# Power dissipation in a Darlington built with an optocoupler and NPN

#### dl324

Joined Mar 30, 2015
12,931
The second graph is not aplicable in my case, I think. There is no 10V
Since you intend to saturate the transistor, you should assume a low CTR and drive the LED appropriately. If you want to minimize dissipation in the LED, you need to determine how low LED current can be to still saturate the output transistor.

Circuit simulator results often don't mirror reality. If you want more meaningful results, you have to change simulation parameters. For such a simple circuit, I wouldn't bother. You should be able to do the design on your own.

#### kubeek

Joined Sep 20, 2005
5,757
I know in my case the 4N35 doesn't allow the Q4 to saturate at 0.2V which is a normal value.
As far as I know you can never saturate the lower transistor in a darlington configuration as it will allways remain an emmiter follower, so ~0.65Vce at best. In other words, even if you short out collector to base, the B-E diode will remain.

Joined Feb 28, 2017
12
Since you intend to saturate the transistor, you should assume a low CTR and drive the LED appropriately. If you want to minimize dissipation in the LED, you need to determine how low LED current can be to still saturate the output transistor.

Circuit simulator results often don't mirror reality. If you want more meaningful results, you have to change simulation parameters. For such a simple circuit, I wouldn't bother. You should be able to do the design on your own.
Thanks a lot for your suggestions!
I've done de calculations taking into account a $$V_{CE(OK3)}=0.2V$$ and a $$V_{BE(Q4)}=0.66V$$ and everithing makes sense!

#### AlbertHall

Joined Jun 4, 2014
11,400
As far as I know you can never saturate the lower transistor in a darlington configuration as it will allways remain an emmiter follower, so ~0.65Vce at best. In other words, even if you short out collector to base, the B-E diode will remain.
Unless you feed the collector of the first transistor directly from the supply, possibly with a current limiting resistor.

#### kubeek

Joined Sep 20, 2005
5,757
Unless you feed the collector of the first transistor directly from the supply, possibly with a current limiting resistor.
Yes then you could have low Vce in Q4, but that is not a darlington arrangement anymore.

#### AlbertHall

Joined Jun 4, 2014
11,400
Is it not known as a split darlington?

#### AlbertHall

Joined Jun 4, 2014
11,400