Problem with driving triac with phototransistor

Thread Starter

daryooosh

Joined Jun 15, 2019
55
Hello
In following circuit:
circuit.png
The triac is always on (mcu = 0 and mcu = 5) but can not deliver sufficient power to the Load (Load is a 32 watt electromotor.)
How can i resolve the problem?
 

Thread Starter

daryooosh

Joined Jun 15, 2019
55
Hello,

Why not use a dedicated driver?
The MOC3043 would be fine to use.
See the datasheet for more info.

Bertus
Thanks, I'm familiar with mMOCxxxx devices but i was create the prototype PCB with TLP521-4 device which is 4 phototransistor in one IC.
 

LesJones

Joined Jan 8, 2017
3,773
Just looking at the datasheet for the T410 shows that it could require up to 10mA gate current to turn it on. Your 150 K series resistor will only give about 2 mA at the crest of the waveform (About 308 volts.)
For the photo transistor in the TLP521 to not provide any gate current is must be able to stand at least 308 volts between the collector and emitter. It is only rated at 55 volts.

Les.
 

Thread Starter

daryooosh

Joined Jun 15, 2019
55
Just looking at the datasheet for the T410 shows that it could require up to 10mA gate current to turn it on. Your 150 K series resistor will only give about 2 mA at the crest of the waveform (About 308 volts.)
For the photo transistor in the TLP521 to not provide any gate current is must be able to stand at least 308 volts between the collector and emitter. It is only rated at 55 volts.

Les.
Thanks
Do you mean that the logic of my circuit is correct and the only problem is that the correct elements are not selected?
 

LesJones

Joined Jan 8, 2017
3,773
You seem to have picked devices at random without checking that their specifications meet the requirements of your circuit and without doing any calculations.

Les
 

Thread Starter

daryooosh

Joined Jun 15, 2019
55
You seem to have picked devices at random without checking that their specifications meet the requirements of your circuit and without doing any calculations.

Les
Thanks
I were be aware of gate current but why is the gate current specified as a maximum in the datasheet? I thought I(gt) is the maximum value allowed and minimum allowable value is "guaranteed at 5% of I(gt) max" (as noted below of the table4 of datasheet).
 
Last edited:

panic mode

Joined Oct 10, 2011
2,215
you need to read the datasheet.
Table 1 at the very begin of the document shows range that Igt is 5-35mA for the product range (more than one product described)
Table 3 shows that absolute maximum gate current is 4A (max 20us)
Table 4 shows actual test conditions with Igt value of 10mA for T410. That is the recommended value for gate current for T410 as tests are used to prove that something works normally. if you want more sensitive device, choose T405 which can do the same thing with 5mA gate current.
 

Thread Starter

daryooosh

Joined Jun 15, 2019
55
you need to read the datasheet.
Table 1 at the very begin of the document shows range that Igt is 5-35mA for the product range (more than one product described)
Table 3 shows that absolute maximum gate current is 4A (max 20us)
Table 4 shows actual test conditions with Igt value of 10mA for T410. That is the recommended value for gate current for T410 as tests are used to prove that something works normally. if you want more sensitive device, choose T405 which can do the same thing with 5mA gate current.
I read the T410 and many other datasheets. I noticed that in some cases the "Maximum Rank" tables show the worst cases rather than continuous operating parameters (like I(gt)-4A for 20 uS). But usually other tables have three columns called "Min", "Typ" and "Max" which show parameters related to continuous work.
However, I now know that Triac datasheets have their own definitions and standards.
Thanks for your help
 

DickCappels

Joined Aug 21, 2008
9,078
Turning on traics and SCRs slowly makes them susceptible to damage. You want a sharp transition from 0 V on the gate to more than the required trigger current. Triggering on the slope of a sine wave can work but is not a good idea. Notice that most lamp dimmer circuit you find on the internet (as below) use a daic to make a trigger pulse for the gate by dumping the charge in the timing capacitor (C1) into the gate with a very low rise time.
1660741032586.png
The MOC3043 uses an internal triac to dump a fasts risetime pulse into the gate. Not only that, but it has circuitry to make that the gate is only driven when the power line is near a zero crossing, thus reducing the stress on the triac and at times other circuitry compared to the case in which the traic can be switched on when the line voltage is high.
 

Thread Starter

daryooosh

Joined Jun 15, 2019
55
Turning on traics and SCRs slowly makes them susceptible to damage. You want a sharp transition from 0 V on the gate to more than the required trigger current. Triggering on the slope of a sine wave can work but is not a good idea. Notice that most lamp dimmer circuit you find on the internet (as below) use a daic to make a trigger pulse for the gate by dumping the charge in the timing capacitor (C1) into the gate with a very low rise time.
View attachment 274028
The MOC3043 uses an internal triac to dump a fasts risetime pulse into the gate. Not only that, but it has circuitry to make that the gate is only driven when the power line is near a zero crossing, thus reducing the stress on the triac and at times other circuitry compared to the case in which the traic can be switched on when the line voltage is high.
Thanks for your great tip(Turning on traics and SCRs slowly makes them susceptible to damage.).
I found the your comment and attached documents useful to work with Triac. One of the documents describe the parameters added in Triac datasheets and another one have 10 golden tips to work with Triac.
 

Attachments

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DickCappels

Joined Aug 21, 2008
9,078
IGT or I(gt) is an indication of the sensitivity of the triac's gate threshold current. IGT (Max) would then be the maximum threshold voltage for the conditions specified, meaning that the actual threshold will not be higher than that.

I could not locate your attached document.
 

Thread Starter

daryooosh

Joined Jun 15, 2019
55
yo
IGT or I(gt) is an indication of the sensitivity of the triac's gate threshold current. IGT (Max) would then be the maximum threshold voltage for the conditions specified, meaning that the actual threshold will not be higher than that.

I could not locate your attached document.
You can check it again.
However the original links are :
https://www.ween-semi.com/en/media/97/download?origin_nid=1651
and
https://class.ece.iastate.edu/ee330/miscHandouts/AN_GOLDEN_RULES.pdf
 

DickCappels

Joined Aug 21, 2008
9,078
From the application note to which you linked:
"Rule 1. To turn a thyristor (or triac) ON, a gate current
≥ IGT must be applied until the load current is
≥ IL. This condition must be met at the lowest
expected operating temperature."

This agrees with my statement in post #14
 
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