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### Topic: 10 Watt high power LEDs. (Read 4643 times)previous topic - next topic

#### LMI

#15
##### Nov 05, 2013, 10:07 pm
May I join too.

If Leds are on the collector and resistors are on the base, there is nothing to limit current to 400mA. Current is Base current * current gain. You need a resistor in series with the Leds.
Quote
and not use any current limiting (Except on base),

#### cjdelphi

#16
##### Nov 05, 2013, 10:50 pm

May I join too.

If Leds are on the collector and resistors are on the base, there is nothing to limit current to 400mA. Current is Base current * current gain. You need a resistor in series with the Leds.
Quote
and not use any current limiting (Except on base),

There's nothing to limit the current?

The resistor on the base does! (Ugh)

#### LMI

#17
##### Nov 06, 2013, 12:04 am

May I join too.

If Leds are on the collector and resistors are on the base, there is nothing to limit current to 400mA. Current is Base current * current gain. You need a resistor in series with the Leds.
Quote
and not use any current limiting (Except on base),

There's nothing to limit the current?

The resistor on the base does! (Ugh)

Perhaps. If you would show a schematic diagram of your circuit here. If not, that is all right too.

#### DVDdoug

#18
##### Nov 06, 2013, 12:19 amLast Edit: Nov 06, 2013, 12:25 am by DVDdoug Reason: 1
Quote
The resistor on the base does! (Ugh)

Have you calculated power dissipation in the transistor?

Without seeing a schematic  I'm not exactly following what you are doing, but linearly controlling or current-limiting a 10W or 3W LED with a 2n2222 "feels like" you are going to fry the transistor.  With 12 Volts, the current limiting resistor (or transistor) dissipates more power than the LED.

You can only (safely) pass the maximum-rated current through a transistor when it's in saturation (with nearly zero voltage across it). So, in a switching application (such as PWM), you CAN use the maximum current & voltage ratings.  In linear applications, you need to consider power (Current x Voltage).

Also, the gain of the transistor tends to increase with heat.   Without a feedback-control circuit you'll get more current through the transistor & LED as the temperature rises.  Depending on where you are along the voltage/current curve, this can result in thermal runaway where more current = more heat = even more current = even more heat, until the transistor dies.    (In other cases, you'll get more current but less voltage and everything will stabilize, or something else will limit the current, or something else will die first.)

#### cjdelphi

#19
##### Nov 06, 2013, 01:00 am
It's like talking to the wall  .....

The transistor is biased and heatsinked and working! Along with the leds thank you.

#### LMI

#20
##### Nov 06, 2013, 05:19 am
I found a schematic diagram of a transistor driving LED.
http://www.sqlskills.com/blogs/paulselec/post/arduino-figuring-out-transistors-and-associated-resistors.aspx
Here R1, the collector resistor, limits the LED current. With high current R2, the base resistor, should be smaller and the transistor bigger. Or use a fet.

#### cjdelphi

#21
##### Nov 06, 2013, 05:35 am
I KNOW!

So how are you mr wall? Having a nice day?

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