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Transistor switching problem Answered

I am having trouble using transistors to switch a some LEDs, I have 5 LEDs in parallel being switched by one transistor but each LED is indervidually switched by another transistor.

I have given each LED its own resistor and the problem I am having is that when all LEDs are on each LED gets dimmer then if only one was on. I don't really understand why this is happening but I'm not 100% comfortable with transistors and I have probably done something stupid.

I have attached a image of how I have wired it up with only two LEDs

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camlv

9 years ago

oww I might be wrong but I think I need to decrease the last resister. The BC338 has a hFE with a min of 110  so a 10k ohm resister and 5V should give 0.5mA then with a gain of 110 that gives max collecter current of 55mA when I need to draw 100mA. Is that right? I'll try it and see anyway.

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gmooncamlv

Reply 9 years ago

Now you're on the right track.

Bipolar junction transistors like those NPNs are generally current in / current out devices.

What the heck does that mean? Just that the amount of current controlled through the collector / emitter is proportional to the amount of current applied to the base. I.E., the load (current draw) determines the how much current is required to switch it.

So the correct base resistor chosen for each individual LED switch would be higher resistance than the resistor needed to switch the entire bank of transistors / LEDs. Switching all those LED / transistor combos is definitely a larger load.

Since the base resistor has to be low enough to switch all five LED combos, regardless of how many might be lit at one time, there are limits to using BJTs in this type of application. I think it will work if you lower the resistance on the "master" switching transistor, though. If not, try a MOSFET instead...

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camlv

9 years ago

Sorry forgot to add that i'm running a 3A power supply so more then enough

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NachoMahma

9 years ago

.  My guess is that you are drawing more power than your power source is able to put out.