triacs

Trust me, the view isn't worth the climb.

It means, why would you want to, when there are so many easier ways (wrong part for the job). Brian

(So you haven't sweated up a steep hill to see the view from inside a cloud? Oh well.)

Tough possible, it is not worth the effort. The circuitry needed to commutate the TRIAC off is more than enough to switch the load without the TRIAC. Why do you feel a need to use a TRIAC for this application? Bipolar junction transistors or MOSFETS would work so much better.

Why in the world would you even consider a triac in that application?

The obvious answer would be that you have yet to grasp what a triac is.

Back up a bit, and ask about an SCR. You trigger it, it conducts, and stays conducting until voltage is removed from the anode (or the cathode is removed from ground). It's a latching device.

A triac is basically two back to back SCRs, with the triggers connected together. In a DC situation, it would just be an SCR, since only one would come into play since the polarity across it never changes. So again, you trigger it, it turns on and stays turned on until voltage is removed from the anode (or the cathode is removed from ground).

Unless you need the latching nature of the device, you don't want an SCR. Unless you need to control AC, you don't want an SCR.

Michael

Sure. Get a 12VAC transformer and a 12VAC coil relay, have the triac turn the relay on and off, and use the relay contacts to control the load.

Unless you want something simple. Then use a transistor or power MOSFET to control the load.

Cheers Chris

that can be done with 2 SCR's using a pulse coil xformer method.. #1 scr (on state unit) will operate through one winding of this xformer. the other SCR will derive its anode source from the on state of the first one., the #2 SCR is turned on, it will cause a current pulse in the xformer of the other winding to cause a nill current effect that forces the first SCR to turn off which will also for the #2 to turn off afterwards.

can describe how to use the transistor and part numbers needed?

John P> > what does this mean?

I suggest any device on this data sheet:

Figure 9 on page 5 shows the on state voltage drop for various load currents and base drive currents. Since you want to switch 2 amps, lets use the 2.5 amp curve, to be a bit conservative. For all base currents higher than about 70 ma, the voltage drop across the transistor is almost the same and less than .4 volts. That is the part of the 12 volt supply that is wasted by having this transistor switch the load current. Before getting into the exact circuit to make use of this transistor as the load switch, can you tell me if 70 mA of drive current is available to turn this transistor on, or do we need to add something else to bring the drive current from what is available up to 70 mA needed to switch this device?

Another alternative would be to use a MOSFET transistor, which needs very little current to keep it on, since its input looks like an open circuit, except when it is being turned on or off. Just as with the above bipolar transistors, MOSFETs come in two polarities, and we need to know whether you prefer to switch the positive side of the supply or the negative side, to pick the best type. (Hint. This is another question for you to answer.) But something like these might work: N-Channel (to switch the negative side):

P-Channel (to switch the positive side):

What signal do you have available to operate the switch?

Just for the sake of not confusing the newbies, the SCR (thyristor) turns itself off when the current through it reaches zero or so. This is why they need commutating circuitry for switching DC.

If the supply was AC, it'd be trivial - just provide some gate current, and since the supply is AC, when you remove the gate current and the supply voltage switches polarity, it turns off.

For DC, like the others have said, just a transistor should do the job.

Good Luck! Rich

No, read what I said. An SCR will latch until you remove voltage from it. Unless you need that latching function, an SCR does not at all sound good, just as your original post about a triac was not at all a good choice.

MIchael