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Optocoupled Triac Help

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StudentSA

Member
Good Day,

I am really struggling to get a "simple" Triac circuit to work. I started by consulting the typical application from the optocoupler MOC3043 datasheet:
TriacDriver.PNG
I chose the MOC3043M as is was suggested for 240Vac operation and had the lowest "turn-on" current (5ma).

I've constructed the circuit exactly (Fig 13) as is except I have pin 2 to ground and Vcc is Vin of a 5v source.

The symptoms presently is a load that is always ON. ( I have checked the circuit for shorts/misconstructions 100 times). If i remove the MOC from the board. The load is still on. The load only switches off when the 330Ohm gate resistor is removed. however with this removed (and the MOC replaced) Applying the correct 5ma to drive the optocoupler does not ON the load. I have also tried removing the "snubber" portion (39Ohm,0.01uF Cap combination).

The Triac I am using is a T2512MH. Can anyone suggest debugging this issue?
 

MaxHeadRoom78

Well-Known Member
The MOC3043 is a zero crossing opto, you need the Random phase style MOC301x/302x etc see Fairchild app AN-3006.
max.
 

StudentSA

Member
Hi Max, I've been over the application note and it is interesting however it seems to me that the zero crossing optoisolator is more superior for power transfer. It does not mention why I cannot use it in my application or why it is not working.
 

spec

Well-Known Member
Most Helpful Member
Hy SSA,

The symptoms that you describe are rather odd; if anything, a resistor between a TRIAC gate and MT1 (alternatively named, anode 1) should inhibit triggering rather than enable it.

Here are a few questions/observations:

(1) I suspect from your other thread about transformers that you are probably driving a transformer with the TRIAC. http://www.electro-tech-online.com/threads/transformer-theory.149125/#post-1275375 If that is the case, you have a highly inductive load which can cause triggering problems.
(2) I suggest that you try the circuit with a restive load (not a filament lamp as the resistance is only a tenth when cold) just to see what happens.
(3) The T2512MH TRIAC is a fairly old design and is not as well behaved as some of the newer 'Hi Com' TRIACS.
(4) Try putting a capacitor in parallel with the TRIAC gate and MT1. Start with 47p and work up until, hopefully the TRIAC operates normally
(5) It is possible that you have a faulty TRIAC; have you tried another TRIAC
(6) Driving a transformer (inductor) with an SCR/TRIAC is fraught with problems if you do not take adequate precautions. One of the main precautions is to use soft start so that the transformer core does not saturate and blow the SCR/TRIAC due to excess current.
(7) Another danger with driving transformers is that you can exceed the maximum rate of change of voltage which can lead to false triggering or even damage.
(8) Inductive loads can also lead to an excessive rate of current change which can damage the SCR/TRIAC
(9) I suspect that, if you are driving a transformer, you will need a couple of back to back SCRs (which are available in higher power ratings) rather than a TRIAC.

There is another thread, presently open on ETO, about phase control of a welding transformer where some of the problems of driving a transformer with SCRs are discussed: http://www.electro-tech-online.com/threads/ac-control-circuit-question.147790/

spec
 
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MaxHeadRoom78

Well-Known Member
Hi Max, I've been over the application note and it is interesting however it seems to me that the zero crossing optoisolator is more superior for power transfer. It does not mention why I cannot use it in my application or why it is not working.
You mention the Triac is always on, because the MOC3043 is a zero crossing device, you need the random phase control type, as shown in the App .
Max.
 

ericgibbs

Well-Known Member
Most Helpful Member
hi Max,
I 'think' the OP means its always ON ie: always conducting, not just over a full cycle, [which you would expect with a ZCD device]

His App should work OK.

Eric
 

StudentSA

Member
I really appreciate that you guys have taken the time to assist, Thanks.

spec,
1) Yes, I will be driving the MOT Spot welder primary coil with this Triac driver.
2) Will try, however I have even tested with no load, i.e I just probe the voltage where the load would be.
3) I have just been past my parts store and picked up some of their high current TRIAC alternatives:
  • BTA16-600 TRIAC TO220 600V 16A
  • BT139X800 TRIAC TO220P 800V 16A
  • MAC223A8 TRIAC TO220 600V 25A
  • MC223A6FP TRIAC TO220 400V 25A
  • JST41Z600B TRIAC TOP3 600V 40A
I also picked up a sprinkle of alterative optocouplers:
  • H11AA1M
  • MOC3011
  • MOC3022M
  • MOC3021M
I will put together a test circuit and see what I can get going.

4) Will try
5) Will swap and test the alternatives
6) I hope that using zero crossing would assist in saturation prevention
7-9) I will look into protection mechanisms.

Besides Triac based driving, what alternatives would be a good idea for transformer driving from a micro.
 

spec

Well-Known Member
Most Helpful Member
Hy SSA,

Wow! we don't normally get such concise and definitive answers: obviously someone with a logical and scientific leaning.:cool:

(1) You have a massive task on your hands to drive a microwave transformer and I think you will need to use at least 50A, 800V, high peak current rating, high com SCRs in a TO247 case mounted on a substantial heatsink with ceramic insulating washers. I can make some suggestions for suitable SCRs which cost around £5 UK each.

(2) Testing with no load and just a probe will be misleading due to TRIAC leakage current: you need a restive load of around 25W (= 2.5K Ohms at 100mA) or so. Once that is working OK I would increase the load to about 250W (= 250 Ohms at 1A) or so. A small heater would do. Or even the wire from a heater. Once that is working you can increase the load to 1KW restive and, once that is working, try driving the transformer but with a resistor in series (we can discuss that further down the line).

(6) Certainly, zero voltage switching has many advantages, and will reduce core saturation but it will not totally eliminate it. One problem with microwave transformers, from what I understand, is that they are operated pretty close to the saturation region anyway so as to use a minimum core cross section and keep costs down. I suspect you will still need soft start.

I think SCRs are your best bet for driving a transformer. But other choices would be IGBTs and possibly high power MOSFETs, from IXUS for example.

Just a bit of cracker barrel advice: take precautions and try to develop your circuit logically and progressively or you will end up knee deep in expensive blown parts, especially TRIACS and SCRs.:arghh:

I have seen this happen many times before and have done the same thing myself, but with audio amplifiers and inverters.:D I even did a bit of modern art by Aralditing blown TO3 semiconductor cans on to a board, complete with picture frame.

Lecture over.:)

spec
 
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MaxHeadRoom78

Well-Known Member
If using a MOT remove the magnetic shunts between the coils, if it has these, they can be punched or pressed out.
Max.
 

spec

Well-Known Member
Most Helpful Member
Max,

Perhaps leave the magnetic shunts in for initial testing as they will limit the input current.

What do you think?

spec
 

Tony Stewart

Well-Known Member
Most Helpful Member
If you notice in the datasheets, all the loads are on the Line side of the Triacs.
Yours is on the Neutral side. In theory both should work, but in practice this method is known as Quadrant IV or 4 where a positive gate trigger latches a negative anode current requires about twice the gate current on typical general purpose Triacs.

So rather than fiddle high higher power lower R gate resistors, just move the load to the Line side.

In addition to other sage comments from fellow advisors, here are some more details on Triacs and make sure you are choosing the correct Anode closest to the gate for triggering.

I suspect this is why your TRIAC won't turn off.
 
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StudentSA

Member
So..... I figured out the problem... and yes I'm ashamed of myself... but in the hope that someone stumbles across this in the future here goes:

A Triac's "Main Terminals" are labled T1, T2 or A1,A2 or MT1,MT2 ... and even though the schematic representation shows them as back to back "diodes" they are not... THEY ARE NOT INTERCHANGEABLE! This was figured out after significant hair loss.

Tony, The schematic I posted above is from the datasheet, but your point is taken.

Anyhow, I tossed the zero crossing opto and instead opted for the random phase opto (as it makes more sense to implement the zero crossing in the microcontroller).
Here is the final result:
DriverTop.jpg
DriverSide.jpg

spec and max, shunts were removed to fit the nice 70mm^2 cable. I connected it up and managed to do a good spot weld, no troubles.

Thanks for putting up with me guys.

Cheers,
StudentSA
 
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Tony Stewart

Well-Known Member
Most Helpful Member
it's an easy mistake ;) and shame on them for not making that clear about which Anode to connect.
 

ericgibbs

Well-Known Member
Most Helpful Member
Anyhow, I tossed the zero crossing opto and instead opted for the random phase opto (as it makes more sense to implement the zero crossing in the microcontroller).
Morning SA,
I would not agree with that conclusion.
ZCD reduces mains radiated interference and its simpler to let a ZCD Opto do the switching,especially when you have a ZCD MOC in your stock.
E
 

StudentSA

Member
Hi Eric,
Indeed, however let me qualify my statement further. The Spot welder that I am driving will be driven in "pulses". either 1 or 2.
Two examples would be:
we may want to drive it using 1 pulse of 1 second <- Sheet metal
2 pulses, first being 100ms and second 250ms. <- Battery Tab

These timings will be set using an Arduino and some dials etc.

Anyhow, if I use the ZC Opto we could end up in a situation where the micro starts the 100ms pulse, but due to the phase of the AC voltage the actual "On" only occurs up to 20ms later.
I will do zero crossing detection but using an auxiliary optocoupler (H11AA1M) to the uC, by doing this I believe I will ensure that any timed pulse starts from the zero crossing point.
 

tomizett

Active Member
A Triac's "Main Terminals" are labled T1, T2 or A1,A2 or MT1,MT2 ... and even though the schematic representation shows them as back to back "diodes" they are not... THEY ARE NOT INTERCHANGEABLE! This was figured out after significant hair loss.
I've done exactly the same myself! I wondered if that was whad had hapened here.... glad you got it sorted out!
 

Tony Stewart

Well-Known Member
Most Helpful Member
Morning SA,
I would not agree with that conclusion.
ZCD reduces mains radiated interference and its simpler to let a ZCD Opto do the switching,especially when you have a ZCD MOC in your stock.
E
I agree

a ZCS opto that costs an extra 50 cents (10pc) is more reliable than any DIY ZCS unless you have done this before and know all the parameters that cause false triggering.

You can use the software to choose how many half cycles to turn on and when. Using random phase may cause grief if you don't know all the root causes of false triggering and early/late triggering around the zero crossing. This still needs care to avoid susceptibility to stray EMI.
 

schmitt trigger

Well-Known Member
So..... I figured out the problem... and yes I'm ashamed of myself... but in the hope that someone stumbles across this in the future here goes:

A Triac's "Main Terminals" are labled T1, T2 or A1,A2 or MT1,MT2 ... and even though the schematic representation shows them as back to back "diodes" they are not... THEY ARE NOT INTERCHANGEABLE! This was figured out after significant hair loss.


StudentSA
Don't be ashamed of yourself, all of us here -experienced guys- have made silly mistakes at one point of time.

The best definition of experience, are the lessons one learns from those mistakes.
 
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