MOC3083 in Series Connection.

What are you trying to do?

If you want 2 MOC outputs in parallel giving an OR function, it looks like it would work. Putting 2 in series for an AND function looks like it will also work. It's easy enough (and cheap enough) to just build it and see what happens.

But why not use just 1 MOC and do the logic externally with the result driving the MOC's input LED.

I do not understand what you are proposing, however if you try to place two MOC3083's in series to switch a voltage greater than their individual rating they will fail because they will not switch simultaneously and the full voltage will be briefly applied to a single device during switching. Failure will result.

Hi WAWAUS,

The off-state voltage of MOC3083 is 800. So if I have two in series, with two balancing resistors of 1 meg ohm each parallel to each of them, the peak voltage each will be seeing will be 660 X 1.414 / 2 = 446.62; or 490 considering a +5% increase in supply voltage.

MOC3083 is a a zero crossing device. The typical inhibit voltage of the device is 12 with a maximum of 20V. Chances of both MOCs turning simultaneously is next to impossible. Let us consider devise "A" has a larger inhibit voltage than device "B". For ease of understanding, let us say, the inhibit voltage of A = 18 and that of B = 17.95.

Now when the total voltage across the series connected devices fall slightly below 35.95, A will be turned dropping the voltage across it equal to the on-state voltage (Max 3).

The instantaneous voltage across "B" just prior to the turn ON of "A" occur must have been slightly over 17.95. Just after the switching takes place, the voltage across "B" will increase to a value very close to 35.95 - on-state voltage of A. This will be a minimum around 32.95. Device "B" stays in its OFF-state until the voltage across it falls just below 17.95.

If my logic is correct, the device never will be subjected to a voltage above 490 (Say 500) volt which is 300 V below specified OFF-state voltage of the device and therefore no failure will occur.

Regards,

Job Thykkoottathil

What you say seems correct, but what if the switch control occurs when the voltage across them is near zero, rising, and within the threshold of one of the devices but greater than the threshold of the other, then that device would see the full peak voltage of the next cycle, it would not survive.....If you can arrange for the switching control to be synchronous to the waveform and only occur at or close to peak voltage, all may be well......

Hi Wawaus,

The LEDs of the devices are also in series so control signal is fed to both devices simultaneously. Also remember that the devices have zero-crossing detectors which keep the devices inhibited at a voltage above the inhibit voltage and therefore both devices will be in their off state when the instantaneous voltage rises to 36 vols from zero.

Regards,

Job

Regardless of the LEDs being in series the two couplers will have different sensitivities, CTRs and different propagation delays, they will ALWAYS switch at different times and LED currents

Hi Wawaus,

None of the two devices will turn ON until the voltage across the two devises fall below 40. Suppose the one with less propagation delay and more sensitive device turned On at when the voltage across it fell below 20. The maximum voltage across the other devise will be a maximum of 37 volts. the voltage across that devise only fall below 37 Volt due to propagation delay. So none of the device will be damaged.

Regards,

Job.

Well go ahead and try,

Most of the time what you say will work, but I wouldn't trust it because I believe that under some very specific timing it can fail, and since the switch timing is likely to be random that specific timing is likely to occur at some stage.

I think that instead of connecting [for example] 2 diodes of lower PkInv to work on double or even higher voltage will burn one of the diodes; which is slower to conduct.

The faster will have a little fwd-drop and put all the rest on the other to breakdown.

Now the era to connect 2 or more such devices have gone as single devices to work on required voltage are available

In old times such connections were not simply to connect in series but a spacial voltage-dividing NWs were used.

Hi Haajee,

MOC3083 is not a diode. It is an opto-coupler for driving a triac or alternistor. You are correct in suggesting that diodes of high voltage is available but opto-couplers having off-state voltage greater than 800 V is not available. That is why it is necessary to connect them in series with balancing resistors connected in parallel with them.

Regards,

Job.

Thanks