voltage trip based on high d/t(voltage)

[excalibur313]

Hi Everyone!
I currently have a simple circuit where I apply a voltage to a piece of wire submerged in a solution and have a counter electrode that has a lower potential that current flow from to the wire. A process occurs where I etch the wire to create a tip at the solution surface. When the tip is created the bottom of the wire drops and the current drops to zero.
I was curious if there is something I can put in the circuit either to directly stop applying a voltage to the wire once the derivative gets high enough or to send a signal to the power supply to shut off. Can I use an op amp differentiator? The trick is that I would want this to be done as rapidly as possible like micro or nanoseconds if that was possible.
Thanks!
Stephen

 

[dick_freebird]

The simplest "good enough" differentiator is a series cap,
shunt resistor (probably followed by a comparator such
as a LM339, one that works with inputs about ground).

However, maybe you do not need to sense the derivative
but simply the voltage jump when current ceases. That
could be even simpler, provided that the etch process
is fed constant-current rather than constant-voltage.

I would not expect chemical etch processes to have a nS- or
uS-regime time signature. Further, if current has dropped
to zero then a really fast turnoff of the source probably
adds little value (mS would do as well, I'm sure).

If you want to sense change in current then you'd need
to add a sense resistor or something. If your supply had an
open-collector compatible shutdown line, then your sense
resistor (in the cathode leg, electrode to ground) in parallel
with a NPN base-emitter path could be a 2-component
solution.

 

[excalibur313]

The issue is that you get thinning at the surface of the solution and eventually it breaks off and you are left with two tips that should be atomically sharp. The problem is that due to capillary action if you don't turn off the power supply fast enough you get slight etching right at the tip and it acts to dull it so it isn't atomically sharp but dulled very slightly. If I could immediately detect when the current starts to drop to zero it could be possible to turn it off right before the submerged piece falls in the water.

Thanks for your help!