NPN transistor switch circuitry, looks a bit odd.

[Yamin]

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.

[magic]

By its nature, BJT is somewhat symmetric but not fully symmetric. If you swap collector and emitter it will still work, but with much lower beta and with breakdown voltage typically less than 10V.

Probably the designer of this circuit made a mistake, I see no good reason to reverse this transistor deliberately.

[Jay_Diddy_B]

Hi,

Check your pin connection on the BJT.

The 2N3904 is not the same as BC547.

Not all TO-92 transistors are the same.

Your coffee grinder circuit make more sense if you switch the collector and emitter.

Jay_Diddy_B

[Zero999]

A multimeter's diode test function can be used to distinguish between the emitter and collector. Connect the anode lead to the base and the cathode to one pin, then the other. The emitter-base junction will have a slightly higher voltage drop, than the collector-base.

Another way is to connect the transistor's emitter/collector  to a 5V supply, via a 10k resistor and the base to +V, via a 1M resistor. The correct direction should give the lower voltage drop cross the transistor's emitter and collector.

[Yamin]

Hi Guys I have pretty much double checked the circuit and pin outs. Here is the actual PCB layout. I have assumed correctly right?

[Yamin]

Here is the full circuit diagram.

[Jay_Diddy_B]

Hi,

Is the transistor marked with a part number?

It could have this pinout?



Jay_Diddy_B