Using a transistor with AC?

Is it possible to use a transistor with AC? if you put a diode in there so no current flowed through it when the current reversed, but what could I do about the difference between the base and collector?

• Can you explain in more detail what you're trying to do? An FET can pass AC, as used in transmission gates. en.wikipedia.org/wiki/Transmission_gate – endolith Mar 24 '10 at 16:33
• Sorry, I didn't provide much detail. I know a triac would b the 'correct' item for this, but I'm wanting to know if a transistor could do it also. My problem is that if a transistor passes the AC, the voltage will vary over time; If the voltage at the base is constant, I would have to make sure the this difference never exceeds the max base-collector voltage; But if this voltage also varied, at the same frequency, I cold use higher AC voltages without worrying about the base-collector max. – CH Mar 24 '10 at 19:06
• The problem, as I see it, is going to be power dissipation. When the transistor is swinging through its conduction area, it is required to drop a significant amount of voltage. This is not true if the transistor is switched completely on or off, like it is in DC switching applications. Essentially, what you would have is a half-wave amplifier. 100 watts is a lot of power for a speaker, but not so much for a toaster. – Robert Harvey Mar 24 '10 at 21:17
• Actually, I do want to use the transistor for switching, not amplification; Just I can't see how I can use DC switch AC. – CH Mar 24 '10 at 22:29
• I am trying to us LV to switch HV, but using a transitor (not triac or relay). I am not making a dimmer control. The transistor need only operate on positive cycles, I would put a diode in parallel with it so it doesn't block negative ones (and to protect it from reverse current). I am wanting to use it as a switch, not to amplify anything. – CH Mar 26 '10 at 0:22

Yes, there are several ways to build an "AC switch" entirely out of transistors.

two diodes and two transistors

You can put a diode in series with the collector of one (nFET or npn) transistor that conducts when the "hot" line of the mains is positive relative to "neutral", so when the CPU turns on that transistor, the transistor turns on the light bulb during the positive half-cycles. (zero electrons flow through this diode+transistor connection when the transistor is off, or during the negative half-cycles).

You can put a diode in series with the collector of another (pFET or pnp) transistor that conducts when the "hot" line of the mains is negative relative to "neutral", so when the CPU turns on that transistor, the transistor turns on the light bulb during the negative half-cycles. (zero electrons flow through this diode+transistor connection when the transistor is off, or during the positive half-cycles).

Normally the CPU would either turn both transistors on, or both transistors off.

The "neutral" line and the emitter of each transistor are all connected, and the CPU power supply and the gate of each transistor stay within a few volts of neutral.

This approach may be simplest to understand, but in practice other AC switches have lower net cost.

two transistors and a bunch of diodes

You can put a pnp and a npn transistor together so the combination acts identical to a SCR. (See the Wikipedia SCR schematic).

Then you can use a bunch of diodes to steer the current so the SCR can turn on and off both the positive and negative half-cycles. (See the Wikipedia SCR dimmer schematic).

four transistors

You can build two SCRs out of a pnp and npn transistor each. Then connect them in antiparallel so the combination acts as a triac. (See Wikipedia: TRIAC and Wikibooks: Power Electronics: Triac).

details

You'll need to use transistors that can handle voltages higher than your average mains voltage and also currents higher than your average load current. Your favorite supplier should be able to hook you up with several kinds of transistors that meet that criteria. You might also look at a triac or IGBT or a solid-state relay -- they have (in effect) several transistors combined into a convenient package.

Timing the off-to-on transition or the on-to-off transition to occur close to the zero-crossing (when the voltage across the transistors is close to zero) puts less stress on the transistors, so they stay cooler.

Much like the acronym LASER has become an ordinary word "laser", the acronym "TRIAC" has become ordinary word "triac" -- both "TRIAC" and "triac" refer to the same thing.

No, as you said a transistor acts like a diode when it is "on" but as a subsitute you could either use a triac for low current applications or a solid state relay for higher current applications.

• In regard to the diode, I mean putting it in parallel with the transistor in order to protect it from reverse voltage, which I expect to exceed the transistors maximum; This also means it only handles the positive part of the AC sine wave. – CH Mar 24 '10 at 19:10
• Triac for low current applications. When I think low current I think 500 mA or less, a TRIAC can go much much higher than this. – Kortuk May 2 '10 at 7:42
• What is the difference between triac and TRIAC?!!! – 0xakhil May 22 '11 at 12:10
• @oxakhil - "Much like the acronym LASER has become an ordinary word "laser", the acronym "TRIAC" has become ordinary word "triac" -- both "TRIAC" and "triac" refer to the same thing." (last paragraph in @davidcary's answer) – stevenvh Jun 11 '11 at 6:44