Transistor NPN usato con PNP. Perchè funziona se motato al contrario?

ho volevo realizzare una scheda eLettronica dove 5 uscite dell’Arduino potessero essere attivate insieme e pilotare eventualmente un piccolo relè.

Ho creato lo schema allegato.
Sempre in allegato vedete come doveva risultare montato… se però lo provo come disegnato,
non funziona, ho dovuto invertire il pin 1 col 3 del transistor NPN.
Mi sembra assurdo… su internet ho trovato datasheets dove riportano che il pin 3 è l’emettitore e andrebbe a massa.
Il transistor riporta il codice: c33740 ph 12.
Sono abbastanza convinto che il circuito si corretto, è possibile che il transistor che ho io ha il pin 1 e 3 invertiti?
Altre idee?

Domani provo ad aumentare le 2 resistenze da 560 a 1K perchè più di 3 insieme l’Arduino si siede.

I would like to create a board with 5 outputs to Arduino.
I created the schema attached and what is really strange to me is that the transistor NPN
works if mounted at the contrary!
See attached pictures.
The transistor is c33740 PH 12.
The datascheed I found on the web reported pin 3 as emiter, so connected to ground.

Why the transistor should be monted at the contrary?

Tomorrow I will try to increase the resistence to 1 Khom so the current is smaller and I can
activate all 5 outputs.




The NPN transistor (BC548) has a base current of about 2-5mA (estimated, because of CTR and Vsat of the opto).
That could result in a collector current of ~500mA, which could destroy that transistor (and the BE junction of the other transistor).
You must use a current limiting resistor in the collector line of that BC548, e.g. 100ohm.

The BC559 is a small signal transistor. Not suitable for large output currents.
Something like a BC327 or BD136 would be better.

Thanks Wawa for good hints.
I will consider for sure the resistence.

However my question is more related to the mounting direction of the NPN transistor. Does it make sense for you that (maybe also considering your suggestions) the circuit works but inverting Collector with Emitter?

..or I'm interpreting the datasheet wrongly?

Please consider that finally, how I specified, I mounted a BC 337 instead of BC548, the exact code is C33740 PH 13.


The "C33740 PH 13" is probably a BC337.

I did a quick test now and it seems working adding a resistance in the base of BC559.
Attached the schema. In this way placing the transistor BC337 in the correct orientation it works!

Most probably, because of high current in the collector it could not work and inverting Emitter and Collector probably it was working in a wrong area… could be?

Last question: if I place the resistance between the collector and +5Vdc of BC559, it’s the same or I should put it in between the base and Collector of BC337?
It would be much better for the board design…

The posted diagram is correct.
Moving the 10k resistor to EB of the BC559 is also correct.

Thanks a lot!


Last question.. really.. :stuck_out_tongue:

Would be better if I increase resistors from 560 to 1K for both: input of opto and base of BC337?

I think the current should be enough and in general better because is smaller..

Makes sense?

You didn't tell us what load (current) Q1 is driving.

Q1 needs a base current of about 10% of the collector current for maximum saturation (for a BC327, BD136, etc.).
Then the optimum value of R15 can be calculated.

Base current needed for Q6 depends on the current through R15, and can be ~20% of that (for a BC547, BC548, etc).
Then optimum value of R32 can be calculated.

Opto LED current needed depends on the minimum CTR of the opto coupler (50% for this one).
So twice the current needed through R25 as through R32.

Allow for the saturation voltages of the transistors, and Vf of the opto LED and BE diodes when you calculate the resistors.