# Zero point switching

Gold Member
I found this scheme of a zero point switching in a notebook

http://img402.imageshack.us/img402/1313/schemew.jpg [Broken]

I'm a bit confused. Look at the SCR on the right with the wide-arching gate... is it really possible to connect an SCR gate like that?

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No, You got the circuit wrong the SCR2 should not be controlled by external pulse. There should not be any third SCR, only a diode in place of that. The gate of the SCR2 will be connected between the cathode of diode and the capacitor. Then in the negative cycle f2 will have positive gate current due to charged capacitor. This fires SCR2 making it conduct.

Hello

Maybe this might help: www,onsemi,com/pub/Collateral/HBD855-D.PDF (I can't post links, yet, replace the commas with periods)

Gold Member
No, You got the circuit wrong the SCR2 should not be controlled by external pulse. There should not be any third SCR, only a diode in place of that. The gate of the SCR2 will be connected between the cathode of diode and the capacitor. Then in the negative cycle f2 will have positive gate current due to charged capacitor. This fires SCR2 making it conduct.

Thanks, you're right.

As far as the PDF, a bit beyond me and contains too much unrelated material to sift through,, but thanks at any rate.

NascentOxygen
Staff Emeritus
Perhaps Femme_physics could write a few sentences of explanation, explaining what zero point switching is, and its benefits?

i guess zero point switch can not be used in dc circuit . Isn't it?

wat is zero point switching?

In ac circuit we want to open the main contacts and specialy in inductive load a huge opposite voltage will induces Ldi/dt cusing spark in the main contacts and zis spark will be harmful if the wave of current in the top positive
But what will happen if we open the contacts when the sinsoidal wave is in zero point ? No oppsite emf will induced and so no spark and by zis techique we protect our circuit's contact

Gold Member
Perhaps Femme_physics could write a few sentences of explanation, explaining what zero point switching is, and its benefits?

I'm gonna use this drawing which contains another capacitor (don't ask me why!)
http://img687.imageshack.us/img687/8634/thecircuitxx.jpg [Broken]

You get pulses of 0 and 1. Regulating the power on RL will be done via changing the amount of output pulses.

1) When you get "1"
SCR1 is shortcircuited so you can't fire it.

2) When you get "0"

A) For positive wave: SCR1 is permitted, SCR2 is not, and diode (D1) is conducting. There's always a current flowing into the gate of SCR1 through capacitor C1. When the current at C1 is max, SCR1 is fired.

Vin = VRL
The capacitor is charged with positive energy.

B)

For negative wave, SCR1 is off, D1 is at cutoff, and C2 discharges energy it stored from the positive wave, through R2 into the gate of SCR2 thereby firing it. As a result, Vin = VRL still. This action is done periodically till the moment where the output at the control circuit shows "1"

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great

Gold Member
I'm asked for 2 advantages of this method ("Zero Point Switching") over regulating firing angle power system.

I would say:

1) We can regulate the firing angle more (till almost 180 degrees)
2) You can add a transformer in the entrance to the circuit to reduce the power lost at the control circuit

Is this correct?

jim hardy
Gold Member
Dearly Missed
uhhhh ... i think for zero point switching the firing angle is by definition zero..

zero point switching into resistive loads avoids the rapid current rise at instant of switching, reducing harmonics and RF interference.
It also avoids inrush into a capacitive load.

Here is a really good paper on thyristor control
http://www.onsemi.com/pub/Collateral/HBD855-D.PDF

from page 24
Zero−point switching is a technique whereby the control
element (in this case the triac) is gated on at the instant the
sine wave voltage goes through zero. This reduces, or
eliminates, turn−on transients and the EMI. Power to the
load is controlled by providing bursts of complete sine
waves to the load as shown in Figure 2.18. Modulation
can be on a random basis with an on−off control, or a
proportioning basis with the proper type of proportional
control.
In order for zero−point switching to be effective, it must
indeed be zero point switching. If a triac is turned on with
as little as 10 volts across it into a load of a few−hundred
watts, sufficient EMI will result to nullify the advantages
of adopting zero−point switching in the first place.

note that for inductive load it is preferable to close in at the sinewave peak thereby establishing 90deg offset between voltage and current. Manufacturers sell solid state relays in both zero and peak switch characterisitcs for that reason.

hope this helps
i saved myself a copy of that Onsemi appnote it's a good one.

old jim

Gold Member
Thanks ol' Jim,
I summarized it for a possible test answer:

Zero point switching is a technique whereby the control element (such as a TRIAC) is blocked by the gate as soon as the sinusoidal wave passes the 0 point. That is to say, the power to the load is done by providing pulses of full sine waves.
Two advantages of this system (over regulating power via firing angle system) are:

1) Decreases and even eliminates electromagnetic interferences
2) Avoids inrush (too fast of a flow) into a capacitive load.

Looks good?

EDIT: Also, is this a period as well?

http://img32.imageshack.us/img32/3580/periodquestion.jpg [Broken]

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NascentOxygen
Staff Emeritus
I summarized it for a possible test answer:

Zero point switching is a technique whereby the control element (such as a TRIAC) is blocked by the gate as soon as the sinusoidal wave passes the 0 point. That is to say, the power to the load is done by providing pulses of full sine waves.
That looks about right. Power can be allowed in bursts of multiple cycles. You have drawn them to be a single cycle.

EDIT: Also, is this a period as well?
That "gap" is 1 or more full periods, i.e., nT where n ≥ 1

(Actually, if using a triac, I can't see a reason that the gap can't be a multiple of half-periods, though I think it rarely is designed that way.)

jim hardy
Gold Member
Dearly Missed
i'm not sure "blocked by the gate" will be received as you intended it,

perhaps words to effect ' caused to pass complete line cycles by applying gate drive at the instant of zero crossing, thereby avoiding the sharp current rise times incurred when turnon occurs later in phase ' .
Does that make it less ambiguous?

Nascent's point about half cycles is right on.
It is conceivable that a malfunction or Murphy's Law could allow firing on every other zero crossing so that half wave rectification occurs. That wreaks havoc with inductive loads and warrants peventive design. But it won't hurt resistive loads.

We had a situation in my power plant where triac relay drivers intended to deliver zero point switching would, when they got very warm, incur a turn-on delay on one or the other half cycle creating a DC offset in their output. That DC component though small would after a few weeks burn out the coil of the relay they were driving . Some of these relays could trip the plant so it was a real nuisance.
On the bench cooled down the triacs worked fine so at first we thought we had a batch of defective relay coils. But when the triacs were in their operating environment which was ~110degF you could see the DC offset in their output with an osciloscope. We learned how to find it with a voltmeter and eventually got all the temperature sensitive triacs out of our system.

Sorry for the boring anecdote . But this stuff is real, and Mother Nature just loves to push us humans.
I guess if we stayed in the Garden of Eden where everything comes easy we'd never develop our potential.
but golly these lessons come hard.

old jim

Gold Member
perhaps words to effect ' caused to pass complete line cycles by applying gate drive at the instant of zero crossing, thereby avoiding the sharp current rise times incurred when turnon occurs later in phase ' .
Does that make it less ambiguous?

What do you mean by "gate drive"?

Power can be allowed in bursts of multiple cycles. You have drawn them to be a single cycle.

You mean I should write that this could be an advantage, as well?

We had a situation in my power plant where triac relay drivers intended to deliver zero point switching would, when they got very warm, incur a turn-on delay on one or the other half cycle creating a DC offset in their output. That DC component though small would after a few weeks burn out the coil of the relay they were driving . Some of these relays could trip the plant so it was a real nuisance.
On the bench cooled down the triacs worked fine so at first we thought we had a batch of defective relay coils. But when the triacs were in their operating environment which was ~110degF you could see the DC offset in their output with an osciloscope. We learned how to find it with a voltmeter and eventually got all the temperature sensitive triacs out of our system.

Good thing your plant has engineers ;)

jim hardy
Gold Member
Dearly Missed
What do you mean by "gate drive"?

oops swapped terms on you didnt i ?

Gate drive is the signal to turn the device on, ie make it conduct.

You mean I should write that this could be an advantage, as well?

i dont think so, but perhaps explain that's how modulation is usually accomplished with zero point switching - a few cycles of conduction followed by a few cycles of non-conduction,
as opposed to conduction every cycle but just for a fraction of each.

Good thing your plant has engineers ;)
thanks. and great technicians !

Gold Member
Hmm... I think my issue was with translating the words "gated on" to hebrew.... eventually I think I found the right words. I like the textbook explanation...especially since the word "line" in your "caused to pass complete line cycles" is a bit confusing to me... what is the meaning of line. I was also not sure where do I start writing it because your started with the word "caused". Ahh... as long as I got an understandable explanation.... :)

Much appreciated you both

jim hardy
Gold Member
Dearly Missed
....what is the meaning of line.

sorry - it would be in the context of a power line cycle, 50 or 60 hz. I'm so entrenched in power plant stuff i forget everybody else isn't.

translating the words "gated on" to hebrew

indeed - to switch the thyristor (SCR or Triac) into its conducting (or ON) state, one applies some current to the gate termnal
which phrase becomes shortened to "Gated On".

phrase 'caused..... later in phase' i thought might replace your "blocked..... 0 point'

Anyhow congratulations on completing your course !

femme thanx alot for your great explaning