# Voltage limiting without altering waveform

#### arg733

Dec 14, 2010
89
Hi.
I want to make a DIY oscilloscope with an usb sound card (max 20kHz).
The problem is that i will use it to analyze waveforms with voltages from 50 to 400VAC , so is there a way of limiting that voltage to 1v without dramatically altering the waveform? I will use voltage dividers but i need some sort of protection, in case i forget to set the voltage divider right and the voltage goes to 2 or more volts. The more i think about it the more i realize that this is not possible so what i need is something that will actually cut (turn off) the signal to the sound card if the voltage exceeds 1v.

Thank you very much.

#### duke37

Jan 9, 2011
5,364
If your voltage dividers have high value resistors in them, then it is only necessary to lilit the voltage with some diodes. The wave form will be distorted but you will be able to see if it is clipped and, if so, increase the attenuation.

Do you think it a good idea to put 400V anywhere near a computer?

#### arg733

Dec 14, 2010
89
If your voltage dividers have high value resistors in them, then it is only necessary to lilit the voltage with some diodes. The wave form will be distorted but you will be able to see if it is clipped and, if so, increase the attenuation.

What do you mean by that?

Do you think it a good idea to put 400V anywhere near a computer?

Last edited:

#### (*steve*)

##### ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
Moderator
Jan 21, 2010
25,508
Well, since you always start with maximum attenuation when you have an unknown signal, it will be obvious when the signal starts to clip.

#### arg733

Dec 14, 2010
89
You could add some additional circuitry, say 2 x 741 op-amps with the same clip limiter on one and a higher clip limiter on the other, then into a LM339 comparator while attenuating the output from the higher clip limited op-amp slightly, and have the comparator turn on an LED if the clip limit is exceeded.

I have no idea how to do that i don't have any experience with op amps and comparators.
I think that it should be ok without cutting off the signal to the sound card. But i need that led to show me when the signal is being clipped cause i am not experienced with oscilloscopes either.

P.S. I've been looking into the Digilent Analog Discovery as a fairly inexpensive electronics tool, $200 gets you your sound card input and more and the software to run it. When I get one I'll let you know whether I like it or not. Well i got the software (free) and the sound card costs me about 1.50$

And if you are referring to one of those usb devises that let you read up to 1MHz , someone told me that they have a time delay but i don't know that this is true.

#### arg733

Dec 14, 2010
89
Well, since you always start with maximum attenuation when you have an unknown signal, it will be obvious when the signal starts to clip.

That's a good tip steve, thanks

#### KrisBlueNZ

##### Sadly passed away in 2015
Nov 28, 2011
8,393
This thread is going round in circles.
This is my suggestion.
It consists of a range switch, an overvoltage protection circuit, and an op-amp buffer.
There's a negative rail generator as well. You can delete that part if you already have a negative supply rail available.

It needs a DC power supply of 5~9V.

C1 blocks any DC on the input signal. R1~3 provide selectable division ratios to support higher-voltage inputs. D3 and D4 clip the waveform to the positive and negative supply rails; this limits the maximum positive and negative voltages seen by the op-amp, and therefore the sound card, to prevent damage to either. D1 and D2 ensure the supply rails do not get pulled up or down by current from the input.

U1 is a unity gain buffer that feeds the sound card input. U2 is a negative rail generator with a maximum supply voltage of 10V. It can be omitted if the negative voltage rail can be provided in some other way.

This circuit clips the input voltage to prevent damage to the sound card. The sound card will also clip the signal that it measures; this is a natural result of converting an analogue signal to its digital representation. Whenever the digital value reported by the sound card reaches the maximum or minimum reportable value (+32767 and -32768 respectively, for a 16-bit sound card), the input is being clipped. This is the best way to detect clipping by the sound card.

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