Go Down

Topic: question about zero cross octocouplers (Read 1 time) previous topic - next topic

heyarn

Hi,

What does a zero-cross octocoupler do? How is different from a regular octocoupler?

I'm trying to pulse 110 volts dc 3 millisecs apart using an attiny, but im afraid that the emf fluctuations from the coils im driving could ruin my attiny... so I want to use an octocoupler to isolate the voltage spikes, but do not know which one to get.

Please give me recommendations as to which octocouple is easily integrateable to my attiny/arduino circuit. :)

PS. Say I have 2 circuits, one is high voltage and one is low voltage.. and the only line they share is GND.. Is there any chance that the high voltage fluctuations from the high voltage circuit could still affect the lower voltage circuit?


Thanks!!

:)

DVDdoug

#1
Jun 20, 2012, 12:56 am Last Edit: Jun 20, 2012, 12:59 am by DVDdoug Reason: 1
The zero-crossing isolator is used for switching AC loads.    It's "smoother"...  It turns on only when the AC voltage is crossing-zero.    Some inductive loads (motors) don't like to be hit with a sudden voltage spike, and the zero-crossing detector prevents this.    And, you are less likely to get noise in the AC line.   (FYI - SCRs & TRIACs, which are commonly used in dimmers, speed controls, and AC solid-state relays,  will turn on any time, but they don't turn-off 'till the voltage goes to zero.)     

Light dimmers & motor speed controls need to turn-on at various points in the AC cycle, so in that case a zero-crossing isolator won't work.

Quote
I'm trying to pulse 110 volts dc
If you are using DC, there is no zero-crossing and it won't work.

Quote
I'm trying to pulse 110 volts dc 3 millisecs apart using an attiny, but im afraid that the emf fluctuations from the coils im driving could ruin my attiny... so I want to use an octocoupler to isolate the voltage spikes, but do not know which one to get.
Well...  You can't have 110V going to chip in any case!  So, you need to isolate that before worrying about EMF.

Whenever you are switching a coil, you need a protection diode (in this case to protect the optoisolator, or whatever is driving the coil.)  About the only time you don't use a protection diode is when you want to use the back-EMF, such as in a switching power supply.

Quote
PS. Say I have 2 circuits, one is high voltage and one is low voltage.. and the only line they share is GND.. Is there any chance that the high voltage fluctuations from the high voltage circuit could still affect the lower voltage circuit?
Yes, therte is a chance.  But, if you are talking about 110V, it should be isolated  anyway (no common ground).   I think the general rule is that anything above 48V should be isolated from humans, and that means it should be generally be isolated from low-voltage electronics too.

There (at least) 3 ways to get undesired "coupling" - Capacitively, inductively, and through a ground loop.  High frequency and high impedance circuits are more vulnerable  than low frequency (or DC) and low impedance circuits.


majenko

Quote
Some inductive loads (motors) don't like to be hit with a sudden voltage spike


This is also the reason why lightbulbs most often blow when you turn the light on.  You switch on partway through a cycle, and the instantaneous voltage jumps from 0v straight up to over a hundred volts (depending on country).  This sudden rise is too much for the delicate tungsten filament, and it goes "Aaaarrrrggghhhh!!!!! *pop*".

By using zero crossing you always switch on when the AC waveform is at 0v, and the load sees a nice gentle rise from 0v to maximum.

The term "rise" here is used to mean either a positive or negative difference to the 0v reference point. A sudden fall of 100v (a rise of -100v) is just the same as a rise of 100v, just in the opposite direction.

Docedison

Zero crossing Optocouplers are switches optically controlled that switch on at the "0" crossing of the AC cycle this is done to minimize turn on transients and sudden surges that occur when inductive loads are switched on, if the turn on occurs any time after the 0 cross the turn on it will control a load that can draw many times it's operating current. When turned on at the zero point that surge is minimized. The device relaxes control semiconductor (usually a triac sometimes an SCR) requirements and prevents  stressing the control device whether triac or scr and lowers line switching transients that occur when inductive loads are controlled.

Doc
--> WA7EMS <--
"The solution of every problem is another problem." -Johann Wolfgang von Goethe
I do answer technical questions PM'd to me with whatever is in my clipboard

Go Up