I am a noobie at electronics and thought I understood MOSFET's and how they work. Then someone said they were using a DIAC and a TRIAC in a sped controller. They looked like they were performing the same function as a MOSFET. Can someone enlighten me (gently)?
Try googling first, then ask again if that turns out to be too deep.
They are no different in the sense that most of the times they are used switches.
They are quite different in terms of what they can switch and how their switching behaviors are controlled.
- Mosfets, by themselves, can only switch DC current. Triac can switch DC and AC current.
- Mosfet is a voltage-driven device and Triac is a current driven device.
- Triac is self-sustaining and Mosfet is (practically) not self-sustaining. Once you have turned on a triac (it starts to conduct), it will remain conducting all by itself until the current reverses. Mosfets, because of its leakage, will turn off over time.
Mosfets can switch at much higher frequencies than a triac can. Triac can switch much higher current than mosfets can - being inherently a transistor, triacs' voltage drop is independent of current it swiches. Mosfet's voltage drop is linear to the current it switches, making the mosfet unsuitable for high current switching applications.
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dhenry - Thanks for a nice concise explanation.
CrossRoads - Thanks for nothing. When I googled, I got several seemingly conflicting pieces of information. One was that a triac is AC only. One was that a triac is isolated from the controller. One was that the triac was not isolated from the controller. ANd one was my original question. I was hoping that this group would have a nice concise answer from a group of intelligent electronics experimenters, which it did.
The problem with the internet in general is that there is a lot of misinformation out there, and if you are not an expert it may be hard to separate the wheat from the chaff. Fortunately this forum is able to do that. With a rare exception or two.
Well, searches don't always turn out to be the most useful.
vectorges:
dhenry - Thanks for a nice concise explanation.CrossRoads - Thanks for nothing. When I googled, I got several seemingly conflicting pieces of information. One was that a triac is AC only. One was that a triac is isolated from the controller. One was that the triac was not isolated from the controller. ANd one was my original question. I was hoping that this group would have a nice concise answer from a group of intelligent electronics experimenters, which it did.
The problem with the internet in general is that there is a lot of misinformation out there, and if you are not an expert it may be hard to separate the wheat from the chaff. Fortunately this forum is able to do that. With a rare exception or two.
Well triacs are (almost?) always used in AC circutis. Triacs along with SCRs are members of the thyristor family of four layer diodes. A triac is just effectivly two SCRs wired in reverse parallel with their gates tied together, allowing each SCR to handle just the negative or positive portion of the AC sinewave. The main difference in that these 4 layer diodes once triggered stay 'latched on' until the external current decreases to zero, where as a transistor properly biased can operate in a linear manner and regulate the current that flows through it by varying the base current or gate voltage. So Thyristors can only act as switches, where as transistor can be operated as switches or amplifiers.
Lefty
Don't forget the middle ground IGBTs 
vectorges:
dhenry - Thanks for a nice concise explanation.CrossRoads - Thanks for nothing. When I googled, I got several seemingly conflicting pieces of information. One was that a triac is AC only. One was that a triac is isolated from the controller. One was that the triac was not isolated from the controller. ANd one was my original question. I was hoping that this group would have a nice concise answer from a group of intelligent electronics experimenters, which it did.
The problem with the internet in general is that there is a lot of misinformation out there, and if you are not an expert it may be hard to separate the wheat from the chaff. Fortunately this forum is able to do that. With a rare exception or two.
Instead of a general search on Google, try Wikipedia first. It isn't guaranteed correct, but in practice it is 99.99% of the time. It is the most important single site on the Web.
It appears to be a really well written article in this instance.
I don't have any TRIACs since I have not done anything with AC yet, but now I want some.... I am so freaking greedy it is hard to believe. ]