Converting polar Electrolytic cap to NP - who is correct?

[Rob Strand]

Would be interested to see what anyone else finds if they can repeat this sort of test.

The LFO on the Boss BF2 Flanger uses the back to back cap arrangement .   The timing works out
that the two back to back 33uF caps acts like the two caps are in series ie C = 33uF/2 = 16.5uF.
That's consistent with your results and Merlins.

What I was getting at in my last post is electrolytics in reverse still work like a cap, and look like the marked value.  The difference is they leak more as the voltage is increases (see graph).   I suspect if you put a *single* electrolytic cap in the LFO  integrator it would work, at least at low voltages.  The leakage would cause the timing to be extended when the cap is in the reverse direction.   When the leakage is the same order of magnitude as the circuit current it will stop working as the cap never charges.

When we put two electrolytics caps in back to back series the second cap blocks the leakage so the circuit doesn't misbehave.

From what I can see the point between the caps should develop a DC voltage due to the leakages (the diodes in the model).  A DC voltage which biases each cap with the correct polarity.

[Rob Strand]

Here's a crude sim which shows that for steady state conditions the back to back series Electrolytics
has a value of C/2.

Under steady state conduction a DC bias forms at the junction point of the two caps which correctly biases
the cap and this makes the combined series cap look like a cap of value C/2.

However, as you can see at the start of the (blue) waveform under transient conditions  the waveform does not
rise correctly.   In this example the rise is more sluggish because we are charging one cap of value C though
the diode.   It's only when the leakage is reduced to a small value do we see the rise of the waveform match the
reference circuit with cap value  C/2.

On a real cap if you keep the voltage across the cap low the leakage will be low.   That follows why we get away
with single electrolytic caps with AC waveforms (see earlier power amp post).

Here's the waveforms and test set-up.   For RL=10k there is a DC voltage on the tap point.  The 10MEG case will also develop a DC bias but it will take a much longer time.  When it does the combined cap will still look like C/2.

The diode and RL resistors are not added they simulate the Electrolytic cap.   The resistor RL sets how much leakage you get when you reverse the polarity of the electrolytic cap.   See link and comments at the bottom of Reply #16 of this thread.

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