I've gone mad. npn question.

[positronicle]

--Edited by Positronicle--

[JPKNHTP]

-JPKNHTP

[dyarker]

Got to work a little early, so I can check here. Glad I was right about others helping.

When setting up to measure current, do it with power off. Then power on, measure, power off and immediately change meter to off or voltage mode.

Because if you accidentally leave the meter in current mode, and try to measure voltage there is a good chance to blow the meter.

The reverse mistake, meter in volts mode, when you want measure current is harmless. The circuit won't work, but nothing goes POOF! Meter resistance is very high in volts mode.

Also, never use a meter set to resistance mode in powered up circuits. Again, POOF bye bye meter. Verify no voltage, make the resistance checks, and change the meter to volts or off.

Cheers,

[JPKNHTP]

-JPKNHTP

[cato]

>About the Base, it seems to be working, I'm not sure I fully understand the issue about Saturating or lack off.

From the data sheet for the transistor --ON CHARACTERISTICS-- , MIN Hfe (current gain) it looks to me like you need to provide about 5 mA to the base pin in order to get about 300 mA through the transistor. That is, if you provide 10 mA base current you could "push" 750 mA through it and at 1mA base current you could "push" 50 mA through it. So, for 300mA you need a base current somewhere between 1 and 10mA.

I doubt that you are getting 5 mA from the "pic pin" Pic Dudes? Also, don't he need a base resistor for safeties sake?

I think you are thinking of the transistor as a switch....but the one you have chosen is more like a valve...it takes some effort to get it all the way open (i.e., saturated)...

<small>[ March 24, 2006, 03:19 AM: Message edited by: cato ]</small>

[jwax]

I'd add a heatsink on that 2222.

[JPKNHTP]

-JPKNHTP

[KamPutty]

Hi all,

Thanks for the reply. And yes, I have been thinking of the transister as a switch and not a valve. I now understand why I need more power to the Base. Very interesting, never knew that.

Here is the URL for the company where I got my LED's

http://www.luxeonstar.com/

I have the "star" ones, white and red.

It is truely amazing at the power, and the heat!

I will now rearrange my design and let you all know the deal.

And again, I truely do appriciate the help!

~Kam (^8*

[KamPutty]

One more gang,

I'm trying to figure out what resister I need from the pic to npn.

My PIC16f688 says that the max output current sourced/sunk by any I/O pin is 25mA.

I tried a 470ohm resister and man oh man, that makes all the difference (thats the 470 on the base). I always thought that the transister was a switch.

Said that, Could I not adjust the base voltage (pwm?) to create a dimmer effect using the npn?

I'm having brain farts about the correct resister. I just replaced with the 470 as I had that there.

How would I determine the correct res from the pic to the base, I'd like to know...

I tried the R=V/I or R=(V1-V2)/I and I just got lost in the specs!

~Kam (^8*

[dyarker]

Base resistor:

Get spec sheet for the transistor.

Find the minimum gain (Hfe) for about the collector current you're planning.

Divide the collector current by the minimum gain. This is the base current needed to saturate the transistor.

Subtract 0.7V (base-emitter voltage) from the minimum drive voltage (probably 4.5V on a 5V PIC). This voltage across the base resistor.

Divide the base resistor voltage by the base current. This is the value for the base resistor. Choose the nearest LOWER standard resistor value. (if the calculated value is like 505.5 Ohms, a 520 Ohm resister is probably okay, but 470 is safer (as far as ensuring saturation)).

<small>[ March 24, 2006, 09:03 AM: Message edited by: Dale Y ]</small>

[JPKNHTP]

-JPKNHTP

[dyarker]

PWM brightness control is on and off faster than the eye can see. A PN2222, 650mW transistor can handle it because at saturation the power disipated by the transistor is 350mW (1V * 350mA).

Linear brightness control would need a 1W transistor with a heat sink. (being conservative)

[rshayes]

You may want to use a heavier transistor than the PN2222. That 650 milliwatt power rating is based on a 25 centigrade ambient temperature and a 150 centigrade junction temperature. More realistic figures are probably a 50 degree ambient and a 100 degree junction. Under these conditions, the transistor would be able to handle 260 milliwatts.

Normally, a saturated transistor is driven with a base current that is one-tenth the collector current. This usually gives a saturation voltage that is in the hundreds of millivolts. This would require a base drive of 35 milliamps, and would add about 30 milliwatts to the power dissipation. This leaves you with 230 milliwatts. The collector voltage would have to be below .66 volts at 350 milliamps to avoid excessive dissipation. From the data sheet, it appears that some parts could be in the .7 volt range at 350 milliamps.

You may want to consider using a heavier transistor or possibly two in parallel.