200mA NPN

[grekson]

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.

 

[Nigel Goodwin]

You've wired it completely wrong, the siren MUST be in the collector, with the emitter direct to chassis. Should be OK if done like that.

 

[audioguru]

The base current must be 1/10th the collector current for the transistor to saturate.
hFE is used for a linear amplifier transistor that has plenty of collector to emitter voltage.

For a base current of 15mA the resistor should be 270 ohms.

 

[grekson]

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?

How do you get the 270ohm? Did you use (5V-1V)/15mA. Where in datasheet you get base/current "ratio"? Need to understand that I won't need to aks every time.

 

[grekson]

This is what I found on Wikipedia;
The common-emitter current gain is represented by βF or hfe; it is approximately the ratio of the DC collector current to the DC base current in forward-active region. It is typically greater than 100 for small-signal transistors but can be smaller in transistors designed for high-power applications.
But in datasheet data are not simple and there are a whole bunch of numbers.

 

[Nigel Goodwin]

I corrected the diagram. Is it OK now?

Yes OK now - but as AG suggests, lowering the base resistor may be helpful - personally I'd probably just try it and see what it measures like.

15mA is too much for PIC MCU, or it is absolute maximum.

15mA is fine, 25mA is the maximum for most pins.

 

[mneary]

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.

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.

Your new diagram uses 2N3904, which at least is NPN. Although it is rated for 200mA, observe that all of the characteristic curves stop at 100mA because its gain all but disappears above that level.

 

[audioguru]

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.

 

[shimniok]

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?

That's not the best approach. You need to understand how bipolar junction transistors work, then you'll understand that the above isn't what you need.

As I understand it -- and the gurus will correct me, I don't doubt -- BJTs can either be "off", operate as amplifiers, or operate "fully on". To act as a switch, that is, maximizing current through the driven device, and minimizing the voltage drop across the collector/emitter and thus reducing heat, you need to run the BJT in the "fully on" or "saturation" mode. Essentially saturation occurs when you can't get any more current into (out of) the collector with additional base current.

To ensure a BJT is in saturation the rule of thumb is to force 1/10th the desired collector current into the base by selecting the base resistor value. As mentioned there's usu. a voltage given in the datasheet as to when the transistor just enters saturation. But you want to push it well beyond that point to ensure it doesn't come out of saturation (e.g., your wall or battery voltage sags and drops Vcc, or resistor is a little out of spec or tolerances on the BJT, or I donno what)

You may know about the key relationships. Ie (emitter current) = the sum of Ic and Ib. Also, Vbe = 0.7V for any silicon BJT. With those two facts, plus ohm's law, and kirchoff's voltage law you can figure out everything else as you may have surmised.

The 270Ω resistor was figured out using both of these, etc. V = IR, Vcc - Vbe / Ib = R, 5V - 0.7V / 15mA = 286Ω 270 would add a little extra current, no problem.

Anyway hope that helps.

Michael

 

[audioguru]

The max allowed output current from a PIC output is 25ma so only 15mA into a 270 ohm resistor is fine. The PIC output goes up to 5V only when it does not have a load current. With a current of 15mA its output voltage is about 4.75V. The base-emitter voltage of the transistor is 0.7V so the voltage across the 270 ohm resistor is 4.05V which produces 15mA.

 

[mneary]

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.

 

[grekson]

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.

Guys you must understand that I an no electrical engineer. I am building some aplication for hobby purposes. I am not retarded but don't expect me to understand all the data in datasheets and that I am able to study curves...

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.

The easies and the quickest solution to my problem will be if you suggest me the right transistor or right way how to control my consumers. It will be very helpfull if you could aslo represent this with simple diagram.

All I have in common with MCUs are their programming. Not in its assembly language but in Basic - I use Mikroe's compiler and their development board to test application. This way I don't need to know much of MCU's internal structures, registers, memmory addreses, stack, addressing, interupt routine...

I can handle few simple equations I can understand few datas but not much. I know you all are very helpfull, I appreciate that but plese keep in mind my limited knowledge.

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?

 

[Nigel Goodwin]

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.

Basically it's a 'rule of thumb' - just connect it like I said, and measure the voltage on the collector when the siren is turned ON. 1K may well be perfectly fine, but by alkl means try 270 ohm and see what difference (if any) it makes to the voltage on the collector.

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?

Your first diagram (with siren in the emitter) could only ever supply 4.3V maximum to the siren, and the transistor would get red hot and die. You need an NPN with the siren in the collector for it to work - this is 'sinking' current. If you want to 'source' current, you need two transistors (NPN and PNP), check my PIC tutorial pages.

 

[audioguru]

A 2N4401 transistor has a max current rating of 600mA and works very well with a current of 150mA. Its datasheet also lists its max saturation voltage loss when its base current is 15mA and its collector current is 150mA.

The datasheet shows its ß as 100 to 300 at 150mA when it is a linear amplifier with a collector to emitter voltage of 1.0V (when it is not a saturated switch). Then its max base current is only 1.5mA.

 

[shimniok]

Right, so use a 2N4401 or a 2N2222 -- something with higher current rating.

Yes, the 1/10th thing is a rule of thumb. Did you read my explanation? I tried to make it understandable. Maybe I didn't succeed.

EDIT:

The first diagram will sort of work, but not optimally. Because emitter current is a sum of both collector and base current, placing the load on the emitter limits both base and collector current. But you don't want to limit base current or you can't select a current to drive the transistor into saturation.

When you put the load on the collector and connect the emitter to ground, then the base current can be anything you want, which is what you need to be able to force the transistor into saturation.

Of course PNPs have reversed polarity (so if you want to know what that looks like let us know)

Michael

 

[caster.cp]

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?

 

[Roff]

The first diagram will sort of work, but not optimally.

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.

 

[Roff]

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?

This is nitpicking, but the extra transistor is required so that the PNP can turn OFF, not ON.

 

[shimniok]

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.

Oh. Right. I forgot we're talking a 12V source and device. Oopsy.

Michael

 

[grekson]

Of course PNPs have reversed polarity (so if you want to know what that looks like let us know)
Michael

Of course you can allways enlighten me. You can tell me in what situations will I use PNP.
When I was searching a little I also found darlington ULN2003. But since it only has input and output legs, plus vcc and gnd I predict that I can only use it for 5V consumers. But its amplification is 1:1000!

A little off topic...
It is hard to make your own PCB, I should just order **broken link removed** from Mikroe and start programing. But there will be no fun.