I corrected the diagram. Is it OK now?
15mA is too much for PIC MCU, or it is absolute maximum.
The 2N2904 is a PNP transistor and will not work in this circuit. As noted before, common collector is not the best way to drive this load.Does hFE tells how many times is the collector-emitter current greater that base-emitter?
I need to control 12V, 150mA siren connected to the PIC MCU putput and I thought I would use 2N2904 NPN and connect like diagram shows. I am not sure if I choosed the right transistor.
I corrected the diagram. Is it OK now?
15mA is too much for PIC MCU, or it is absolute maximum. I need other NPN then, the one that has higher base/collector current ration, at least 3 times higher - can you help to fing some?
The 2N3904 characteristic curves, including the one for saturation, conveniently stop at 100mA like I said.The datasheet for the 2N3904 and most little transistors have a written spec called "Collector-Emitter Saturation Voltage" and it shows the base current as 1/10th the collector current.
The 2N3904 characteristic curves, including the one for saturation, conveniently stop at 100mA like I said.
All of the charts show remarkable deterioration of performance as 100mA is approached. At 150mA I would expect nothing but disappointment.
I still don't even understand where you guys get 1/10 ratio. On wikipedia amplification factor marked as hfe or ß is given as ratio of collector vs base current Ic/Ib. As I said in datasheet datas are not shown in a simple form.
PS I still don't understand why first of my diagram won't work. Isn't same if syren is connected to the emitter or collector? Why current won't flow if syren is connected to colector? The problem is that I only have + wire leaded to the syren. The negative wire is simply syrens housing connected to car's body. So how can I connect syren by second diagram? Do I need PNP then?
Driving the load with an emitter follower, when the base can only go to 5V, will limit the voltage across the load to ≈4.3V. Not only is this not optimum, it probably won't work.The first diagram will sort of work, but not optimally.
This is nitpicking, but the extra transistor is required so that the PNP can turn OFF, not ON.2N2222 are common and would suffice, too, if you could connect it as Nigel said.
But as you only have access to the positive input of the buzzer (is it a buzzer, a piezo, what? I'll assume that polarity matters here)...
As already said, if you need to source current, you'll need a PNP transistor. But as your siren doesn't use the same supply as the PIC, you'll need an extra transistor to do the magic (to turn the PNP transistor ON, for the PIC alone would not handle it).
Castilho
edit: One thing I couldn't understand: you said that your siren was already connected to ground. If that's so, how do you intend to connect it the way you showed in your first post?
Driving the load with an emitter follower, when the base can only go to 5V, will limit the voltage across the load to ≈4.3V. Not only is this not optimum, it probably won't work.
Of course PNPs have reversed polarity (so if you want to know what that looks like let us know)
Michael
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