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?
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?
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.
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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
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
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?
I was imagining paralleled Zeners. I did not note Osval said series connected.
In parallel the diode does not defeat the clamping action.
But the series connected uses fewer components and the effective capacitance is quartered. Go for series connected.
Still, the capacity is very voltage-dependent. So, for an input that has high impedance it might be a problem. Plus, the zenner might be to slow anyway and let-trough som edangerous pulses.
Usually the CMOS IC have protecting diodes inside on every input, so doing this outside is redundant and unnecessary.
If the input is associated with an OpAmp, the better option is two fast-acting diodes, one connected to negative rail, the other to positive rail.
Usually the CMOS IC have protecting diodes inside on every input, so doing this outside is redundant and unnecessary.
If the input is associated with an OpAmp, the better option is two fast-acting diodes, one connected to negative rail, the other to positive rail.
An externally hosted image should be here but it was not working when we last tested it.
That is quite alright. Sometimes deep insights heretofore unseen are derived from misunderstandings.
I submitted a post with an image of the basic circuit yesterday, along with some details of the larger project, but it failed to publish thus far.
Interesting. I need to be at the tentative design stage of the complete circuit to consider this impementation.
I do not trust soundcard makers to provide overcurrent protection. I already blew one card simply by using a boosted zoom mic; however, I am unsure if that is what did it or if it was the cutting edge, beta driver furnished by ALSA to adddress complaints of low mic level inputs. We know aggressive software drivers can blow hardware. I will disect the mic I used when it arrives next week, to see what is in it and how the component are wired.
ITPhoenix, this is the first real information that you've given about what it is you're trying to protect against. Apart from that we knew only that you wanted to keep a 600R input impedance.
I had dismissed Tesla coil performance monitoring as unlikely for this forum and was dithering between lightning strikes and accidental connection to the mains supply, slightly favouring the latter. I appreciate that actually specifying things tends to curtail the discussion but occasionally a bit of focus is nice.
Microphone input usually implies low impedance, low signal level, needing good low noise amplification etc etc.
Will the 'boosted' mic be the the only one you're using? In which case won't it work on the line level input?
Might I suggest that the best way for you to stop blowing soundcards is to stop using the mic inputs? They're never as good as a proper external mic preamp anyway.
It is very true, but you must have access to internal wiring, which isn´t recommendable por people that don´t know exactly they are doing. In place, a simple 5.6V 1/4w back to back may be accomodated inside the plug and don´t need to remove nor a screw from the computer. And nonlinearity, buaaaaa, it may be significant when diodes start acting. Also can be tested a MOV, I don´t know how MOV´s can perform in audio, but a 6V+ varistor may work properly, at the price of (perhaps) increased self capacitance
Agreed, that is why I submitted three paragraphs explaining the application with an image of the basic AC zener clamp. It was never posted. It appears some users are experiencing posting problems at this time.
The mic is a mono, boosted zoom that is normally used in camcorders. This is fine at SPLs originating at a distance. It is also attached to an autonomous robot so there is no way of knowing when a close, loud noise source, such as someone screaming at it may occur, not to mention possible mechanical shock from a fall or a punch in the face.
Whether or not overvoltage/current caused the destruction has not been determined, but we know this is possible if enough energy is available. It still may have been caused by poorly written software. The app needs to pick up speech at a max distance of about 15 meters.
I do not see why I cannot use the line inputs, come to think of it, as long as the input signal is compatible. The only problem might be is the recognizer may only read mic input. It would work for training the recognizer as we can record in mono, but then a different channel would be used in operation. It is critical the same channel hardware be used in training and in the recognition mode.
Gas tubes are the best protection against HV discharges, from my limited research. A friend experienced a direct hit to the pole in front of his house. He was forced to go electronics shopping for almost all he had.....If he only had one on the service panel............
Given the very variable signal levels you're getting, and the application, it would probably be worth having the mic feed an automatic gain control circuit. As the mic has an inbuilt pre-amp, I don't think you need worry about input impedance of the agc too much.
To go into the mic input, a simple resistive voltage divider, after the agc, should do the trick and provide definite protection to the soundcard input.
Hopefully your robot won't be suffering too many lightning strikes, or wandering up too many pylons. My thankfully limited experience of nearby lightning strikes is that gdts etc are of limited value - anything conductive takes a massive common mode hit. Further back, gdts will save stuff that otherwise would have fried. In my case the strike was to the telephone line, entering the house downstairs, where stuff died. Upstairs, ie out of the direct path to ground, stuff survived.
I've added a note about the posting problem to a relevant thread in Forum Problems
To go into the mic input, a simple resistive voltage divider, after the agc, should do the trick and provide definite protection to the soundcard input.
Hopefully your robot won't be suffering too many lightning strikes, or wandering up too many pylons. My thankfully limited experience of nearby lightning strikes is that gdts etc are of limited value - anything conductive takes a massive common mode hit. Further back, gdts will save stuff that otherwise would have fried. In my case the strike was to the telephone line, entering the house downstairs, where stuff died. Upstairs, ie out of the direct path to ground, stuff survived.
I've added a note about the posting problem to a relevant thread in Forum Problems
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