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

[Stewart Pye]

Hi Neil,

Great idea.
I recall seeing a similar circuit in a 100V line mixer amp, but didn't 
realise why at the time.

Regards,
Stewart.


Neil Johnson wrote:
> Hi,
>
>   
>>> Outputs
>>> http://www.sdiy.org/juz/euro_output_03.pdf
>>>       
>>> And for the non-inverting circuits add anti-parallel
>>> Schottky diodes across the op-amp inputs and a resistor to the +ve input.
>>>       
>> I've added Schottky diodes based on advice from you and Harry B - it's also
>> been suggested that I could use 1N4148s.
>> Can you clarify what you mean about the resistor to the +ve input? Where
>> to/from? Why? Value based on?
>>     
>
> In the non-inverting case, definitely on the path to the +ve input to
> the op-amp before the diodes to limit any fault current that might
> flow from the signal source either into the op-amp or through the
> diodes.  Better still a matching resistor on the connection to the -ve
> pin.  10k would be enough.
>
>   
>> You'd ditch the zeners and their associated protection resistor from the
>> output as well?
>>     
>
> Well, since you seem unable to provide any *specific* requirements for
> what you are hoping to protect the outputs from, I'm assuming that the
> worst case is an input of -15V and the op-amp trying to push +15V.
> All you need to do then is put some resistance, say 1k0, in the path
> to limit the fault current.
>
>   
>>> Summing
>>> http://www.sdiy.org/juz/euro_summing_03.pdf
>>>       
>>> 2/ R1 will appreciably load the input pot to give a non-linear control
>>> curve.  This might not be an issue for you (designers choice) but it
>>> irks me.
>>>       
>> So how would you lay out an attenuated input for an inverting summing config
>> or what would you suggest revising about that input with regards to
>> component choice, value, order ,etc?
>>     
>
> Its something I'm working on:
>
> http://www.milton.arachsys.com/nj71/index.php?menu=2&submenu=2&subsubmenu=4
>
>   
>>> 5/ Depending on what the internal load looks like that 1N4001 could
>>> behave in a rather non-linear fashion.  And assuming enough current
>>> passes through it to turn it on, it will drop about 1V from the output
>>> of the op-amp.
>>>       
>> Yes, agreed, but considering it will probably be feeding something along the
>> lines of my beloved single transistor VCA - it does the job just fine. When
>> I need a precision solution I'll use a 'proper' rectifier.
>>     
>
> Well, that's your call.  And its the first time you've given any
> information about what these circuits are supposed to be driving.
>
>   
>>> 6/ When the 5V1 zener turns on its going to provide a nice low-impedance
>>> path to ground to pull lots of current out of the op-amp, which will
>>> overheat and possibly melt.  Not quite desirable.
>>>       
>> I've used this config plenty of times and not even noticed any undue warmth
>> - and I do check. Certainly never had one melt.
>>     
>
> What was the op-amp output pin sitting at when you did the overvoltage test?
>
>   
>> Your impedance point does concern me though. Considering that the aim of the
>> 1N4001/5V1 Zener combo is to allow an AC or negative V input to be used as a
>> CV source, then attenuated, biased, blocked and capped so you only get e.g.
>> 0-5V - what would you suggest for capping the upper DC limit?
>>     
>
> A zener in the op-amp feedback path would be a better starting point.
>
> Neil
>