[sdiy] Modular synth input/output protection.

[Seb Francis]

Most inputs will generally already have summer buffers (i.e. large e.g. 
100K resistor from input to opamp inverting input which is at virtual 
ground).
This is better than a voltage follower buffer because it will not suffer 
over-voltage damage.  A voltage follow input buffer would have the input 
connected *directly* to the non-inverting opamp input which could be 
damaged if over-voltage is applied.

As for outputs, most modern opamps are short-circuit safe, but consider 
if you connected a +10V output to a -10V output.  This is a worse case 
than simply short-circuit.  So many designs have something like a 1K 
series resistor on the opamp output to limit the current.  If you need a 
lower output impedance then you could use 100R instead, but 1K offers 
more protection.  A good trick, especially for critical CVs, is to put 
this series resistor *inside* the feedback loop so the opamp 
automatically compensates for the voltage drop across the resistor (if 
it's a voltage follower, you will also need another larger resistor in 
the feedback loop from the module output back to the opamp inverting input.

Bottom line is: don't worry too much: most modules should already be 
designed pretty resilient.  Best thing is to check the schematic of each 
one and if necessary make any changes .. e.g. in the following cases:

(a) Input is direct to an IC (maybe for some kinds of digital modules?)
SOLUTION: schottky diodes connected from input to Vmax, and from Vmin to 
input.

(b) Output is coming direct from IC output
SOLUTION: if output is an opamp, add a series resistor *inside* feedback 
loop.
SOLUTION: if output is not buffered, add an opamp voltage follower with 
10K + 1K resistors in series in the feedback loop.  Connect the module 
output to the mid point of the 2 resistors.

I hope this helps.

Seb


Dave Kendall wrote:

>Thanks Seb.
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>>................ perhaps if you wanted to protect over-voltage being
>>fed into outputs (can you trust people not to feed outputs to outputs?)
>>    
>>
>
>Knowing some of my friends, er..., that would be the first thing they'd
>manage to do :-)
>Is there a good scheme for protecting outputs? I've been thinking about
>op-amp voltage followers as buffers on both inputs and outputs of all
>modules.
>Some of the modules I'm building have an output directly from an op amp,
>sometimes via a 1k series resistor, sometimes straight out with a
>(typically) 100K resistor to GND. How easily could damage be done ?
>
>cheers,
>
>Dave (with dodgy muso mates...)
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>>Seb
>>
>>
>>
>>Dave Kendall wrote:
>>
>>    
>>
>>>Hi all.
>>>
>>>As my growing modular may be (mis)used occasionally by musician friends,
>>>(and me, come to think of it....) I'm thinking about protecting all inputs,
>>>audio and control, against over-voltage, and general blowing-up type stuff
>>>;-)
>>>I guess CV inputs could be quickly overloaded if several sources are mixed
>>>and fed into them? Most of the modules are ±15V, and a few will be ±12V.
>>>
>>>Is there a "standard" and relatively cheap way of doing this? I was thinking
>>>of setting a ±10V limit on voltages in the system, so modules running on
>>>±12V and ±15V would both be usable.
>>>
>>>Zener diodes come to mind as a way of protecting inputs, and for ICs,
>>>there's Ken Stone's neat +14.4V -0.6V trick which, although explained on his
>>>site, I can't quite get my head around.......
>>>
>>>I'm sure this is all well-trodden ground, but I'm a bit in the dark as to
>>>the best way to stop the modules accidentally frying...
>>>
>>>Any tips gratefully received.
>>>
>>>Cheers,
>>>
>>>Dave
>>>
>>>
>>>
>>>
>>>
>>>
>>>      
>>>
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