Transistors

[mgeno216]

I am not super familiar with transistors, but need to use one in my project. I have a 555 generating pwm and need 800ma and 12v to respond to the PWM. I am not sure exactly what transisitor to use, but I stopped at radio shack and picked up a TIP42
https://www.electro-tech-online.com/custompdfs/2010/05/TI2FTIP42C.pdf
I am not sure if this was exactly right. I am pretty sure I need a PNP, right? This was the only one that had the power ratings for my needs.

BTW:
12v is not going to base, it is going to be less than 5v.

Thanks,
Mike

 

[crutschow]

Do you mean you need drive a 12V, 800mA load from the 555? What is the nature of the load?

 

[mgeno216]

Ya i think that is what I want. I am not exactly sure what your saying, but if you are saying that the allow through 12v and 800ma of electricity when the 555 pulses then yes. This circut is for a fan controller. So will the TIP42 work?
One more question, is there a minimum voltage that needs to be applied to the base inorder for that collector to pass through onto the emitter. The max is 5v.

Mike

 

[audioguru]

The TIP42 PNP transistor will have its emitter connected to +12V and its base needs a voltage much higher than 5V.
Instead, use an NPN transistor plus a current-limiting resistor for its base.

The datasheet for a TIP31 or TIP41 NPN transistor shows that its base-emitter saturation voltage is typically 0.8V to 0.9V when it conducts 0.8A though its collector-emitter but could be as high as 1.2V.
The base resistor should be calculated for 80mA. The output of a 555 with a 5V supply goes up to only 3.2V to 3.4V when it sources 80mA. So the base resistor is (3.3V - 0.85V)/80mA= 31 ohms which is not a standard value. Use 33 ohms.

 

[Hero999]

Why not run the 555 from 12V?

It has a maximum voltage rating of 15V you know.

The only thing you need to beware of is that if you use a PNP device, the waveform across the load will be inverted.

 

[mgeno216]

Well this is for an xbox 360. The transistor's max base voltage was 5v, so to be safe I was just going to use a 3.3v source on the mother board, there are plenty. It doesn't realy matter if the waveform is inverted right, because it is still going to be so fast.

 

[Hero999]

No.

The transistor's maximum reverse base voltage is 5V.

The base is a diode junction and has no maximum forward voltage rating, instead it has a maximum current rating and a maximum saturation specification.

The NE555 has a minimum working voltage of 4.5V so 3.3V is not suitable.

I'd recommend running the 555 off 12V, if possible.

 

[mgeno216]

Ok. It is fine runnning the 555 off 12v. So I still want to know if this transistor is ok.
@audioguru
You were saying that I should you a TIP41 NPN. Did you mean you is in conjuction with the TIP42 or only use the 41. I am not too familiar with the electrical jargon, so forgive me if i am slow to understand what you are saying. But since now I have decided that I want to use the 555 with a 12v output I will test it to see what the 555 ouput is and then calaculate the appropiate resistor. You also said that the emitter needed to be connected to 12v, now my understanding of transisitors may be wrong, but I thought that the collector stopped the current until a voltage was applied to the base, which then allows the current to flow freely through the transistor

 

[Hero999]

You'll be fine with PNP, as long as you accept that when the 555's output is high, the load will be off and when the output is low, the load will be on.

I think 80mA of base current is overkill and at 12V would require a huge 1W base resistor which is ridicules.

With a forced beta of 10 and a current of 1A, the collector saturation voltage is 100mV, which is more than good enough for a 12V load so we can stand it being a bit higher, if it means the base current is lower.

I reckon a base current of 40mA will be fine, so use a 270R base resistor, if the 555 is powered from 12V. The resistor will need to be rated for 0.5W, if the duty cycle is 100% but it can be derated if the duty cycle is lower.

 

[mgeno216]

Thanks Guys

One last question. What decides how much current the 555 will draw. I have 555CN's from RadioShack and on the package it says "Output Current (sink or source): 200ma"
Does this mean that if there was no limiting resistor it would pull through 200ma. If not, what determines it

 

[Hero999]

It means it's the maximum output current it can safely provide without destroying the IC.

IF you didn't use a current limiting resistor, the 555 timer will get very hot and smoke.

The 555 will use whatever the output current is, plus the supply current given on the datasheet.

 

[audioguru]

The output voltage of a 555 does not go high enough to turn off a PNP transistor. Therefore another resistor must be calculated and added.
That is why I said to use an NPN transistor with a single base current-limiting resistor instead.

 

[Hero999]

This is the problem when you look at numbers rather than schematics.

The output voltage of a 555 does not go high enough to turn off a PNP transistor. Therefore another resistor must be calculated and added.
That is why I said to use an NPN transistor with a single base current-limiting resistor instead.

I almost believed that to be true, until I looked at the internal schematic for the 555 and realised that virtually no current will flow into pin 3 when it's connected to +V and the output is high.

 

[mgeno216]

SO i have to do the doulble transitor?

I set up the 555 part last night and the output voltage was around 10-10.5. I didn't think about testing current though

 

[Hero999]

Don't worry about it, it'll be fine, if you have a problem, add a 10k resistor between the base and emitter of the switching transistor, but I don't think you'll have a problem.

 

[audioguru]

Add a pullup resistor to the output of the 555 to see if the transistor turns off. Don't let the base of the transistor float when it is suppposed to be off.