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Using opto-triac to trigger SCRs.


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#1 schow

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Posted 24 May 2006 - 12:39 PM

Hi,

I am having a circuit where Fairchild MOC3052 opto-triacs are used to trigger the gate of some SCRs.

Fairchild Semiconductor guarantee that the opto-triac would be fully triggered with 10mA (max) LED forward current but they did not mention what is the typical LED forward current (Toshiba TLP3052 stated 5mA as typical LED forward current).

The problem I am facing now is that due to the limitation of my +5VDC power supply, I am able to provide these opto-triacs a max LED forward current of 5mA only.

I have done some tests on few tens of Fairchild opto-triacs and it seems that the max LED forward current in order to fully turn on the triac is approx 3.8 mA.

My circuit seems to work fine now but I am a bit worry what if after certain time of operation and when the opto-triac gets older, it might require much higher LED forward current to turn on the triac (10mA as stated in the datasheet).

Is anyone out there have any idea what is the typical LED forward current of MOC3052 or some other opto-triacs with much lower LED forward current?

danke! rolleyes.gif

#2 marke

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Posted 24 May 2006 - 08:06 PM

Hello schow

Be careful using opto triacs to control SCRs. There are a number of things to consider.
  1. The SCR will be triggered by a very fast current pulse. You need to ensure that the capacitance of the opto triac is not able to generate such a pulse under a high dv/dt transient.
  2. Many opto triacs have a poor dv/dt rating causing the effective dv/dt of your power circuit to be very low.
  3. The voltage rating of the opto triacs are typically quite low. This will set the voltage rating of the power circuit.
  4. Opto circuits suffer from gradual degradation of the LED in the opto device. You need to ensure there is sufficient drive to make it work after years of operation, not just at commisioning.
  5. Protect the SCR gate against an unlimited negative gate current.

I would probably go with the 10 mA drive unless I had good long term life specs from the manufacturer.

Best regards,

#3 schow

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Posted 01 June 2006 - 10:36 AM

Hi Marke,

Thanks for your advices!

I have found another opto-triac, which LED only requires 5mA to fully trigger the triac but the problem is it's peak blocking voltage is only 400V.

May I connect 2 of this opto-triac in series so that the blocking voltage could be increased to 800V? Does it work this way? Or can I use 1 opto with 600V and the other one with 400V blocking voltage to achieve 1000V?

These opto-triac would be placed accross a 430VAC supply, unfortunately it is higher than the 400V blocking voltage of the 5mA device, thus I am thinking of putting 2 in series.

Kindly comment!

danke! laugh.gif

#4 marke

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Posted 01 June 2006 - 07:26 PM

Hello schow

You will need to have optotriacs with a blocking voltage equal to or better than the blocking voltage of the SCRs that you are using. For a 400 volt supply, it is usual to have SCRs rated at 1200V or higher.

If you used 400 volt blocking devices, you would need to have at least three in series. You can put them in series provided that they have the same turn ON speed and you drive the LEDs at the same time. It is also preferable to use some form of balancing network to ensure that the OFF state voltage is equally distributed across them.

Best regards,

#5 schow

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Posted 02 June 2006 - 05:16 PM

Hi Marke,

Thanks for your reply again!

Just one question about the off state peak blocking voltage of opto-triac, if there is a device with 400V off state blocking voltage, is this means that the triac would not conduct with any voltage below 400V, what if the voltage accross the triac is higher than 400V? Is the triac going to be turned on without any trigger signal from the LED? Will it be damaged when applying voltage higher than 400V or it is just short circuit accross the triac terminals only and will return back to normal after the 400V is removed?

If I put 3 of these 400V devices in series, as you said, the off state blocking voltage would now be increased to 1200V, but when I apply a 430V accross the first & last triac terminals and when all the 3 LEDs are turned on at the same time, I suppose each triac will now see 430V accross each individual opto-triac, will this damage any one of them? Why the datasheet of opto-triac will only specify off state blocking voltage but not the on state max voltage?

Kindly advice.

danke! rolleyes.gif

#6 marke

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Posted 02 June 2006 - 08:57 PM

Hello schow

If you exceed the rated voltage of the opto triac, it will begin to break down and conduct current. Depending on the rate of rise of current and the magnitude of the current this may be destructive. If the current is very low, it may survive.

If you have three devices in series and you put 430 volts across the string, the voltage across each device should be one third provided that there is a voltage balancing network used or the devices have exactly the same leakage current.
When you turn the triacs on, there will be about one volt across them unless the current is too high. Provided that the current is within the operating current of the triac, the voltage will be low and not be an issue. There is no ON state blocking voltage because in the ON state, the device is not blocking voltage.

Best regards,

#7 schow

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Posted 03 June 2006 - 02:20 AM

Halo Marke,

Thanks for your comments!

When you said "When you turn the triacs on, there will be about one volt across them unless the current is too high", your one volt is it "1" volt or you are referring to one same voltage (eg. 20V for all the 3 devices)???

I am not quite sure if the voltage is balance among these devices but if I am using a 600V device in series with a 400V device, just in case the voltage is not balance across these 2 opto-triacs (eg. 20V across the 600V part and 410V across the 400V part) even the applying voltage is higher than the 400V off state blocking voltage of the 400V part, I assumed the SCRs would still not be triggered since the 600V part is blocking the current from flowing thru the gate of the SCRs, am I right? But if this is what happened, will the 400V device be damaged since it is being protected by the 600V part?

If the other way round, if the voltage across the 600V part is 410V and 400V part is 20V, this will definitely not be any problem, right?

danke! cool.gif

#8 marke

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Posted 03 June 2006 - 02:49 AM

Hello schow

I wopuld expect that the ON state voltage across each triac would be in the order of 1 volt. i.e. three in series would give about 3 volts total.

I wouod not consider putting different types or different voltage ratings in series. If you are using a 600V device, use 2 x 600 V rather than a 600V and a 400V.
It is important that the two parts are fairly well matched to ensure balance during the switch ON and switch off times.
Remember, the voltage rating is not an absolute rating. It depends on the batch, the manufacturer etc. Do not even bother to consider what would happen with a 400V device at say 420 volts. A 400V device is designed to bused where there will be a repetitive OFF state voltage of less than 400 V. You may find that in many cases, parts rated a 400V are actually good for 600V. Some manufacturers have a high safety margin, others do not. The other problem is that you can not predict exactly what is going to happen in the field.
Experience tells us that it is best to use devices rated at least 3 times the average RMS voltage when connected on to the standard supply. In bad areas, this needs to be higher, 3 - 4 times.

Best regards,




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