[sdiy] Op Amp LED driver / Current Source problem/solution

Neil Johnson neil.johnson71 at gmail.com
Thu Nov 27 09:51:19 CET 2014


Hi Justin,

> That version doesn't turn the LED fully off when the input signal gets to 0V...
>
> ...but adding a big old 1M R to the supply rail fixes that:
>
> http://www.sdiy.org/juz/unipolar_LED.png
>
> Is that a valid fix or is this potentially going to cause problems elsewhere in the curcuit/make bad things happen?

You've got a couple of problems with the circuit as presented.  You've
specified a TL072 so at this stage we can discount input bias
currents.

With just one diode in the feedback loop you are going to get awful
open-loop behaviour under half of the input range (when the diode is
reverse-biased).  The impedance of the diode becomes very large when
reverse-biased, so the op-amp gain will be very large too.  The end
result is that as the control input approaches zero it is highly
likely the op-amp output will peg to the negative rail.  As an aside,
this is probably not a good thing for the LED to experience - over
time it will fail if it sees too much reverse voltage across it (they
are typically specified for 5V or so).

Why does the resistor "fix" the problem?  You need to consider the
op-amp input offset voltage (Vio) - think of it as a little voltage
source in series with the Vin+ pin.  Now, depending on the sign of the
offset voltage you wil either find that even with the control input at
0V you still have some total control voltage due to Vio.

If you have a very efficient LED then even a few microamps could be
enough to produce a very small glow (I have some blue LEDs that need
microamps to glow nicely).

Your extra resistor introduces an external bias current that pulls the
Vin- pin up a little bit, so that when your control input is at 0V the
op-amp is trying to pull the Vin- pin down, but it can't due to the
reverse biased LED, which gets turned off very hard.  So no glow, but
not good for the LED.

The simpliest solution is a 1N4148 anti-parallel to the LED to protect
it, and keep the 1M resistor to bias up the Vin- pin.

Phil's suggestion of a capacitor would help stabilise the op-amp
during the transition between diode conduction (the point where both
diodes are not forward biased so the op-amp is operating practically
open-loop).  And check if your "0V" really is 0V as Roman suggested -
any output offset from the DAC just adds to the problem.

Hope that helps!

Neil
--
http://www.njohnson.co.uk


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