[sdiy] The annual I/O impedance/protection thread! (bumped)

[Harry Bissell]

Well OK. Now you know that you can't exceed the supply voltage. IMHO if you make all your signal inputs using an inverting configuration,
you would have input impedance of 100K (presumably) and the opamp itself would maintain the voltage differential across the inputs to
0V (so there would be no chance of exceeding the common mode range of the TL07x (08x) opamp. That in itself should prevent you from blowing any input tied to any control voltage source, any output, or any of the power supply rails directly.

Now... if you want to plug OUTPUTS together, you'll need to protect them from each other. That should be simple as well. Use a resistor in series
with each output, such that you can't draw too much current from the opamp output with the output jack tied to the opposite rail from what the opamp
wants to put out.

Example: You are trying to put out a maximum positive signal (with a 12 supply that would be 10.5V) from there to the negative rail is 22.5V.

With a 1K resistor that would give you 22.5mA. If you assumed that there is a 1K in series with each output, that would be a 2K load if two outputs were shorted together and trying to drive in opposite directions.  The part is specified to drive a 2K or larger load... so it looks like you would not
blow any parts that way. You have to figure out the internal heating by using the voltage drop across the device and see that the package doesn't get too hot.

Watch out that you don't have two outputs shorted, sharing the same package... that might give you a high enough power dissipation to burn the part.

I'd say no need for zeners, anywhere.

You might consider using a single opamp (TL071) so that the full package dissipation is available just for that output. Even then, a mixer is the right
way to combine outputs... even a passive mixer with 10K resistors would work quite well...

H^) harry





----- Original Message -----
From: Justin Owen <juzowen at gmail.com>
To: synth-diy at dropmix.xs4all.nl
Sent: Tue, 12 Jul 2011 15:34:53 -0400 (EDT)
Subject: Re: [sdiy] The annual I/O impedance/protection thread! (bumped)

-----Original Message-----
From: Neil Johnson [neil.johnson97 at ntlworld.com]
Received: 12.07.2011 20:16:52
To: synth-diy at dropmix.xs4all.nl
Subject: Re: [sdiy] The annual I/O impedance/protection thread! (bumped)

Justin Owen wrote:
> Now everybody's back - I hope nobody minds me giving this a bump...
>

>>Well, I think it would help if you could explain *what* you are trying 
to protect the inputs and outputs from.  Mains voltage (what voltage? 
120V, 240V, 415V..?).  ESD??  RFI??  Just synth voltages (12V, 15V)? Its 
difficult to offer specifics without knowing what you're trying to achieve.
>>

Fair point.

It annoys me when people assume that everybody on this list is building modulars - so I guess it's my own fault for assuming you would've known that I was talking about modulars...

So - I'm talking about modulars. Specifically +/-12V power supply, 10V PP AC waveforms, 0-5V CV and a level of protection based on the fact that I will have no control over who will be plugging what into where.

Thanks,

J



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-- 
Harry Bissell & Nora Abdullah 4eva