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Old 26th April 2012, 12:04 PM   #1
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Default Sound Card Input Circuit Protection

I was wondering what the consensus is on protecting the inputs of PC sound cards from overvoltage/current. I am looking for a cheap, effective solution. The only one I have seen that meets this criteria, is a 5.1v zener in series with a 100 ohm resistor from output to ground. The max input voltage of the ADC is 5v.; Creative refused to provide the max current spec., but my research indicates it quite low, and there is a current limiting resistor already in the card.

One person argued, "The current needs to be limited, not the voltage." But limiting the voltage will limit the current, so it appears a matter of semantics.

Obviously, this theoretical protection circuit cannot introduce any detrimental noise. Would the presence of a zener do this? How about a varistor?
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Old 26th April 2012, 12:32 PM   #2
AndrewT is offline AndrewT  Scotland
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Series resistor followed by a parallel Zener is good protection.
A resistor ladder attenuator in front will give a more flexible input controller.

Always start with the highest attenuation. Much as you did when Multimeters were analogue.
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Old 26th April 2012, 12:45 PM   #3
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Quote:
Originally Posted by AndrewT View Post
Series resistor followed by a parallel Zener is good protection.
A resistor ladder attenuator in front will give a more flexible input controller.

Always start with the highest attenuation. Much as you did when Multimeters were analogue.
I cannot visualize the "series resistor/parallel zener circuit. Additionally, the mic impedence should remain around 600 ohm. Hence the attraciveness of the clamp.

The ladder attenuator is good for testing. I am looking for a permanent addition to the mic circuit.

Last edited by ITPhoenix; 26th April 2012 at 12:46 PM. Reason: Forgot something
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Old 26th April 2012, 01:05 PM   #4
AndrewT is offline AndrewT  Scotland
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Is the sound card input a mic compatible input for 600ohms microphone? I doubt it.

The Zener is in parallel to the input impedance. i.e. across or between signal and return.
The resistor is in series with the signal. i.e. fitted in line with the signal.
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Old 26th April 2012, 01:20 PM   #5
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I prefer to use two zeners in reversal series to short both negative and positive signals.
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Old 26th April 2012, 01:20 PM   #6
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Thank you for the clarification.

600 is the number given by Creative.
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Old 26th April 2012, 01:22 PM   #7
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I prefer to use two zeners in reversal series to short both negative and positive signals.
Thanks. True indeed. I may actually have overlooked that!
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Old 26th April 2012, 08:10 PM   #8
Simon B is offline Simon B  United Kingdom
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Your reverse series zeners shouldn't start making much noise til they start conducting, so no problem there. They will add a little non-linear capacitance, but across a 600R input unlikely to be a problem.

I'm not sure what sort of peak voltages you want to protect against, or from what sources, but one scheme would be much as suggested so far:

Reverse series zeners across the soundcard input, (limiting voltage). Series resistor, 100R, between the protection input and the soundcard (limiting current through zeners during input overload).

If you want the impedance looking into the protection input to be 600R, rather than 700R, though I doubt it will be vital, a parallel resistor, 3k0 across soundcard input (ie parallel with the reverse series zeners) restores it to 600R.

The soundcard will see an apparent source impedance of:

3k0 // (100R + Zmic)

which should not be a problem


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Old 27th April 2012, 08:42 AM   #9
AndrewT is offline AndrewT  Scotland
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The Zeners will conduct in the forward direction starting from ~400mVpk. You will need diodes to block this forward current.
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Old 27th April 2012, 10:18 AM   #10
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The Zeners will conduct in the forward direction starting from ~400mVpk. You will need diodes to block this forward current.
Any conduction before the zener voltage may be a problem. However, the zeners are back to back, in series, so in any polarity of potential, one of the zeners will be reversed biased.

I have yet to consider the specifics of component operation to that degree, so I do not know precisely what behavior the pair will demonstrate, except the pair will clip at about .7v above rated zener voltage.

Would not the introduction of standard diodes defeat clamping operation?
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