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Weird PCB stuff

by _Bd_
Tags: weird
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Apr3-14, 06:55 PM
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I wonder if the problem is component tolerances. Looking at the MMBT3904 data sheet, it gives a 0.2V spread (0.65 to 0.85V) of base-emitter voltage at 10mA, which is roughly the collector current in U1.

With a 3.3V supply and 2.6V across the LEDs, there is no supply voltage headroom above the 0.7V needed to turn the transistors on. Maybe some of them are stuck at "off".

I expect this would work fine in a simulator where all the transistors are mathematically identical, but real world electronics isn't like that.

If you can isolate this circuit (so you don't fry anything else) and slowly increase the supply voltage up to say 4V, that would check if this is the problem.
Apr3-14, 07:03 PM
P: 475
Quote Quote by davenn View Post
how do you figure that ?

so the way its wired the transistor would ALWAYS be on in full saturation so what's the point of having it there ?

because the junction of R12, C6 is going directly to the other transistors
that transistor is being bypassed anyway
but because the transistor is fully on, the C6/R12 junction is effectively always grounded

just wanna know

The C6/R12 junction is not grounded at DC. It is ground from the point of view of noise which is what you want. It is just the input device of a simple current mirror.
Apr3-14, 08:05 PM
P: 93
What is the exact part number of the transistor chip you are using? The one on the schematic is a chip but an individual transistor (1). Unless the batch is the same, there is a high likelihood that the Hfe is not the same among the transistors and some are not getting enough current to conduct as much as the others, though I would expect some current to flow. If your part is a chip with multi transistors in one package, then you need to make sure all emitters are connected to ground or common(some chips will show GND but require all GND pins be connected to GND.
Apr3-14, 08:47 PM
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P: 1,781
I would check the pinout on the mirror chip very carefully. Maybe the numbering convention is different from that expected, the chip may be pictured inverted.

AlephZero's suggestion of increasing the supply voltage is worth trying.

Another possibility would be to momentarily short two transistor collectors.
If one LED was off and the other on then; if both come on at half brightness, the mirror transistor is the problem. If both turn off, one LED is the problem.
Apr3-14, 09:43 PM
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Quote Quote by mjhilger View Post
What is the exact part number of the transistor chip you are using?
Quote Quote by Baluncore View Post
I would check the pinout on the mirror chip very carefully.
There is a part number on the circuit diagram. The data sheet I found implies they are separate transistors, not an array of matched transistors on one chip (which would be a better design option IMO).
Apr3-14, 11:07 PM
P: 1,074
I certainly would probe as I described to prove it is the mirror. You will have to do that eventually. It's ridiculously simple, and isolates the problem.

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