Odd looking NPN transistor switch circuit. Thoughts?

Thread Starter

Yami

Joined Jan 18, 2016
282
Hi guys,
I came across the following npn transistor circuit in a coffee grinder. The led D1 is the led of a MOC3063. It looks kind of peculiar to me as normally the 'load' this case the led and the resistor would be normally placed on the collector side and I haven't seen the collector tied to ground with these kind of setup before.
Something else I noticed was this - the supply voltage is 5.1V (comes from a 5.1V Zener regulator). But measured from the Cathode to ground of D1 the meter shows around 5.6V how can this be?
This unit works which means the circuit works, Ltspice simulation also does work apart from the 5.6V measurement. Why might have the designer choose this particular order, might there be an advantage to this approach? Hoping for some more interesting thoughts.
Thanks in advance for the help.
p.s: I have searched the internet for a similar circuit but didn't get anything identical to this.
npnSwitch.jpg
 
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KeithWalker

Joined Jul 10, 2017
902
The circuit will not work as drawn. If the circuit works correctly then the power supply is connected the opposite way to what is shown in the diagram with the collector connected to the positive supply. Check the polarity of your measurements..
Regards,
Keith
 

Thread Starter

Yami

Joined Jan 18, 2016
282
Hey guys, yes those were my initial thoughts as well as I was tracing the circuit. I have double triple checked it.
Also I'll add that the simulation also does work.
 

Thread Starter

Yami

Joined Jan 18, 2016
282
Hey guys, I'm attaching the rest of the circuit I have hand drawn. Maybe there is something thing there.
@AlbertHall my thoughts exactly, it does not make any sense. However I did double check it.

IMG_1714.JPG
 

Papabravo

Joined Feb 24, 2006
13,741
I don't care what you think you see. You can't make a transistor work with the base-collector junction forward biased and the the base-emitter junction reverse biased.

EDIT: Apparently you can...sorta. Not as good as it works when done correctly however.
 
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Thread Starter

Yami

Joined Jan 18, 2016
282
Hi guys, here I am attaching the actual PCB. Could be the designer actually made a mistake? The first photo is topside and the second picture is the bottom side.(I didn't change the orientation of the board just flipped it).
Photo1.JPG

Photo2.JPG
 

Thread Starter

Yami

Joined Jan 18, 2016
282
Thanks so much for the link, I was hoping that you could clarify somethings you just mentioned - I'm not sure I understand
The applied voltage is less than the reverse base emitter breakdown so that is OK.
and also :
The base resistor is only twice the resistor in the (now) collector path so only low gain would make this work.
What do you mean by the 'now'? is it because "The collector acts as the emitter and the emitter as the collector " quoted from the link.
Thanks in advance,

Can we agree that this is a stupid way to design this circuit? Or is it a more 'clever' way to do it in this instance?
 

AlbertHall

Joined Jun 4, 2014
9,875
In this circuit the pin usually performing the emitter function is actually being used as the collector.
I suspect it is a mistake but still works well enough so wasn't spotted.
 

atferrari

Joined Jan 6, 2004
3,962
How comes the emitter in both pictures is on the left side? The picture shows in the transistor itself is correct for the 2n3904. Emitter (on the left) base and collector.
 

Thread Starter

Yami

Joined Jan 18, 2016
282
How comes the emitter in both pictures is on the left side? The picture shows in the transistor itself is correct for the 2n3904. Emitter (on the left) base and collector.
I didn't change the orientation of the board, just flipped the board upside down thats why the emitter is on the left side.
 

Thread Starter

Yami

Joined Jan 18, 2016
282
You can turn the 2N3904 around and have the circuit work the way it was supposed to work.
NICE!!! funny I didn't think of that, I was just too worried to figure out why this was working.
However I learned something new and interesting thats always good - 'Reverse Active Region'
 
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