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I am thinking of using a BTA04 type TRIAC in a project to switch a power hungry mains appliance, which could draw up to 12A. I can't find a forward voltage anywhere in the datasheet, or an on-state resistance. I presume, looking at the symbol, and internal construction, that it drops about 1V (silicon diode drop). In which case it would dissipate 12W at this kind of current level and require a good heatsink. Is this correct?

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I'm not familiar with that part so I did a search for "BTA04 triac" and found a datasheet right away.

First, this part is obsolete, so not a good idea for new designs. Second, it's only rated for 4A RMS continuous, so using it with a load that could draw 12A is a bad idea.

The forward drop is in the datasheet, and is listed as 1.65V. That's about as expected since a SCR has two junctions in its conduction path. That's roughly what you'll get from most SCRs and TRIACs.

The method of your power calculation is correct. It will dissipate power equal to the current thru it times the voltage accross it, which would be 12W at 12A and 1V. However, that is considerably out of spec for this part.

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  • \$\begingroup\$ Hmm. I didn't notice this was a 4A only part. I'll have to choose another one. Either way, I should expect around ~1.6V drop so about 18W dissipated, which is quite a lot, but not impossible with a good heatsink. \$\endgroup\$ – Thomas O Jun 5 '11 at 23:20
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The first google hit for "BTA04 voltage drop" takes me here.

There, in the product specs, it states a forward voltage drop of 1.65V

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  • \$\begingroup\$ The only reference to 1.65V I can find in the datasheet is Vtm. Which would be why I didn't notice it I guess - Vd or Vdrop would be what I would be expecting. Assuming Mouser are right, though... \$\endgroup\$ – Thomas O Jun 5 '11 at 23:14
  • \$\begingroup\$ I see the same quoted on other sites: state voltage drop VTM <1.65V (ITM = 5.5A) \$\endgroup\$ – Majenko Jun 5 '11 at 23:16
  • \$\begingroup\$ Definition found on-line: "VTM: PEAK FORWARD ON−STATE VOLTAGE (SCR) / PEAK ON−STATE VOLTAGE (Triac): The maximum voltage drop across the main terminals at stated conditions when the devices are in the on−state(i.e., when the thyristor is in conduction). To prevent heating of the junction, the VTM is measured at a short pulse width and low duty cycle." \$\endgroup\$ – Majenko Jun 5 '11 at 23:20
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I know it is an old subject, but I thought I could a drop my 2 cents for future reference.

  1. Indeed the voltage drop is between 1.5V and 2V on triacs.
  2. Before choosing the rating of your triac, you need to analyze the load of your power supply: inrush current and permanent regime. If 12A is just the inrush current, then a 4A triac is enough, as ITSM (inrush current) is around 30A. If 12A is your RMS current, then I suggest that you prepare a good heatsink (5°C/W) with a 20+ A triac like the BTA24.

NB: There is no such thing as a "forward voltage" on a triac because it is a bidirectional device; hence, the notion of forward is irrelevant. In a triac the equivalent parameter is the VTM, which stands for Voltage drop of Triac (Maximum).

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