Triacs and Optocouplers

[Max-Paul]

Even pulsing DC voltage will still show a half wave peak of 120 volts. Gee all of those meters do not prove me wrong. I believe I said that it takes a o'scope to show the pulse voltage. And if you try to build your own LED string, you best take that peak voltage in account or you will burn out your LEDs.

 

As to why you are seeing lower voltages is cause of average. And that average is based on either two peaks (full wave) or pure (filtered) D.C. voltage. Anything else is an average. RMS is what energy would be seen if a full wave is rectified.

 

Well looks like we both are old farts. Cut my teeth on a simson 260 without protection. Later years I worked with the 260P. I also have a Fluke 71 and 83.  But anyway you slice it, a half wave is still peaking at 120 volts. Now averaging will lower the voltage seen on a meter, but not an o'scope. And if your such a electronics / electrical guy as you say you are. Then you damn well recognize what I  meant by o'scope.

 

Let me ask you this. How many white LED can you connect up in series and put them across an A.C. line plugged into a wall? I am going to send a message to Plasma with the number I have found out. Will give you a couple this way or that leeway. Lets see if you really know what you are talking about. I have my doubts. So far you have only shown me that you know the surface stuff, not the deep down stuff. Not reconizing the fact that there exsist a peak voltage in your half wave rectifying circuit can burn up things like LEDs.

[a31ford]

.

Max...

 

Sorry, BUT, 120 volts AC is BOTH portions of the AC wave (above 0 volts, AND below) If only 1/2 (above OR below 0 BUT NOT BOTH PORTIONS) the voltage would only be 60 volts.

 

zero ramping up to 60, then ramping down to  zero again, then moving into negative ramp, down to negative 60, then ramping back to zero again. SUM is +60 AND -60 = 120

 

Removing 1/2 of the ramp will only provide 1/2 of the voltage. (AKA diode makes meter show only 60 volts.)

 

IF SCR's provided ALL the voltage, then WHY do we need TRIACS ???????

 

COME ON MAX, 

 

Enough is enough, just admit it already.

 

Greg

[-klb-]

I've forgotten where this even started. But I'm not sure where 120V comes from in relation to peak voltage. Isn't the peak voltage in a 120RMS waveform something like 170 volts?

[-klb-]

To add more confusion to the mess, presuming a linear load, which is sort of implicit in the context of RMS, a string of incandescent lights will behave the same on just the positive (or negative) half cycles of a 120VRMS (clipped sinusoidal) source as on a full sinusoidal 60VRMS source. They will both light up at the same brightness. Though the one on the alternating half cycles will fail somewhat faster, and too much load will cause more heat in the supply transformer.

But, you replace that linear load with a non linear one, and RMS voltage no longer tells you how the system behaves. You plug a typical string of CDI LEDs into the first source, and you get 50% brightness. Plug it into the second one, and the cumulative voltage to meet the current knee for the LED string may not exist at peak voltage, possibly yielding only 2% intensity.

[Santas Helper]

Are we all getting a degree when this is done?

We should after reading all this high-tech stuff going on.   

 

Seriously, this is some great stuff being hashed out.

[Max-Paul]

I've forgotten where this even started. But I'm not sure where 120V comes from in relation to peak voltage. Isn't the peak voltage in a 120RMS waveform something like 170 volts?

Right about the about 170VAC is the peak of 120VAC. I found that when I put a full wave bridge on the wall outlet. I got 155VDC, what I call dirty D.C. Based on that 155 volts I was able to build several custom LED display items for my display. I also use 14mA RMS cause thats what my digital meter reads. For LEDs that max out at 20mA on a clean DC power supply.

[Max-Paul]

.

Max...

 

Sorry, BUT, 120 volts AC is BOTH portions of the AC wave (above 0 volts, AND below) If only 1/2 (above OR below 0 BUT NOT BOTH PORTIONS) the voltage would only be 60 volts.

 

zero ramping up to 60, then ramping down to  zero again, then moving into negative ramp, down to negative 60, then ramping back to zero again. SUM is +60 AND -60 = 120

 

Removing 1/2 of the ramp will only provide 1/2 of the voltage. (AKA diode makes meter show only 60 volts.)

 

IF SCR's provided ALL the voltage, then WHY do we need TRIACS ???????

 

COME ON MAX, 

 

Enough is enough, just admit it already.

 

Greg

I see what you are getting at. But there is something in the back of my mind that will not let me come over to the DARK side. I'll give you this. Once the crunch is over (Thanksgiving night). I'll drag out my cheater cord, Fluke 83 and a Fluke digital O'scope and read up if it is true peak / pk2pk or RMS. So a truce for the time being.  At this time I see someone that didnt realize that it is still 60Hz. So, why should I give in about the voltage? But as I said a truce till I can take time to mess with this and prove or disprove your statement.

[Santas Helper]

 So a truce for the time being. 

 

Noooooooooo, not a truce!!! I haven't gotten my degree yet.   

 

Okay, just kidding.

 

But hope we get to the bottom of this Thanksgiving night.

[a31ford]

Max... 

 

Truce (For now  

 

RMS and IPP (like the way Sears use to advertise their guitar amplifiers, 250 watts IPP, (6 watts RMS)) or something like that.

 

Think of the way an audio crossover works... woofer @ 12db/octave cut, midrange at 12db/octive ramp up, the SUM of the two speakers outputs is GREATER than one or the other (In the crossover band), this is much like the SUM of both the + and - waveforms of the AC cycle (IPP) instead of RMS.....

 

Greg

[ErnieHorning]

Geez it really pains me to say this but Max is right.    170 volts positive and 170 volts negative.  Greg you really do need to fire up that old scope, those meters are fooling you.  Your meters are all measuring RMS and not peak voltage; you’re just seeing the average.

 

Try your meters on a full wave bridge and see what you get.   The bridge can’t double the voltage; it just adds the other half of the AC wave to what you get with one diode.

[a31ford]

Ernie...  Really ?

 

 So we call the line voltage we use 170 AC ???  

 

Can I ask you to RE_READ Max's first post  (post #3) and tell me again that we do NOT need triac's then ?  

We can use SCR's in our controllers and get the SAME results as TRIAC's ? PPPLLEEAASSEEEE !!! (Sarcasm intended)

 

This topic has gotten WAY off-topic (PEAK voltage ?? who cares)

 

I guess the multi-meter companies should re-invent how meters read then.....

 

It (this topic) was about TRIAC's people..................... NOT peak voltages.

 

Greg.

[ErnieHorning]

Nope, it’s the peak voltage.  We call it 120 because it’s the RMS value or what the value would appear to be to a resistor in a DC circuit.

 

With your single diode, you basically have a 50% duty cycle DC circuit.  Your meter is averaging that to half the voltage.  If you just put a capacitor across your meter probes, you’ll see the voltage go much higher.

 

I use 24 volt transformers every day at work.  The unloaded voltage is about 26 VAC.  Place a full wave bridge on it, add a capacitor 10µF or bigger and you get about 34 VDC not 17.

 

Check your LOR controller.  I’m pretty sure that it’s got a 12 VAC transformer on it and a full wave bridge.  Check the voltage on the big capacitor.  It’s not 6 VDC.

 

BTW, Beckman also makes peak reading meters, at least they used to.  We had a few at work that read the line voltage as 177 VAC.  It was very confusing, so we ended up just tossing them in the trash.

 

You really should swallow your pride for a moment and try the scope.  You could also do a Google search and tell the rest of the world that they're all wrong too.

[ErnieHorning]

I should never do this before morning coffee….

 

Greg is exactly right!  Thinking about the LOR supply, I believe it’s a center tap supply.  So it will be half the voltage.

 

Max, it really is 60 volts; actually 54 rectified DC.

[ErnieHorning]

You really should fire up that scope though.  I bet it doesn't work very well if it's been sitting there unpowered for several years.

[plasmadrive]

We use, in the real world, values for things as standards.  For AC it is RMS (Root Mean Square).  True RMS is the real or effective value of AC voltage or current for a circuit.

.707 x peak. Our voltage in the USA is around 165-170vpk. That is why the 117vac -120vac we use nominally.
 

For pulsating DC we use Average.  1/2 wave avg is .318 x peak  for full wave it is .637 x peak.  Digital meters that read DC read the value as an average. 

 

The freq for a 1/2 wave pulsating circuit is the fundamental x1 and for a full wave it is the fundamental x 2.  In our case the fundamental freq is 60hz.

 

For filtered DC we use the voltage it is. 

 

All of these have tolerances and variations.

 

Certain things don't care about peaks so much, like incan lights.  They care about the effective power they can use.  They are very slow to react to peaks in voltage because of their thermal properties.
 

Certain things do care about peaks if they respond fast enough.  Like LEDs. 

Using the formula of 1.57 x Avg with Max's 14mA on the LED, the peak current is about 22mA.   That is about 2mA higher then what they like to see as a norm, (for a normal T1 3/4 LED), but it will take it.  It will stress it just a bit. You won't exceed the thermal rating of the junction short term, but it does use it all. 

 

I keep seeing the words Avg and RMS interchanged.  They are not actually interchangeable in reality of a technical discussion of this nature.

[ErnieHorning]

Can I ask you to RE_READ Max's first post  (post #3) and tell me again that we do NOT need triac's then ?  

We can use SCR's in our controllers and get the SAME results as TRIAC's ? PPPLLEEAASSEEEE !!! (Sarcasm intended)

 

Actually, technically you could.  It would require a full wave bridge before the SCR.  You then would have rectified DC and then the SCR would operate with exactly the same end results.  Many of the old multi-function strings used this method.  It requires one extra part though.

 

You could substitute two SCR’s for a TRIAC but then you would need to drive one of the gates with a negative voltage.  Kind of complicates the circuit though when TRIAC’s are so cheap.

 

You can substitute an SCR with a TRIAC since most TRIAC’s will accept both a positive or negative gate voltage.

[plasmadrive]

Actually, technically you could.  It would require a full wave bridge before the SCR.  You then would have rectified DC and then the SCR would operate with exactly the same end results.  Many of the old multi-function strings used this method.  It requires one extra part though.

 

You could substitute two SCR’s for a TRIAC but then you would need to drive one of the gates with a negative voltage.  Kind of complicates the circuit though when TRIAC’s are so cheap.

 

You can substitute an SCR with a TRIAC since most TRIAC’s will accept both a positive or negative gate voltage.

There are caveats to those statements, but for the most part, yes. 

 

However, if you were to use back to back (anti parallel) SCRs, you most likely would not have most the dimming or flickering issues with the LEDs compared to the triacs.  This is because each SCR would have 1/2 a cycle to turn off and the capacitive nature of the twisted wire strings would not matter so much. 

[Santas Helper]

[plasmadrive]

LOL @ Santa's Helper..

[ErnieHorning]

Careful...

 

[plasmadrive]

OMG!!   LMAO

[Ken Benedict]

Max... 

 

RMS and IPP (like the way Sears use to advertise their guitar amplifiers, 250 watts IPP, (6 watts RMS)) or something like that.

...

 

That IPP is Instantaneous Peak Power that is usually achieved milliseconds before the amplifier loses it's Magic Smoke.

 

It's quite misleading, but the sales people like it.

[plasmadrive]

LOL.. 

[plasmadrive]

instantaneous puffing power?
 

[Max-Paul]

Plasma,

 

 

Where did you come up with the 1.57 X Avg.?  I know of only two sets of numbers to use with RMS and peak. They are .707 and 1.414   So if you take 14mA X 1.414 you get 19.976mA.

 

Really not familiar with your numbers, please enlighten me.