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EDIT: DO NOT even think about using the naive example of a circuit to switch three phase. As someone rightly pointed out this is a complex issue and will almost certainly result in burnt finger or death :S without being a very competent engineer. I am a hobbyist and this was meant to get some feedback and guidance on the subject

EDIT: Just found out about triacs so replace NPN with triac. What else do I need to know about?

I am looking for some help in designing a very simple circuit using a transistor to switch a load based on a 300 hz pulse width modulation circuit. Could someone give me some guidance on values etc for the componenets.

I think the circuit should be something like this:

enter image description here

Could someone give me (ideally and I would really be very pleased) the values for RB, do I need that RL or is the motor itself the load? and do I need anything else?

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You can't control AC with transistors. And is that really three-phase? – Federico Russo May 19 '12 at 7:10
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To control motors powered with AC you need to have complex biasing arrangemants, you need to give more info about your application. IT is not true that you have to use MOSFETS or Triacs, you can use BJT transistors (it's more difficult though) There are also devices called GTO's Gate Turn Off transistors, which are like a combination of a FET (or transistor) and a Triac. – Jason Morgan May 19 '12 at 7:28
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There is an excessively high chance of killing yourself before you learn enough if you aim at any "doing" before you have learned more about the subject. A scattergun of comments: Motro shown with 2 leads will not talk to 3 phase as is. If you have more leds (and you must have) theh the diagram should not be a block diagram that is unable to actually built. You draw an NPN transistor and show its symbol but talk of TRIACs. Diagram must match reality. You do not say what you are trying to do and we can't tell. How do you know the circuit is simple (it may be)? Why 300 Hz? Who says? Details pse – Russell McMahon May 19 '12 at 7:31
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@Paul - Yes, I've seen that one. But it creates the 3 phases from a DC power supply, it's not working with 3-phase mains. – stevenvh May 19 '12 at 8:03
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@PaulSullivan - why don'y you tell us what you actually want to do instead of wanting to look at random circuits and iC's. You are so far wasting your time and hours. More usually than not when a question follows the lines this one has it turns out that what the questioner wants is quite different than what everyone thinks. What voltage? What power? Where is the 3 phase coming from? Does it always have 50% power but speed varies? - possible but not usual? What does it do? One or many? More ...? – Russell McMahon May 19 '12 at 9:54
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1 Answer

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No, nobody can give you a value for Rb since the circuit it is in makes no sense whatsoever.

No, you probably don't need Rl, although again, it is in a nonsensical circuit so it is hard to tell what you intend. However, you should not need any explicit additional resistive load in series with the motor.

Yes, you need something else. It's impossible to say what since you haven't provided a clear description of what you want. However, it is clear that the circuit you show isn't going to do anything like motor control. At best it will function as a pyrotechnic transducer, once.

Obvious things wrong with your circuit:

  1. You say "3 phase AC", but there are only 2 wires. 3 phase requires at least 3 wires, sometimes four.

  2. You mention a 3 phase motor, but that motor only has two connections, so it can't possibly be three phase.

  3. You say the power is AC, but are trying to use a single NPN to switch it. At best that will turn on for half the wave, but bipolar transistors are generally not designed for large reverse voltages. A single bipolar here makes no sense.

  4. I have no idea what you think Rl is supposed to do.

There are just too many things wrong here to try to fix.

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The accepted answer is an answer that says that the question makes no sense :) – Telaclavo May 19 '12 at 13:41
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It is true - the question was posted by me and now I realise that a) the circuit does not deal with 3 phase and was purely to get some pointers as to what I was dealing with - actually all the (semi-negative and non-negative) answers gave me a lot of reference and reading material. It all helps – Paul Sullivan May 19 '12 at 13:47

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