Yep, I didn't notice the ground leg. But, it's still messed up, the ground side of the regulator filter caps are raised to 4 volts (plus noise) above ground. Looks to me like a 9v regulator and a zero ohm resistor is in order.
Mikek
They are, but it doesn't matter in this particular case because the constant-current generator is in a well-damped feedback loop and is unaffected by a small amount of voltage drift and noise.
This was a one-off design, so the cost of the regulator wasn't an important factor. When I was building it, i found I didn't have a 78L09 in stock so I used a 'stood-up' 78L05 instead.
In a conventional configuration where it was important to maintain stability and low noise to the 0v rail, I would agree with you. This circuit is a little unconventional and the caps are only there to maintain the stability of the regulator, any noise is taken out by the time constant in the Base circuit of the BC212L.
The 470uf capacitor decouples to the +ve rail to ensure a noise-free current through the 560 ohm Emitter resistor, so it doesn't matter if the regulator generates a bit of noise relative to the 0v rail.
Yes, if you have a 9v regulator you can use that and omit the resistor - that's what i would have done if I had had a 9v regulator in stock at the time.
-- ~ Adrian Tuddenham ~ (Remove the ".invalid"s and add ".co.uk" to reply) www.poppyrecords.co.uk
Re-visiting that circuit, it looks as though the diagram may have a capacitor missing - there should be an electrolytic (probably 47uf) between the 9v rail and ground. I can't get hold of the prototype to check at the moment because it is in use elsewhere, but I think I intended that capacitor to be there.
The advantage would be that any noise generated by the 5v regulator would appear across its own 'tail' resistor and not on the 9v line. As mentioned before, it won't make a lot of difference because the Base of the BC212L is tied to the 9v rail by a capacitor soany noise on that rail doesn't appear in the current through the Emitter resistor.
-- ~ Adrian Tuddenham ~ (Remove the ".invalid"s and add ".co.uk" to reply) www.poppyrecords.co.uk
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