[sdiy] Driving LEDs from CMOS

[Martin Czech]

:::My problem comes from trying to drive an LED from this Q output.  I went from 
:::the Q output through a 47 ohm resistor to the base of a 2N3904.  Emitter to 
:::ground and collector through a 680 ohm current limiting resistor, then to the 
::: LED.  The other LED leg goes to V+

Dave Maguson, the evil CMOS gate torturer B->>

47 Ohm into Base? Let's assume Hfe of 70. 
I assume 5V supply/logic high, this means CMOS. How much base current?

(5V-0.7V)/47Ohm= 91mA !

Quite a load current. Should lead to Ic of 6A !!
Of course this is rubbish, 6A is impossible.
But it shows that you need Ic=20mA -> Ib=20mA/70 ~ 280uA.
=> R= (5V-0.7V)/280uA= 15kOhm.

Problem: Hfe varies widely through individuals, and also
with larger current. 20mA is large for a tranny like that.

So you should measure a few trannys to get an idea,
then cut down R by 2, means 6.8kOhm, should be ok even for the
weakest Tranny you'll find. Of course, that will load the CMOS
gate more. Tradeoff. And slower up-slope. That could wreck
your logic, because logic needs FAST slopes, especially for clocks.
A good method would be not to use that gate output for further
logic if you feel that the load is still too much.

Are you still in SOA of tranny? What's the maximum allowable dissipation?
I FEEL it should be ok.

m.c.