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

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- acannell
  
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posted
15 years ago

Fri, Apr 17, 2009 7:32 PM

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

formatting link

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

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- John Larkin
  
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posted
15 years ago

Fri, Apr 17, 2009 8:38 PM

That upper opamp circuit is very strange and arguably unstable.

John

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- Jim Thompson
  
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posted
15 years ago

Fri, Apr 17, 2009 8:46 PM

Looks like a twist (mis-connect?) of the Howland current source/"pump"

...Jim Thompson

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| James E.Thompson, P.E.                           |    mens     |
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- A
- acannell
  
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posted
15 years ago

Fri, Apr 17, 2009 9:09 PM

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.

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- JosephKK
  
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posted
15 years ago

Fri, Apr 24, 2009 5:02 AM

I strongly suspect that is not an optimal solution. If you are of an experimenting mood please try removing both the cap and the 10K between the gate and the non-inverting input. Also i would like to see the traces on the outputs of both opamps as well, both before my suggested change and after. .