Dangerous Prototypes

General Category => General discussion => Topic started by: nowait on March 22, 2013, 10:32:34 am

Title: Transistor question
Post by: nowait on March 22, 2013, 10:32:34 am
Good Morning,

I have a circuit with some RGB LEDs connected (each with a resistor) going into a transistor. Looks something like the attached image. Each resistor is for 20mA at 2.1V. (150R) This is the red leds only.
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My question is, it works but the LEDs are not as bright as I was expecting. The voltage across the transistor is only ablot 3.5V and should be 5. Am I missing something?
The transistor is a http://http://www.fairchildsemi.com/ds/KS/KSP45.pdf
Thanks
Ryan
Title: Re: Transistor question
Post by: sqkybeaver on March 22, 2013, 02:26:51 pm
the voltage across the emiter-base should be close to zero.
20mA through a 150 ohm is a 3 v drop across the resistor alone.

what is the forward voltage of the LED?
Title: Re: Transistor question
Post by: dolabra on March 22, 2013, 02:37:56 pm
Also, how are you biasing the transistor,  are you sure you are driving into saturation?
Title: Re: Transistor question
Post by: nowait on March 22, 2013, 02:42:02 pm
Sorry the LEDs are 2.1V forward voltage and 20mA typical current. Supply is 5V.
I did some more reading and I think the problem was that I needed to add a resistor to the input of the transistor to limit the base-emmiter current. I did that and it now seems fine, just not totally sure why :)
Title: Re: Transistor question
Post by: sqkybeaver on March 22, 2013, 02:53:37 pm
[quote author="nowait"]I think the problem was that I needed to add a resistor to the input of the transistor to limit the base-emmiter current. [/quote]

that will do it just about every time.
Title: Re: Transistor question
Post by: doub on March 22, 2013, 02:58:00 pm
[quote author="nowait"]I did that and it now seems fine, just not totally sure why :)[/quote]

Without a resistor to limit the current going into the base, you probably exceeded the maximum current that the PIC can source, which may have put the PIC pin in an unstable state (also possibly damaging it).
Title: Re: Transistor question
Post by: Bertho on March 22, 2013, 10:01:24 pm
[quote author="nowait"]...I did some more reading and I think the problem was that I needed to add a resistor to the input of the transistor to limit the base-emmiter current. I did that and it now seems fine, just not totally sure why :)[/quote]

With an average current gain (Hfe) of 90, according to the datasheet, and a Vbe(sat) drop of 0.75V, you need a base-resistor to match the currents.

The minimum base current is 8*20mA/90 or about 1.8mA. However, the minimum Hfe is indicated at 40, which means that you would need to push 4mA into the base for guaranteed collector current (without going into complete saturation).

With a PIC driving at 5V, you need a resistor of maximum value (5V-0.75V)/4mA ~ 1kOhm. To be guaranteed in the saturation region, you should add 10..20% driving current to the base, which would result in an 820Ohm base resistor. The PIC pins can drive this without problem.

The Vce saturation voltage will be hovering between 0.1V and 0.25V, but can be as high as 0.75V according to the datasheet (max value). At a saturation voltage of 0.2V, you will see about 18mA current per LED ( because (5-0.2-2.1)/150=18mA).
Title: Re: Transistor question
Post by: kdittyr on March 25, 2013, 07:05:57 pm
@Bertho - that is great information, thanks.