Run general triac as Logic Level Triac

[skmdmasud]

Hi..
as shown below, i am following this design and using Atmaga8 to control load using triac BTA16 which is not a logic level triac, therefor i am unable to fully switch on my triac. i dont want to use optocoupler because

1. my circuit uses transformer less power supply therefor isolation do not make sense.
2. cost (anyway...just wanted to add a point, 2 is better than 1).

My transformer less power supply can give a max of 70mA, and i need to run 3 triacs, I have 10 BTA16 in hand. What can i do. Can we device something like a mosfet driver using general purpose transistors to run the triacs gate from 220VAC .

 

[ronsimpson]

logic level triac

logic level is 0/5 volt or 0 to 3V some thing around TTL levels.
Your driver is 0 to 15V and not "logic level"

 

[skmdmasud]

logic level is 0/5 volt or 0 to 3V some thing around TTL levels.
Your driver is 0 to 15V and not "logic level"

Actually BTA16 its not a sensitive gate triac, i need some way to turn them fully on using my 5V from uC

 

[alec_t]

i need some way to turn them fully on using my 5V from u

Use the uC to control a transistor which has the 15V as its supply. The transistor then switches the 15V to the triac gate.
But for safety's sake you really should provide mains isolation.

 

[skmdmasud]

Use the uC to control a transistor which has the 15V as its supply. The transistor then switches the 15V to the triac gate.
But for safety's sake you really should provide mains isolation.

Hi..
ok i will change my powersupply to 15v and feed it to the transistor for switching as you have said.

 

[audioguru]

The datasheet for the BTA16 shows that the maximum gate voltage is only 1.3V but some of them (the suffix code) need up to 100mA. Where will this high current come from?

 

[alec_t]

Oops. Didn't check the BTA16 spec. So 15V won't do for its gate. .

 

[MrAl]

Hi,

A "regular" triac should really NOT be driven with a constant source current anyway, it should be pulsed every time it needs to be turned on. This is the classic solution to triggering a triac when the gate current has to be high enough to make sure it always turns on when it should while at the same time not overpowering the gate.
So if you provide a pulse instead of a constant drive, you can afford to use 80ma if needed and not overdraw power from the power supply even though it may only be able to deliver 50ma constant current. A slightly larger filter capacitor is needed to store more energy that's about it.
100us pulse is probably good enough. That means low duty cycle for both the gate dissipation and the power supply dissipation.

Also, whenever possible it is better to drive the gate with a negative pulse rather than positive, because most regular triacs turn on with less negative current than positive current into the gate meaning you need less current to begin with. This just means that the triac will be biased to look like it's Main Terminal 1 is being driven, not the gate, but it will actually still be the gate that is being driven.

 

[tunedwolf]

I usually drive triacs with a burst of pulses to ensure it turns on even if an inductive load kicks it off again. As Mr Al already mentioned, negative drive usually results in less gate current being required

 

[ChrisP58]

The max gate threshold voltage of the BTA16 is only 1.7V so, from a voltage point of view, it would be considered a "logic level" device. However, it does need more current than you'll safely be able to get from most uController pins.

But I think your main problem is that the BTA16 only triggers in three of the four quadrants, and the missing quadrant is one of the two you are using. So, at best, you can only guarantee triggering 50% of the time. Kind of like a half wave rectifier. That would explain why you're only getting ~ half the output voltage you expect.

So, you need to change your circuit connections around a bit. This will drive the triacs in quadrants 2 and 3.

1) Make MT1 of your three triacs common hot.
2) Tie the +5V rail of your low voltage power supply to MT1 (This makes 'logic ground' 5V negative with respect to MT1)
3) Drive each of your three triac gates with a common emmiter NPN transistor to give you current gain. Add a gate to collector resistor to limit the gate current to ~50mA
And, as has been said.
4) Pulse the gates with at least 50mA for ~100uS immediately following zero crossing.

Now, I understand that you feel that what you are trying to do does not need zero crossing, but it will give you the ability to properly give each of your three triacs a brief, healthy pulse without exceeding your 70mA supply limit.

P.S.
You now have at least three threads with different questions, but are on the same project. It is better to keep them in the same thread, instead of multiple threads. Especially when new questions build upon info that has already been covered in previous questions and answers. Members who join in down the line can have access to the previous discussions.

Info for this reply came from https://www.onsemi.com/pub_link/Collateral/BTA16-600CW3-D.PDF

 

[skmdmasud]

The max gate threshold voltage of the BTA16 is only 1.7V so, from a voltage point of view, it would be considered a "logic level" device. However, it does need more current than you'll safely be able to get from most uController pins.

But I think your main problem is that the BTA16 only triggers in three of the four quadrants, and the missing quadrant is one of the two you are using. So, at best, you can only guarantee triggering 50% of the time. Kind of like a half wave rectifier. That would explain why you're only getting ~ half the output voltage you expect.

So, you need to change your circuit connections around a bit. This will drive the triacs in quadrants 2 and 3.

1) Make MT1 of your three triacs common hot.
2) Tie the +5V rail of your low voltage power supply to MT1 (This makes 'logic ground' 5V negative with respect to MT1)
3) Drive each of your three triac gates with a common emmiter NPN transistor to give you current gain. Add a gate to collector resistor to limit the gate current to ~50mA
And, as has been said.
4) Pulse the gates with at least 50mA for ~100uS immediately following zero crossing.

Now, I understand that you feel that what you are trying to do does not need zero crossing, but it will give you the ability to properly give each of your three triacs a brief, healthy pulse without exceeding your 70mA supply limit.

P.S.
You now have at least three threads with different questions, but are on the same project. It is better to keep them in the same thread, instead of multiple threads. Especially when new questions build upon info that has already been covered in previous questions and answers. Members who join in down the line can have access to the previous discussions.

Info for this reply came from https://www.onsemi.com/pub_link/Collateral/BTA16-600CW3-D.PDF

Thanks for the detailed reply.
I tried your step 1 and 2 first, the moment i applied power my fuse blew and the zener silently died (some how the HOT(live) and 5V derived from it dont mix). So i gave up this method and decided to go for opto isolator to drive my 3 triacs, power requirement will be like driving 3 leds, and i can also use a smaller X rated capacitor.

About the different thread, yes i should have had one thread but as i posted i also learned new things and so i posted another with different requirement as per my new learning.

Regards.

 

[KeepItSimpleStupid]

Smart move - opto drive for the triacs.