Circuit not working?

Hey Guys I just made this circuit (schematics provided) and it doesn't work. The problem is that the  at the output of the 2N3906 transistor the voltage is only 0.5-0.7. So that isn't powering anything. I was wondering if anyone could tell me what the problem is. (the first pic is the original and the second one is the modified one with i built).

Thanks in advance.
-Prickly Potato

Picture of Circuit not working?
1t7792 (1).jpg
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iceng2 years ago

Is this what you asked to see ?


OK first of all 2N3904 is an NPN transistor and a 2N3906 is a PNP transistor and the schematic uses only the symbol for a PNP transistor which makes me wonder if the designer even knows what he is doing.

PNP transistors need negative charge on the base if you have 2.2 k ohm from source to base and 2.2 k ohm from base through the 2N3904 to ground you have a positive charge on the 2N3906 transistor base.

The reason you have a positive charge on the base of the 2N3906, is you have a voltage drop across the 2N3904, think of a transistor as a voltage controlled pot and the voltage drop across the transistor like the voltage drop across a resistor, then apply the voltage divider rule and you find that at your base on the 2N3906 has a positive charge.

- 2.2 k ohm + 2.2 k ohm = 0 + voltage drop of the 2N3904 positive charge on base

So from the base of the 2N3906 to the 2N3904 change the 2.2 k ohm to something smaller, or change the 2.2 k ohm from the source to the base of the 2N3906 to something larger.

You will know you have it right when you have a greater voltage drop from the emitter to the base then you do from the emitter to collector.

Last the TIP120 which they used the right symbol for a NPN is connected backwards the collector should go to the led strip and the emitter should go to ground.

Last the 1N4148 diode is backwards it shorts out the source to ground at the 2N3904 base.

If you want some good advice don't build their circuits.

+1 on the NPN being wrongly wired and not part of the original schematic.

It could make sense as an emitter-follower but the strip needs reversing to a negative supply...

shahryar.adil.3 (author)  iceng2 years ago

Well I have already built it so what do I do now? I havent wired the last TIP 120 yet so thanks ill wire that right. Can you give me step to step instructions on how to fix the diode and transistor please thanks. oh and in the video ( they wired it like i did and its working.

I am no expert but I have never seen this configuration for a TIP120 or similar.
Emitter goes on ground, collector to the negative of the LED strip and postive from the strip to the supply power.
You reversed the transistor and feed negative to the negative of the LED strip.

shahryar.adil.3 (author)  Downunder35m2 years ago

Ok sorry. Anyway forget about that the problem is i don't see changing voltages at the base of the 2N3906 as i should.