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[SOLVED] Can I use MOC3052M as triac?

IonelButar

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Hello,
I know that MOC3052M is designed as triac driver, but I have to drive a small load of 10mA and I tried to drive it directlly from MOC, without using the second triac, as sugested in datasheet.
It is working without any problem. I limit the current trough triac as a double safety measure with a 680Ohm series resistor.
With or without the resistor, circuit work just fine.
Load sinks about 100mA inrush, stabilize after about 10sec to 20mA and stays at 10mA after about 3 minutes.
I attached the schematic, first channel is designed according to MOC datasheet, the others are the ones I am wondering if they are working or not.

Is it a problem to use the opto-triac to drive directlly the load? Do you recommend to keep the series resistor?

If I cannot use it this way, do you know why it cannot be used?

Thank you!
MOC3052M.PNG
 

betwixt

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There is no problem using it directly as long as you observe the ratings. The biggest issue is that 'big' triacs have a surge rating because the primary cause of damage is overheating the junction and being bonded to the heat sink/tab they can dissipate it easily. In the MOC3052 and similar devices, if you exceed the maximum current it will be damaged almost instantly and unfortunately will probably stick in the 'on' mode.

Brian.
 

IonelButar

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Hello,
Thank you for your fast response. Indeed, I did not considered the power dissipation calculation on the device.

According to datasheet:
- Peak Non−Repetitive Surge Current (Single Cycle 60 Hz Sine Wave) = 1A
- Total Power Dissipation at 25°C Ambient = 300 mW
- Total Power Dissipation at 40°C Ambient = 300mW - ( 4mW * (40°C-25°C) ) = 240mW (condition plausible to be met)
- Total Power Dissipation at 60°C Ambient = 300mW - ( 4mW * (60°C-25°C) ) = 160mW (I consider this as worst case scenario)

According to my calculation and measurements:
- inrush:
P = 100mA*2.2V = 220mW (for about 10s)
- stabilization:
P = (40mA->20mA)*2.2V = 88mW->44mW (for about 60s)
- maintaining:
P = 10mA*2.2V = 22mW

According to above data:
- in my worst case scenario, near to the device limits, high probability to fail on power dissipation during inrush
- normal operation condition, power dissipation is ok

What would you do in this conditions? Would you place an additional power triac?
What about the series resistor, would you keep it? I was thinking to keep it to limit the inrush current bellow 1A, but I think doesn't make any sense after I see the power calculations.

Thank you!
 

betwixt

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I think using it as the manufacturer intended is the best solution. When used to trigger a bigger triac the dissipation is normally very low because the voltage across the MOC3052 is essentially the same as the voltage across the main triac so it drops to just a volt or two when it is conducting.

The other consideration is the duration of the peak current, it is specified as 1A for a single cycle at 60Hz, if you are in a country where the AC is 50Hz, each cycle lasts longer so it needs further de-rating. Note also that 1A is the 'absolute maximum' rating, in other words the current at which damage may start to occur, the (Lite-On data sheet) current graphs for constant use do not go beyond 100mA and suggest no more than 100mA for temperatures up to 40C.

Brian.
 

IonelButar

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Thank you very much!
I will take your advice.
 

KlausST

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Hi,

If I remenber right I ´ve read that one should no tuse the MOCs directly.

I´ll try to find the source of informations....

Klaus

Update:
Similar thread: https://www.edaboard.com/showthread.php?362619-MOC3021-drive-small-load-directly

MOC3021 datasheet says:
Note:
This optoisolator should not be used to drive a load directly. It is intended to be a trigger device only.
But in the MOC3052 datasheet I don´t find this remark.
Thus I agree with Brian: "...as long as you observe the ratings. "
 

IonelButar

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Hello,

I also received today an answer from Onsemi company which states:
"This is not a problem at all. The internal triac can handle non-repetitive pulses of up to 1A. If you are driving just 10mA then it will not affect the device in any negative way."

But considering the power dissipation for inrush and worst case scenario, I will go with the safe side. That means, I need to add the power triac.
Thank you @betwixt for opening my eyes into that direction!
 

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