[sdiy] Running a Bipolar Comparator..

Busby Bergson busby.bergson at gmail.com
Sat Mar 25 15:26:07 CET 2017


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? 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...







On Sat, Mar 25, 2017 at 8:56 AM, Mattias Rickardsson <mr at analogue.org> wrote:
> FWIW, the 339 has only an upper and a lower rail, no middle ground in
> between. It doesn't know that it's connected to a bipolar supply, it
> doesn't do anything in relation to ground potential if you haven't
> connected its pin called "GND" to ground. it just does its thing
> between the two rails. :-)
>
> /mr
>
>
>
> On 25 March 2017 at 13:32, Tom Wiltshire <tom at electricdruid.net> wrote:
>>
>> On 25 Mar 2017, at 07:36, Busby Bergson <busby.bergson at gmail.com> wrote:
>>
>>> Hi everyone -
>>>
>>> I'm currently trying to run an LM339 on a bipolar power supply. The
>>> application notes suggest this is possible, as long as the input
>>> signals don't go negative. I'm feeding it with a fully-rectified input
>>> and a variable threshold from 0-12v.
>>>
>>> Attached to it is a pretty standard Attack/Release circuit, feeding a buffer.
>>>
>>> There are two issues:
>>> 1. The "Release" circuit shorts at the far end of its range.
>>> 2. Diodes are getting blown out.
>>>
>>> Does anyone have any idea why this is happening?
>>>
>>> Here's a drawing of the circuit:
>>> http://imgur.com/a/U0F8e
>>
>> Given that the input is positive, and the threshold voltage is positive, and the output across the cap is positive…I'm not really seeing why you're using a bipolar supply for this. It looks like Ground and +V would be sufficient, except for the final output buffer op-amp, which probably wouldn't go fully to ground unless given a bipolar supply.
>>
>> You need to make sure that the current that flows when the pots are at zero is low enough that the rest of the circuit can cope. That 100R resistor next to the cap is there to limit this current. The downside is it also limits minimum attack/release times. Thus it needs to be as small as safely possible, but no smaller. Some juggling of timing cap/pot values may be required to get a good compromise, though 1uF/1M is a combination I've seen before.
>>
>> If the LM339 is on a bipolar supply, does its output get pulled down all the way to the negative rail? That'd really screw things up…cap suddenly turned upside down, etc.
>>
>> HTH,
>> Tom
>>
>>
>>
>>
>>
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