## Controlling half amplitude of 230AC

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Hello,

i have to turn on/off half pulse of 230V, at 20mA load (a bistable solenoid valve for coolant). When i turn the load on, it is for max 20ms.

So thats a normal AC 230V, feed through a P600 diode. I need top or bottom half of amplitude, to change stance of the valve. Triacs are no use here, bcs basically i am dealing whit DC voltage, although it is pulsing. I dont want to use mechanical relays, maybe a solid state, or simply a high voltage mosfett? Any better ideas?

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Last Edited: Mon. Sep 11, 2017 - 06:45 AM

It may help to know more about your solenoid value (Datasheet?) and more about the project, what the need is (describe what you actually want to do).

Jim

Valves datasheet is in attachment, i just need to switch beetwen stance 1 and 2.

I am using p600 diode to only obtain half of amplitude, and then turning it on for 4 halfwaves, which according to datasheet ensures the position has been set and at 50Hz voltage thats 80ms, if my calculatiosn are correct.

## Attachment(s):

And here is also the control section of circuit (i wanted to use optocouplers, but could not find any for voltages of 325V(max votlage of 230V AC)):

## Attachment(s):

Last Edited: Fri. Sep 8, 2017 - 01:17 PM

How about a SCR?
Surely you can write 'because' rather than bcs?

You can use the opto triac and only fire on one half. No diodes needed.

Yes i can write because :D I cant use SCR, because i dont have common ground whit load.

So i would have to sense the amplitude of AC voltage and then "open" the optotriac only at the amplitudes i want?

Whit is actually with. With a scr or triac, you fire the gate and the device stays on u til the current drops below the hold current. You would need to arrange a circuit to only fire the opto triac on positive or negative cycles.

Last Edited: Fri. Sep 8, 2017 - 01:25 PM

Any idea on how to sense the AC cycles?

Just to clarify someting about triac. If i use lets say a MOC3061, for DC votltage and i open it, it will stay open for as long as there is a voltage on the output side.

If i use triac to control AC votlage, and i open it, it will stay open until the voltage/current FALLS BELOW A LEVEL, or until IT CHANGES POLARITY?

I see that MOC3063 has a holding current of 500 uA, so if i put in front of it a diode that has  a leaking current less then 500 uA, the triac will "close", then the AC polarity changes?

Last Edited: Fri. Sep 8, 2017 - 01:39 PM
This reply has been marked as the solution.

Your circuit in #4 should do what you need to do with a couple of caveats:

1. Some opto-triacs are only "specified" to drive thyristor gates, rather than direct loads like solenoids, lamps, etc.  (even though they usually work fine doing these things). So, carefully check the data sheets so your device won't be used contrary to manufacturer recommendations.

2. DV/DT!  Opto-triacs like any thyristor are sensitive to dv/dt.  Even very small inductive loads like a small solenoid or relay coil can cause an opto-triac to re-trigger when the load switches off, causing erratic/disappointing/dangerous operation.  A simple rc snubber network should be added to prevent this.

3. There are gobs of devices rated for 230 vac applications.

4. You may want to consider a "random" triggering device rather than one with "zero cross" detection.  Sometimes a zero cross detector circuit can get confused and won't trigger properly in certain applications.  Zero cross devices are really only for simple on-off AC loads.

Tom Pappano
Tulsa, Oklahoma

I used a MOC3023, it is supposed to have good dv/dt protection, i also add a rc snubber and it is also non-zero crossing device. So far it seems to work, there are some instabilities in 5% of switches, but i  can jsut switch 3 or 5 times to ensure the end position of valve is reached. If nothing else will work, i will add another thyrystor beetwen MOC and valve. Thanks fo r all help all of you.

Something like an IL420 might be a better choice as the moc3023 isn't specced to switch loads other than triacs. Sharp probably have something as well. The snubber is recommended as it soaks up nasty spikes that kill the opto.

Ok i will also order couple of IL420`s and test them to see stability. Since i need to finish the PCB till thursday, so there is no time to manufacture pcb,  im jsut gonan hand solder it on protoboard, so i paln on using this snubber: http://si.farnell.com/roxburgh/r....

According to my clumsy calcualtions it should fit.

Klemko wrote:
turning it on for 4 halfwaves, which according to datasheet ensures the position has been set and at 50Hz voltage thats 80ms

Note that the attached datasheet says it requires a minimum of 4 halfwaves; it doesn't need to be exactly 4 halfwaves ...

Yes i set it up to turn it on for 120ms(3x4 halfwaves on 50Hz network) and for 96ms(3x4 halfwaves on 60Hz network), to ensure the position has been reach. After testing today i can set on time higher if it shows out to be necessary. I will have to find the lowest time possible for reliably switching+few ms of safety margin, to get the minimum ON-time, to prolonge life span of optocoupler.