Thanks for that Ken - a very useful post.
When comparing components, it can sometimes be difficult trying to work
out what the really significant parameters are on a datasheet.
Cut-and-pasted into the archives ;-)
cheers,
Dave
When comparing components, it can sometimes be difficult trying to work
out what the really significant parameters are on a datasheet.
Cut-and-pasted into the archives ;-)
cheers,
Dave
On 4 Nov 2012, at 12:27, sasami@... wrote:
> >Does anyone know if I can sub a BAT42 for the BAT48 in the CGS 81 dual
> processor?
>
> Look at the datasheets:
> http://www.vishay.com/docs/85660/bat42,%20b.pdf
> http://www.datasheetcatalog.org/datasheet/vishay/85664.pdf
>
> 1)Reverse voltage 40 for the BAT48 vs 30. How much signal will it
> see? As it
> is back to back with another diode, it will only see the maximum of
> the
> other diode's forward drop, which is in the order of half a volt.
> Well, we
> ain't getting anywhere near 30 of 40 volts, so that won't make a
> differece.
>
> 2)Forward voltage drop. Both are around 400mV for 10mA through them.
> No
> difference there. The drop for the is greater at 50ma though. Will
> this matter?
>
> 3)Forward current. 350ma for the BAT48. Only 200 for the BAT42.
> Hmmm....
> Looking at the circuit we have 270k at one end connected to a summing
> node
> on an op-amp. Summing node is always zero volts. Other end of diodes
> goes to
> pots that can go between +/- 15. So maximum forward voltage across
> the 270k
> is 15 - diode drop, which we will ignore for the moment. Using ohms
> law
> V/R=I. 15 volts over 270000 ohms = 0.055555 micro amps. Okay, our
> current is
> so tiny we aren't going anywhere near the BAT42's 200mA limit, so
> that isn't
> a problem.
>
> Going back to 2) datasheet doesn't give forward voltage drop values
> for the
> current we are using, but we know it will probably be close enough
> between
> the two diode types. We have worked out that we won't damage the part
> because it is being used well within its specification. The only
> concern
> will be its behavior at these undocumented levels.
>
> Conclusion. Stick it in and try it. No harm will come of it. It will
> probably do the job.
>
> Reality: All these diodes are doing is providing a tiny dead band in
> the
> middle of the pot's rotation, so that zero is easier to select. Any
> variation in the diodes will only vary the width of this tiny gap. In
> all
> likelyhood, you won't be able to tell the difference.
>
> Hopefully next time you have a similar problem, you will have an idea
> of how
> to find out the answer yourself.
>
> Ken
>
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> >[Non-text portions of this message have been removed]
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> >------------------------------------
> >
> >The CGS Modular Synth home page: http://www.cgs.synth.net/
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> >
> >
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> __________________________________________________________
> Ken Stone sasami@... otherunicorn@...
> Modular Synth PCBs for sale <http://www.cgs.synth.net/>
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> <http://www.blaze.net.au/~sasami/>
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