Replacing a relay with a Transistor

[Hero999]

That will work but move R1 and put it in series with Q1's collector and Q2's base, it should act as a current limiting resistor rather than a pull-up.

 

[sazerac99us]

In this case, figure 2, the base of Q2 will never go low and Q2 would never conduct.

 

[audioguru]

DON'T draw an ordinary transistor when you are talking about a Mosfet. I pasted this one with Microsoft Paint.

 

[sazerac99us]

In this case, I was meaning a PNP BJT.

 

[audioguru]

In this case, I was meaning a PNP BJT.

Are you ever confusing!!

Do you understand that you drew a circuit with an NPN transistor driving a PNP transistor that will smoke because there is no current-limiting resistor between them?
Do you understand that you drew an NPN transistor driving a PNP transistor that will also smoke because the PNP transistor has its base shorted to its emitter, then the NPN transistor has no current-limiting resistor?

 

[sazerac99us]

No, I did'nt understand, which is precisely why i am writing to this forum. Figure 1 was my idea for this....someone else suggested something, so I made figure 2, to clarify what he said, because it was'nt clear in my mind.

If you have a suggestion, im all ears, if not, and you only wish to state the obvious, that I don't understand all this, its not necessary. I'm trying to get a BJT as well as a MOSFET solution to this. I apologize for any confusion.

 

[audioguru]

I fixed your circuits in my sketch:

 

[sazerac99us]

I did'nt realize that current would flow from the emitter through the base of a transistor.

Now I have both solutions, the P-MOSFET and BJT. Thanks for your help!

If you ever need some help in any of my areas of knowledge (mechanical testing, motorcycles, women<smile> etc), I'd be glad to help.

Seriously, THANKS!!!

 

[Tarsil]

No, I did'nt understand, which is precisely why i am writing to this forum. Figure 1 was my idea for this....someone else suggested something, so I made figure 2, to clarify what he said, because it was'nt clear in my mind.

If you have a suggestion, im all ears, if not, and you only wish to state the obvious, that I don't understand all this, its not necessary. I'm trying to get a BJT as well as a MOSFET solution to this. I apologize for any confusion.

TRy the circuit in my earlyer post!!

 

[audioguru]

TRy the circuit in my earlyer post!!

Your circuit is exactly the same, except it is missing a resistor to turn off Q2 when it is hot, and to turn off Q2 quickly.

 

[sazerac99us]

Tarsil,

Yes I saw your circuit and was wondering how the base of Q2 would ever be high and noticed it was essentially the same as the one we ended up with.

Thanks (both of you) for your posts!!!

Which is preferable, using an NPN driving the PNP or just driving a p-channel MOSFET directly?

 

[Tarsil]

Your circuit is exactly the same, except it is missing a resistor to turn off Q2 when it is hot, and to turn off Q2 quickly.

Yup...my mistake . Th for pointing that out.

 

[Tarsil]

Tarsil,

Yes I saw your circuit and was wondering how the base of Q2 would ever be high and noticed it was essentially the same as the one we ended up with.

Thanks (both of you) for your posts!!!

Which is preferable, using an NPN driving the PNP or just driving a p-channel MOSFET directly?

I would go with NPN/PNP because i allready have them . But i guess the mosfet wil disipate a bit less heat and the circuit is simpler.Your choice.
And....If is too complicated.....most of the thimes is the wrong way