[sdiy] Running a Bipolar Comparator..

Tom Wiltshire tom at electricdruid.net
Sat Mar 25 16:18:20 CET 2017


On 25 Mar 2017, at 14:26, Busby Bergson <busby.bergson at gmail.com> wrote:

> Thanks for all of the great information..this helps a lot.
> 
> Been testing all morning, and for the time being, the 1N4148 diodes
> seem to have solved the issue.
> However, when I run the comparator from ground, instead of -12v, the
> circuit no longer seems to work. Any idea why?

"no longer seems to work" in what way exactly? Doesn't do anything? Doesn't do what you expect? Does what?

How close can the comparator operate to its supply rails? lowering the negative supply to -12V rather than ground effectively gives you a lot of "headroom" below zero. If the comparator struggles when close to the negative rail, that might be what you're seeing.

Another issue you might hit (and this is presumably why there were germanium diodes in the circuit originally) is that as the voltage decays away, it will drop below the forward voltage of the diodes (about 0.65V) and then they'll stop conducting. What this looks like is that the release curve is doing fine down to 0.65 volts, and then only drops very slowly after that. The diodes will leak even if they don't conduct properly, so it carries on decaying, but it's slow.

Germanium diodes will mitigate this problem since they have a lower voltage drop, so it won't kick in until later in the curve, by which point you maybe don't care so much.

> Presumably, if I replace the electrolytics, +/-12 will ultimately be fine, right? Also,
> electrolytics should be able to handle a small-ish reverse voltage,
> shouldn't they? That might just be wishful thinking…

They might be able to handle it (for a while), but exposing them to it isn't good design. Work out a way that doesn't involve toasting things!

HTH,
Tom





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