Is this reasonable power dissipation when driving transistor base?

[DaveB]

Hi,

I'm just getting to grips with a 555 PWM circuit driving an array of LEDs via a 2N2222A transistor, and am not sure if I'm doing the right thing to get the right current to the transistor base whilst managing the power used/dissipation.

The circuit is at http://www.flickr.com/photos/dave-b/6931772670/sizes/l/in/photostream/.

With R1 = 1K, I get 1mA to the base of Q1, but I need 4mA (to get 0.4A with beta=100) so need a lower value for R1.

With a multimeter I measure 10V maximum over R1 so, given 10V * 0.4A = 4W, R1 needs to be 25R, and rated for at least 4W (although I'll probably use several higher value resistors rated for 2W, due to package size).

This seems to me like a lot of power dissipated over R1 - is it the right approach to getting enough current to Q1 base?

Cheers,
Dave.

NB: There's background and more details in my rambling about it on my blog: http://occasionalrandoms.blogspot.co.uk/search/label/lightcanvas

 

[gorgon]

If you measure 10V over 1k(R1) you are sourcing 10mA into the base of Q1, more than enough to saturate Q1 to supply 360 mA you'll need for your 12 LED panels, each drawing 30mA.(you'll drop a small voltage over the transistor)

I think you mix base current with collector current in your calculation, even if you talk about Hfe of 100. BTW, the LM555 will never supply 400mA from it's output.

TOK

 

[(*steve*)]

You also calculate the power incorrectly.

10V across 1000 ohms is 10^2 / 1000 = 100/1000 = 0.1 W

However, if you measure the voltage across the transistor when its turned on, you need to multiply that by your load current (360mA?) to get the power dissipated by the transistor.

If the voltage across the transistor is more than about 1V then you may well overheat the transistor.

If that is the case, the simple solution is to connect fewer panels (say up to 100mA of current -- 3 of them?) then replicate the transistor and base resistor for each other set of 3 or 4 panels.

Using a slightly lower value of base resistor will increase the base current and force the transistor further into saturation which will in turn reduce the voltage across it and therefore the power dissipated as heat. I wouldn't go much lower than 560 ohms.

 

[DaveB]

Hi gorgon, steve,

Thanks for the replies, I've definitely mixed up with my readings and calculations.
I'll have to try again when my brain is working a bit better!

Cheers,
Dave.