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Help with a simple transistor circuit

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Froskoy

New Member
Hi,

I'm totally new to analogue electronics, and am trying to teach myself about transistors.

I need to drive 20 LEDs, connected in parallel, from the output of an IC, that can source 150mA.

Would the attached circuit be correct or am I wrong? I've drawn 4 LEDs - there are actually 20.

The transistor data sheet show a min h(FE) of 50 (max is 250).
I calculate that the 625Ω resistor from the 5V output of the IC will provide a base current of 8mA (to a first approximation - I've ignored the 0.7V drop on the base-emitter diode).

With a gain of 50 (worst case), this means that the collector current will be around 400mA, which is needed to drive each of the LEDs in parallel, with around 20mA through each.

Is this correct or are my calculations wrong?

Now a few beginner's questions:

1) If I increased the base current, then I guess the collector current will also increase. But there will still only be 20mA through each LED because of the resistors, and therefore only 400mA will be drawn. What happens to the extra current?

2) Do I actually need the 150Ω resistors? The 400mA from the collector will be shared equally between the LEDs, meaning that only 20mA will pass through each, which does not exceed the maximum of 30mA. I guess I would have to measure h(FE) exactly for my particular transistor and use a computer simulation to take into account higher order effects?

Many thanks, and apologies if my questions seem silly,

Froskoy.
 

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ronv

Well-Known Member
Most Helpful Member
Usually when you are driving a transistor switch you use a gain of 10 so that the voltage drop from collector to emitter is very low (this keeps the transistor cooler).

You still need the resistors because the LEDs are not exactly the same, so the one with the lowest voltage drop (Vf) will hog all the current.

There is no "extra current" It will be fixed by the voltage and resistance.
 

audioguru

Well-Known Member
Most Helpful Member
I calculate that the 150 ohm resistors limit the current to 20mA in red 2.0V LEDs when the transistor turns on as hard as it can so it is saturated. The datasheet for every American transistor shows its Vce max saturation voltage loss usually when its base current is 1/10th the collector current. The datasheet for some European transistors shows the max saturation voltage loss when the base current is 1/20th the collector current.

So for a collector current of 400mA then the base current should be 40mA.
A 2N4401 transistor will have a typical base voltage of 0.95V when the collector current is 400mA and the base current is 40mA.
We don't know how low the source voltage drops when it is loaded but assuming it does not drop then the base resistor should be (5V - 0.95V)/40mA= 101 ohms. Use 100 ohms.
 
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