[sdiy] Voltage-limiting a comparator output.

[David G Dixon]

I haven't heard of zeners failing, and certainly not failing by shorting.
Maybe I just haven't been around long enough.

Personally, I'd go with the divider, as it consumes far less current.  With
a zener, your comparator opamp will by pumping out maximum current (25mA or
more) in the high state.  If you're using CMOS logic downstream, then the
"on" voltage range is pretty forgiving.  I believe they turn on with
anything higher than about 2.5V.  Just make sure that the voltage will be
higher than about, say, 3.3V for a 12V supply, and Bob's your uncle.


> -----Original Message-----
> From: synth-diy-bounces at dropmix.xs4all.nl 
> [mailto:synth-diy-bounces at dropmix.xs4all.nl] On Behalf Of Dave Kendall
> Sent: Saturday, August 27, 2011 1:56 PM
> To: synth-diy DIY
> Subject: [sdiy] Voltage-limiting a comparator output.
> 
> Hi all.
> 
> I'm working on a circuit that aims to accept a gate anywhere 
> between ±15V, and convert it to a positive voltage between 0V 
> and +5V, for driving logic circuits.
> The first bit seems simple enough - an op-amp based 
> comparator, followed by a series diode to give half-wave 
> rectification.
> Then I had thought of feeding the comparator's output into a 
> simple voltage divider, to bring the output down to +5V.
> The other option would be to use a 4.7V or  5.1V zener. I'm 
> unfamiliar with using zeners, and the drawback of the first 
> solution is that using ±12V or ±15V will affect the divided 
> voltage. It would be nice to have the thing equally happy 
> with either ±12V or ±15V power rail voltages, without 
> swapping out resistors if changing between the two.
> 
> Are there any drawbacks to setting the voltage divider to 
> kick out say 7V at ±15V (which would be a bit over 5V ±12V), 
> and then have a zener limit the output to  +5.1V or 4.7V? How 
> robust is a zener? - I guess if it failed, the next IC in 
> line is toast.
> 
> Probably a really basic question, but maybe there's a 
> well-known, proven scheme already out there....
> 
> Any suggestions welcome.
> 
> cheers,
> Dave
> 
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