my "proportional/derivative" led driver? comments please!

[[email protected]]

i am new to PID theory and circuit design, and I threw this together
over a few days.

http://i242.photobucket.com/albums/ff197/acannell/leddriverpid.jpg

the goal is to create a voltage controlled current driver for an LED,
where the current is directly proportional to the voltage, i.e. 100mV
= 100mA, and the driver is useful up to at least 50khz, including
square waves.

what mystifies me is how well adding the 1nF cap worked. without it
there was about 15% overshoot (instead of rising to 100mV, it rose to
about 113mV).

i tried everything i could think of to make a derivative section, but
none of it worked as well as that cap.

so, is this circuit close to being good? what have I done wrong?

Asa

 

[Eeyore]

i am new to PID theory and circuit design, and I threw this together
over a few days.

http://i242.photobucket.com/albums/ff197/acannell/leddriverpid.jpg

Isn't an LM318 a bit old ? Like the 2N / PN 2222s that crop up in ancient
scrolls all the time.

Graham

 

[[email protected]]

Isn't an LM318 a bit old ? Like the 2N / PN 2222s that crop up in ancient
scrolls all the time.

Graham

my circuitmaker 2000 had it already as a spice model, plus its
actually got the specs I need as far as I can tell....bipolar supply
to +/-20V, fast enough, etc..

 

[GregS]

Isn't an LM318 a bit old ? Like the 2N / PN 2222s that crop up in ancient
scrolls all the time.

Graham

I was thinking it was obsoleted!

Lately I dug out some LM837's I had and made a buffer out of it
using all 4 channels.

greg

 

[Eeyore]

I was thinking it was obsoleted!

Good Lord, they still make it. I recall it being used in some of the earliest IC audio gear. Better
than a 741 !
http://www.national.com/mpf/LM/LM318.html

Can't say I'd choose it though even though some of the specs still aren't too bad.

Graham

 

[[email protected]]

That upper opamp circuit is very strange and arguably unstable.

John- Hide quoted text -

- Show quoted text -

Lets say I have 5 apples, and I want ten apples, I tell you, "give me
5 apples".

Actually...it says "give me 5 MORE apples"...thats what the feedback
from the gate of the IRF510 to the opamp positive input does.

The resistor values of 10k/500 ohms takes into account the sense
resistor voltage being 1/10th the current through the sense resistor,
and the IRF510 having a 2 volts per amp gate voltage relationship.