[sdiy] OA95 as protection diode?

[Seb Francis]

Yeah, this seems to be what they are trying to achieve.  But it doesn't 
seem like very good design.  The voltage drop on a germanium isn't so 
low when it's got some current running through it.  Seems like their 
protection design depends on the driving module having a fairly high 
output impedance.

A schottky sounds like a much better bet!

However, I have had other problems with this EG - although it claims to 
accept gate voltages in the range 3-15V, in actual fact if the gate 
voltage is too low (I'm using 5V) the attack portion of the envelope 
keeps retriggering repeatedly.  I have made a fix to this problem which 
as a bonus adds proper protection for negative voltage inputs - I will 
post this info in a new message with appropriate subject in case it's of 
interest to other Digisound 80 owners.

Seb


Ken Stone wrote:
> they would appear to be more concerned about the lower voltage drop the
> germanium offers. Perhaps a schottkey (sp?) would better.
>
>
> Ken
>
>   
>> Hi,
>>
>> I'm just giving my new Digisound 80 modules a good overhaul, and on the 
>> EG module I've noticed one of the diodes is physically broken.
>>
>> It's an OA95 (germanium) and clamps negative input gate signals to 0V so 
>> they don't damage the LM358 and CEM3310.  The docs say it "gives fast 
>> protection against negative voltages, for example if the module is being 
>> gated by a +/-V pulse or other waveform".
>>
>> Is this really the best type of diode for the job?  Is there something 
>> different one would use in a modern design?  In particular the datasheet 
>> for this diode gives 150mA as the abs max peak forward current.  Since 
>> there is no resistor between the gate input and the protection diode, it 
>> seems likely this diode would just get killed just by running a negative 
>> voltage from a module that was able to sink more than 150mA.
>>
>> Seems like bad design to me, but maybe I'm missing something.
>>
>> Seb
>>
>> P.S. Schematic is here:
>> http://www.digisound80.co.uk/digisound/modules/80-18/construction/80-18.pdf
>> The diode in question is D1
>>
>>
>>
>>
>>     
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