified sine waves a few days ago that you said that using RMS for dc waveforms was incorrect, and that only average applied for DC waveforms?

You mean this?  "The RMS value is useful only for AC as the avg value of and AC sign wave is 0.  The RMS value does give you the usable power equal to the Avg of the pulsating DC.  That I agree on.  But Avg is used for DC and RMS for AC.  Terminology counts".