I have a small project and I need to use 2N3904 to drive FOD814 (which is then used to drive some load).

Here's the circuit:

Could somebody please help me calculate currents etc to see if I can drive 20mA load at OPTO_OUT?

Here's my math (and please be lenient/understanding...):

At 2N3904:

Vbe (base-emitter) = 0.7V

Ib (base) = (3.3-07)/10k = 0.26mA

Vce (collector-emitter) = 0.2V

As for FOD814 part:

Ir1 = (12V - Vd - Vce) / R1 = (12V - 2.1V - 0.2V) / 1000 = 9.7mA

Ir2 = (12V - Vfod - Vce) / R2 = (12V - 1.2V - 0.2V) / 1000 = 10.6mA

So Ic (collector current) = 20.3mA

Considering hFE = 60 I will need to increase Ib (eg. changing R3 to 3k4) to be able to achieve 20mA at the collector.

Now - at 20mA FODs CTR is about 110% so I should be able to drive 20mA load at the OPTO_OUT...?

Does that make _any_sense...?

Thanks,