Which Transistor To Use ??? (simple)

[iso9001]

I have an automotive circuit that is running at 5V, has a pic, some other stuff and whatnot. I have to use this to drive a 12V lamp inside the dash panel... Problem is ofcourse that this lamp is 12V and probally requires mre power then my pic is going to give.

This lamp always has 12V going to it, but the ground is only connected when the body computer tell it to, thus conrolling when its on. I dont think I can sink -12V into a pin on my pic. So I was thinking a transistor would be the right tool for the job.

Problem is, I dont know which kind tho. NPN, PNP, FET ??

I have an NPN here but I played around with it some but can not get it to work they way I think it should. Anyone have any advice or perhaps a simple diagram ?

Thanks!

 

[kybert]

Work out what current the lamp is going to draw:

I = P / V (power watts / volts)

So if the lamp is a dashboard lamp (normally 1.2W) then the current would be:

1.2W
------ = 100mA
12


I would use a BC337 for this, it can switch upto 500mA, which would be 6 Watts.

connect a 1k resistor between the pic pin and the base pin of the transistor (middle pin usually).

Now connect the emitter of the device to ground and the collector the the bulb. the other side of the bulb goes to +12V

 

[Oznog]

An n-channel MOSFET won't need the resistor and won't draw any power off the PIC to drive it. The bipolar NPN will need enough current based on bulb current divided by hfe(min) to keep it saturated. Also, the bipolar cannot drive without dropping out about 0.3v from the load voltage, whereas a MOSFET will usually be far lower (based on rds times the current).

But, a bipolar will work fine.

 

[ante]

Why not swap the bulb for a LED?

Ante :roll:

 

[iso9001]

I like the idea of not drawing any current from the pic b/c idealy this thing has to last a LONG time. (life of the car)

I cant change to an led b/c its in a switch of the dash and I suppose I could use rapid prototyping to make a new switch and then have that investment casted... maybe just pay the half million dollars to have a new injection die made up and produce these in mass myself.... hell, or I could just use a transistor.

So... an N-Channle Mosfet you say hmm ? I love the combonation of usefull people and the internet.

Thanks, I'll give that a try.

 

[ante]

If it is supposed to last the life time of the car the swap to a LED is definitely a must.

Ante :roll:

 

[iso9001]

its a factory bulb deisgned to last a LONG time, they are replaceable, but i have never actually seen one go out.


just image if it wern't a bulb, imagine a relay or somthing instead.