Triac Equivalent - soldering station

Thread Starter

mike69

Joined Dec 22, 2012
83
I have a schematic which uses a triac, for example a BT136. Please have a look at the attached schematic (this is a schematic for a soldering station)
What parameters from datasheet we should take into consideration when we are searching for a equivalent triac ? For example, if I will use BT138 instead of BT136, the soldering station will work correctly ? I searched the datasheet for both BT136 and BT138, and I saw that the gate trigger current for BT138 is highter that the gate trigger current of BT136. Meanwhile, in the MOC3041 datasheet it does not mention the maximum current, only the surge current. So I cant find if the BT138 is compatible with MOC3041. I am wondering if I can use BT138 as a substitute for BT136, in the uploaded schematic ?

Thanks in advance.
 

Attachments

There are couple of series of those triacs. We need to be clear about the ones you're using specifically. For example, BT138-600E, BT136 series E. Have you looked into the datasheets? If you have them, upload them. Some versions are sensitive gate triacs, while others are not. Does your triac marking code say crudely BT136 or BT138?
 
Generally speaking, there's a main difference between them. For the BT136, IT(RMS) is 4A and for BT138 it's 12A. As said above, I need to know the specific serie in order to see other details.
 

Thread Starter

mike69

Joined Dec 22, 2012
83
The part numbers are:
BT136 600
BT138 600
And the optotriac is:
MOC3041
I cant upload the datasheets right now.
I am waiting for an answer.
 

Dodgydave

Joined Jun 22, 2012
9,912
Both Triacs are 70mA @1.5V max triggering , so either will do for your soldering iron, the opto-coupler has 60mA max current.
 

KeithWalker

Joined Jul 10, 2017
1,528
Both Triacs are 70mA @1.5V max triggering , so either will do for your soldering iron, the opto-coupler has 60mA max current.
I designed an almost identical circuit that avoids the complexity of isolating the trigger circuit. I powered the soldering iron from the un-smoothed, rectified 24 volts ac. That way I could switch it with a power MOS FET. The diode shown in the diagram connected to R8 is a red LED.
I used an inexpensive 2.3 to 30 volt digital meter to indicate the temperature. It works very well.
SolderingIronControl_2.jpg
 

Dodgydave

Joined Jun 22, 2012
9,912
I designed an almost identical circuit that avoids the complexity of isolating the trigger circuit. I powered the soldering iron from the un-smoothed, rectified 24 volts ac. That way I could switch it with a power MOS FET. The diode shown in the diagram connected to R8 is a red LED.
I used an inexpensive 2.3 to 30 volt digital meter to indicate the temperature. It works very well.
View attachment 159079
Looks good, does the circuit switch rapidly as there is no Hysteresis on the op amp, if not i would put a 470k to 1M on pins 5,7.
 

KeithWalker

Joined Jul 10, 2017
1,528
Looks good, does the circuit switch rapidly as there is no Hysteresis on the op amp, if not i would put a 470k to 1M on pins 5,7.
It doesn't switch very rapidly. The red LED dims briefly at switchover but it is not enough to cause the FET to overheat. I have been using it for about a year now without any problems. I am so satisfied with it that I have built a backup unit. I'm just waiting for the connector so I can complete it.
 

Thread Starter

mike69

Joined Dec 22, 2012
83
1. So in the original schematic from the first post, using BT136 600 or BT138 600 will work fine ?
2. Are there any methods to calculate the current which flows through the optotriac (pins 4 and 6) ?
 
Last edited:

ebp

Joined Feb 8, 2018
2,332
The reality is that probably just about any triac rated for at least 50 volts and 3 amps or more would work without any problem as long as the gate trigger current is under about 20 mA.

The zero-crossing optocoupler and resistive load preclude high dv/dt, so that is not an issue. If the trigger current of the triac is too high there could be issues with unreliable triggering because of the low supply voltage. R10 could be reduced to 100 ohms if required for less sensitive triacs. The optocoupler zero cross circuit will hold off triggering above 20 V (max), so the peak trigger current won't exceed about 20V/R10 under any circumstances.
 

Ramussons

Joined May 3, 2013
915
I have a schematic which uses a triac, for example a BT136. Please have a look at the attached schematic (this is a schematic for a soldering station)
What parameters from datasheet we should take into consideration when we are searching for a equivalent triac ? For example, if I will use BT138 instead of BT136, the soldering station will work correctly ? I searched the datasheet for both BT136 and BT138, and I saw that the gate trigger current for BT138 is highter that the gate trigger current of BT136. Meanwhile, in the MOC3041 datasheet it does not mention the maximum current, only the surge current. So I cant find if the BT138 is compatible with MOC3041. I am wondering if I can use BT138 as a substitute for BT136, in the uploaded schematic ?

Thanks in advance.
Yes. In this case, BT138 can safely replace a BT136.
 

ebp

Joined Feb 8, 2018
2,332
No, 90 mA would be perfectly OK for the output. This is not a steady-state current. The actual time that that current would flow would be in the range of several microseconds - just long enough for the main triac to turn on. Once the triac turns on, there is no longer any mechanism for the gate current to flow in that circuit - the voltage between the main terminals drops to something under 2 V so only a very small current can flow from MT2 into the gate. The triac in the driver output will probably turn off completely.

The ON Semi datasheet specs 1 ampere as the absolute maximum repetitive current for 100 µs pulse width at 120 Hz. Normally you stay some way away from the absolute maximum, but I wouldn't hesitate to run it at 500 mA peak. You must also consider the average power, which is a bit difficult to calculate. If you look at Figure 14 of the ON datasheet, you will see that a limiting resistor of 180 ohms is used for 115 VAC. You are running at about a fifth of that, so a resistor of 36 ohms would be equivalent.

It is not uncommon to use the opto drivers to trigger quite large triacs where you might want 200 mA or more bashed into the gate. If gate current is too low the triac tends to turn on in localized areas which can lead to hot-spotting and destruction. By using a high-current gate pulse localized turn-on is avoided and the whole die area tends to turn on at nearly the same time.

You should be able to find applications notes for the MOC series at the ON Semi website and for similar devices from other manufacturers. I know Motorola published ap notes several decades back when the MOC series was new. ON is what is left of that division of the former Motorola.
 
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