I've been playing, trying to create a zener driver current source as described in "The Art of Electronics" (Chapter 2.06 "Transistor Current Sources", 2nd Ed) (thinking towards an LED driver for a static, battery driven string) The circuit is as follows:
Now, for testing, I've disconnected the base of Q1 so that I can check V(R1) and V(DZ1) to check that they stay in bounds that I'm expecting. R1=45k and DZ1 V(fwd)=3.9v
I'm getting the following readings:
| Vs = 3.40 | V(DZ1) = 1.84 | V(R1) = 1.56 | | - | - | - | | Vs = 6.43 | V(DZ1) = 2.087 | V(R1) = 4.34 | | Vs = 9.44 | V(DZ1) = 2.221 | V(R1) = 7.22 | | Vs = 12.66 | V(DZ1) = 2.321 | V(R1) = 10.33 |
As I'm repeating these experiments, I realise that my R1 may be a little high to get a good enough holding current, so I'm repeating with R1=6k8 (which should give me an I(DZ1) of between 300 uA and 1.2mA) (The example in AoE is 10k)
| Vs = 3.39 | V(DZ1) = 2.288 | V(R1) = 1.116 | | - | - | - | | Vs = 6.43 | V(DZ1) = 2.658 | V(R1) = 3.77 | | Vs = 9.44 | V(DZ1) = 2.848 | V(R1) = 6.59 | | Vs = 12.69 | V(DZ1) = 2.985 | V(R1) = 9.67 |
The I(DZ1) @ Vs=12.69 is measured at 1.42mA - which works out about 5mW (1% of rated power). You can get LEDs that will light at 2mA!
So here's my question - why is this not asymptoting to 3.9V? Do I really needs a higher I(DZ1) to get this thing to hold its rated zener voltage?
One closing set of values ...
R1 = 1k Vs = 6.43 I(DZ1) = 3.13mA V(DZ1) = 3.28 V(R1) = 3.144
Over 3mA and it's still not there yet.