How To interface a resistor to the v+ rail of a component with picaxe?

glab123

New Member
How To interface a transistor to the V+ rail of a component with picaxe?

(transistor, not resistor)

Hello,
I have a speaker driver board which is powered by 9V. On its V+Rail There is a sitch bounce which poweres the whole circuit.
I want to replace the sitch with a transistor to control the power with my picaxe which is 5V.
My problem is that in the documentation (menual 3), there are only examples of connecting the transistor on the 0V rail:
schematic.png

And my problem is that I want to connect the transistor on the V+ rail, like that:
schematic.png

Did I misunderstand something? or there is a way to do what I want?
 
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nick12ab

Senior Member
NPN transistors should be connected to the 0V rail when being driven directly from a PICAXE. If the NPN transistor is connected to the V+ rail, the performance of the transistor will be very poor in your application (search 'emitter follower' on the Internet if you want to learn more).

To do what you want, you can use a PNP transistor instead. If V+ is more than the PICAXE supply voltage, then an NPN transistor is also required to drive the base of the PNP transistor, otherwise the PNP transistor will be on all the time because the output voltage of the PICAXE pin will always be lower than V+. The NPN transistor can be a small low-power one; only the PNP transistor passes the large amount of current required by the load.
 

glab123

New Member
Thank you very much, but how can I choose the right transistors for that? and can you make a schematic for that, because I'm not perfectly sure how everything goes.
 

nick12ab

Senior Member
Thank you very much, but how can I choose the right transistors for that?
In general, find out the maximum current consumption of your load (the speaker driver board) then select a PNP transistor that can switch that current continuously (collector-emitter current).

The NPN transistor can be something small. The base current requirement for the PNP transistor will be small (a few mA at most though this depends on the load current and gain of the PNP transistor) so almost any small signal transistor will be fine. Don't forget the base resistor is still required on the PNP transistor.
 

darb1972

Senior Member
Further to what Nick has mentioned, if you need further assistance with circuitry it would be extremely helpful if you could supply a data sheet for the speaker driver board. As Nick said, you need to know the maximum current consumption for the load (the board in this case). Knowing this current will allow for an appropriate switching design. It allows one to calculate the maximum necessary base current needed to ensure saturation and therefore ensuring the transistor/s act like switches and have minimal voltage drop across the Collector/Emitter. Poor design might result in the transistor/s not acting as a switch and this will lead to issues.

An NPN design will be less complicated to implement in the case of switching a load on a higher voltage rail. Even a FET design might be suitable. FETs can be very efficient electronic switches.

In your diagram above, a Darlington transistor has been used. You could use one of these if the load current is high but it's probably not necessary. Often with BJTs higher Ic current results in lower hFE gain so this is where a Darlington might prove useful. It has really high hFE. In the case of the one in your example, the hFE is a minimum of 10000. The maximum Ic for the same transistor is 800mA. A similar non Darlington transistor with an Ic of 800mA might only have a hFE of somewhere between 100 to 400. That's a massive difference.

Again, as Nick said, it really depends on the MAXIMUM current draw of your load.......................
 

Goeytex

Senior Member
The attached circuit should do what you want. If driving an inductive load across the output, add diode protection.

I show a 2N3906 as the main switch. This is rated at 200 ma. IF you need more current use a 2N2907 which is rated at 800 ma.

If you need more than 800 ma. Then provide exact specifications.
 

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