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Triac driving

:: EPE Chat Zone ≠:: ≠Radio Bygones Message Board :: » EPE Forum Archives 2010 - » Archive through 09 February, 2012 » Triac driving « Previous Next »

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echase
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Username: echase

Post Number: 603
Registered: 07-2007

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Posted on Friday, 13 January, 2012 - 01:49 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

What is best way to run a 20 or 30amp triac from a PIC? Logic level triacs seem to be limited to 10amps.

In the absence of better advice I guess an optotriac driving the 20amp triac is the solution but am wondering is there is cheaper/lower component count solution as I donít necessarily need to isolate the mains from the PIC in this case.
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muskrat
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Post Number: 416
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Posted on Friday, 13 January, 2012 - 02:21 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

IMHO it would be preferable to opto-isolate simply in order to also protect the PIC in case of catastrophic break-down of the triac.
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dselec
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Post Number: 380
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Posted on Friday, 13 January, 2012 - 06:11 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

http://www.miscel.dk/MiscEl/miscel.html by dave js

I have tried 2 ways 1) germainum diode or transistor reversed conduct with light on them.i use them as light dect. photo cell .
2)led + ldr

(Message edited by dselec on 13 January, 2012)
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terrym
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Posted on Saturday, 14 January, 2012 - 02:12 am:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

Opto relay is another possibility. Works exactly the same as an opto-coupler and triac but may be more expensive.

TM
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dselec
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Posted on Saturday, 14 January, 2012 - 09:57 am:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

Just found out 10pcs for 1$
or 2000 for 10$
dead cheap
http://www.ebay.com/sch/i.html?_trkparms=65%253A3%257C66%253A2%257C39%253A1&rt=nc&_nkw=opto&_sc=1&_sticky=1&_trksid=p3286.c0.m14&_sop=15&_sc=1
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dselec
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Post Number: 383
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Posted on Saturday, 14 January, 2012 - 10:54 am:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

echase
you did-not say if its on/off action for that u can use a relay .
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gordon
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Posted on Saturday, 14 January, 2012 - 12:47 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

If isolation is not needed, you could use a transistor to increase the current drive to the triac, from the PIC. The circuits on P157 may be of interest.

http://homepages.eee.strath.ac.uk/~bwwilliams/Book/Chapter%207old.pdf
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echase
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Post Number: 604
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Posted on Saturday, 14 January, 2012 - 02:02 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

Gordon, I guess that if I used the unregulated voltage before the 7805, which would be around 8V, that should be enough to drive the triac with one transistor and it does not need not a high voltage transistor.

On the other hand with optotriacs being so cheap might be easy to just use one.
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gordon
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Posted on Saturday, 14 January, 2012 - 02:21 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

As already posted, I would also go the optotriac route. Seems to be the most preferred method for triggering a larger power triac. I have used this method for a few home projects, and it seems to be very reliable.
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james
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Post Number: 31
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Posted on Sunday, 15 January, 2012 - 11:42 am:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

Hi

The reason for using opto-isolation is to protect the low voltage circuitry from the mains voltage. In most designs the switch (Triac) should go between the load and the live. This isolates the load from the 240V when the triac is switched off - a safety feature. However this also means that when the triac is switched on it (the triac) will go to 240V. Opto isolation prevents this voltage being fed back to the low voltage circuit.

Opto isolation can be avoided by placing the triac between the load and the neutral. the Triac gate can then be driven with a non isolated signal. However this method has the disadvantage that the triac is being triggered in quadrants 1 and 4. Triacs are less sensitive to triggering in quadrant 4 and this is best avoided. Quadrant 4 is Gate +ve with respect to MT1 and MT1 +ve with respect to MT2.

A way around this dilemma and avoiding using opto isolation is to use a transformerless power supply to power the low voltage circuitry. The low voltage circuitry can then be "hung" below the neutral with the +ve low voltage supply being at neutral voltage. The Triac gate can then be driven directly. This method is only any good for situations where the low voltage circuitry (including the triac gate) draws less than about 40mA. Above this current the transformerless power supply starts to become impractical. This method triggers the triac in quadrants 2 and 3. (Gate always -ve with respect to MT1).

Cheers

James
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echase
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Post Number: 605
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Posted on Monday, 16 January, 2012 - 06:05 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

James, how do you define quadrant 1 etc?
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james
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Post Number: 32
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Posted on Wednesday, 18 January, 2012 - 08:14 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

Hi Echase.

The usual way of triggering a triac with an optotriac is to connect the gate of the triac via the optotriac and a resistor to MT2 which is connected to 240V. ie. The triac is placed between the load and 240V. MT1 is connected to the load, the other side of which is connected to the neutral.
A triac is triggered by developing a voltage between the gate and MT1.
This means that, with the triac and load connected in the above described way, as the mains voltage goes +VE with respect to neutral the triac is triggered. Also when the mains voltage goes -VE with respect to neutral the triac is triggered. (Assuming the opto triac is switched on).
This means that in this circuit configuration the triac is being triggered in quadrants 1 & 3.

A triac can be triggered in 4 ways:-

Quadrant 1 : MT2 +ve with respect to MT1
Gate +ve with respect to MT1

Quadrant 2 : MT2 +ve with respect to MT1
Gate -ve with respect to MT1

Quadrant 3 : MT2 -ve with respect to MT1
Gate -ve with respect to MT1

Quadrant 4 : MT2 -ve with respect to MT1
Gate +ve with respect to MT1

As I mentioned in my last post, this configuration means that when the triac is in a triggered state the gate will swing between +325V and -325V. Hence the need for opto- isolation to protect the low voltage circuitry.

I hope this is clear enough.

Cheers

James.
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echase
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Post Number: 606
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Posted on Thursday, 26 January, 2012 - 05:34 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

Thanks for that.

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