Soundcard voltage limiter?

[Bill]

I want to use a shareware PC oscilloscope with my soundcard. The inout
would be via a standard CRO probe to the line in socket.

I don't need to pre-amplify or attenuate the input signal, as I can
see the waveform level on the PC scope and adjust to within viewable
range at the source.

Rather, I wish to prevent initial damage to the card from
inadvertently applying excessive voltage to the line input socket.

As I understand, this would be anything over about 1.5V.

What is the best design approach to use for this? Something that does
not distort an audio signal that is of the correct voltage level.

Better yet, an actual circuit would be much appreciated.

Thank you,

Bill

 

[Ian Stirling]

I want to use a shareware PC oscilloscope with my soundcard. The inout
would be via a standard CRO probe to the line in socket.

I don't need to pre-amplify or attenuate the input signal, as I can
see the waveform level on the PC scope and adjust to within viewable
range at the source.

Rather, I wish to prevent initial damage to the card from
inadvertently applying excessive voltage to the line input socket.

As I understand, this would be anything over about 1.5V.

Probably rather more.

What is the best design approach to use for this? Something that does
not distort an audio signal that is of the correct voltage level.

Better yet, an actual circuit would be much appreciated.

A non-inductive 200 ohm high peak power capable resistor in series,
and four fast signal diodes setup as a bridge rectifier with a shorted
output.

This will cope with most spikes.
Or are you looking for something that somebody can accidentally connect
to a 220V line, and it'll be fine for a week?

 

[Tim Mitchell]

Probably rather more.

A non-inductive 200 ohm high peak power capable resistor in series,
and four fast signal diodes setup as a bridge rectifier with a shorted
output.

This will cope with most spikes.
Or are you looking for something that somebody can accidentally connect
to a 220V line, and it'll be fine for a week?

Replacing the series resistor with a PTC thermistor would allow that to
happen. (not sure how inductive thermistors are, but we use this
technique to protect 250Kbaud RS485 data lines from accidental mains
connections)

 

[John Woodgate]

I read in sci.electronics.design that Tim Mitchell

not sure how inductive thermistors are

They are normally bulk material, not spiralled film or anything that has
more inductance than a straight wire.