Positive sensing zero-crossing detector. - diyAudio
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Old 19th April 2017, 11:20 PM   #1
Artie is offline Artie  United States
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Default Positive sensing zero-crossing detector.

This question is in regards to a power supply I'm trying to design. The internet is chock full of zero-crossing detectors, but I can't seem to find one that will only detect as the cycle transitions from negative to positive. Is this really as difficult as I'm finding?

This will be with 60 hz only, but not necessarily 120VAC.

Thanks all.
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Old 20th April 2017, 03:03 AM   #2
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Would be easy enough to take the square wave output of the traditional zero crossing detector and put it into a pulser that only transitions on a positive or negative transition. 555 timers are commonly used to do this.

Larger question in my mind is when do you want the output to reset, if not on the positive transition?
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Old 20th April 2017, 03:24 AM   #3
rayma is online now rayma  United States
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Quote:
Originally Posted by Artie View Post
The internet is chock full of zero-crossing detectors, but I can't seem to find one
that will only detect as the cycle transitions from negative to positive.
http://www.next.gr/uploads/135-9161.png
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Old 20th April 2017, 04:16 AM   #4
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Hi,
Attached is a circuit that worked for me.
Attached Files
File Type: pdf zero-crossing.pdf (17.1 KB, 32 views)
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Old 20th April 2017, 06:40 AM   #5
Artie is offline Artie  United States
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Quote:
Originally Posted by zigzagflux View Post
Larger question in my mind is when do you want the output to reset, if not on the positive transition?
I'm not exactly sure what you're asking me here. But I want an output pulse only when the AC crosses zero, going positive.

Thanks rayma and tauro0221. Both those diagrams got me pointed in the right direction.
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Old 20th April 2017, 07:48 AM   #6
Elvee is offline Elvee  Belgium
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A one-shot like the 4538 would be ideal for task: bias one of the trigger inputs (the (-) one in your case) mid-supply with two large resistors, say 1 Meg, couple the input signal with a sufficient capacitor, say 1µF, and if the p-to-p input voltage is larger than Vdd-Vss, add a series resistor to allow the signal to be clipped by the protection diodes.
You can then define the timing components to suit the desired pulse width.
The 4538 incorporates schmitt trigger inputs, making its operation very reliable

http://www.ti.com/lit/ds/symlink/cd14538b.pdf
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Old 20th April 2017, 10:55 AM   #7
Ketje is online now Ketje  Belgium
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An other idea. Like that you can also separate the two circuits.
Mona
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Old 20th April 2017, 12:04 PM   #8
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Quote:
Originally Posted by Artie View Post
I'm not exactly sure what you're asking me here.
What do you want the duration of the pulse to be.
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Old 20th April 2017, 12:43 PM   #9
Ketje is online now Ketje  Belgium
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Originally Posted by Ketje View Post
An other idea. Like that you can also separate the two circuits.
Mona
The diode was in the wrong way , have to be more carefull
Mona
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Old 21st April 2017, 04:55 PM   #10
Artie is offline Artie  United States
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Thanks everyone for the help so far, and sorry if this is a bit long-winded.

Let me explain better what I'm trying to accomplish. This project is multifaceted. Primarily, I want to experiment with dimming and flashing some of the cool new LED light fixtures that are available now. Perhaps even leading to a 21st century version of the cheesey old Realistic light organ. (Just for the fun of it.) I've also wanted to experiment with Ti's "Little Logic" family of IC's ever since I've discovered them, and I've been playing with the Picaxe series of micro-controllers. And lastly, I just enjoy playing with electronics.

Attached, is the circuit I'm playing with right now. It isn't a schematic, in the classic sense of the word, but more a blending of a schematic and block diagram. (A schemock?) It's just something I use to help me visualize what I'm doing and thinking. It's actually fairly refined from where it started, and it's not as convulted as it might appear. (Maybe.) Beside the Picaxe, there are only three IC's. A dual comparator, and two of the Little Logic series of chips.

At idle, with no control input on the Picaxe, "reset" is held low. Anytime the voltage is above the "trigger" level, U1b will be high. But since the other input of U3 is low, nothing happens. When the voltage drops below the arming threshold, U1a goes high. Again, nothing happens because of U3. U1a and U1b flop back and forth, doing nothing, as long as there is no control input. When the Picaxe gets a control input, it takes the reset line high. Now, when the voltage drops below the arming threshold, U1a goes high. U2's OR gate feeds back into itself via it's sister AND gate and latches itself on. Now, as the voltage rises to the trigger level, U1b goes high, and the pulse is seen at the input of the Picaxe. The Picaxe immediately takes the reset line low again, and sends a 12ms pulse out to the TRIAC circuit. 12ms is approximately 75% of a 60-cycle wavelength, so it holds the TRIAC triggered long enough to get past the 1st zero-crossing point, and then lets the TRIAC shut itself off at the end of the complete cycle.

If I watch the AC output on a 'scope, I should be able to tweak VR1 to compensate for my total propagation delay, to get a perfect zero-crossing point. If everything works as planned, and with proper programming of the Picaxe, I should be able to select any number, and sequence, of individual zero-perfect cycles, from the AC line. In theory.

Artie
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