Probe circuit for measuring higher voltage on oscilloscope

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Amation
Posts: 1
Joined: Wed Apr 11, 2018 6:50 am

Probe circuit for measuring higher voltage on oscilloscope

Post by Amation »

The only oscilloscope I have is a tiny DSO138 (input section is on the top left of page 4 in the PDF). The scope has a limit of 100Vpp (actually displays only 80Vpp). At the highest attenuation (100x), there is 1V input to the first op-amp.

I want to see the output of a step down transformer. The transformer secondary shows 40V AC on my multimeter. Assuming this to be close to the RMS value, peak would be 56.5V and peak to peak at 113V. This is slightly higher than what I should put into the scope.

How can I go about attenuating the signal? Will a pair of 100K resistors as voltage divider work (with input set to 100x on the scope)? Will I need to attach any capacitors? I will probably never use it for more than 200Vpp ever, but there is an audio amplifier project coming up. If I can make a decent probe to view 100-200Vpp, it will be helpful.

All advice will be helpful. My electronics knowledge is rather limited (but I cannot go out and buy a proper probe or scope just yet). Also, I've never used a real scope.

Here is the input section of the scope:
Image

As far as I understood, standard scopes come with a 50Ohm and a 1M resistor inside them, and probes are 1x/10x/100x/1000x with reference to that, with 10x being most common. Is this XXXx multiplier with reference to the first op amp input voltage (somewhere around 0-1V)? That would mean my scope already has the 1x, 10x, and 100x probe circuits built in, right? Would that mean using an additional 10x passive divider give me grossly innacurate results? Are scopes mostly used for seeing waveforms in the 0-10V range?

Another question: The input opamp TL084(http://www.kynix.com/Detail/541497/TL084.html) has a bias current of 30pA (max 200pA). With the input 100K on the lowest range, that would mean my lowest ranges are not protected upto 100Vpp. Only protected upto +=5V and +-50V, even though there is a 100K resistor on the input of the lowest range?
davef
Posts: 78
Joined: Sat Jun 06, 2015 10:01 am

Re: Probe circuit for measuring higher voltage on oscillosco

Post by davef »

How can I go about attenuating the signal?
By buying a proper 10X probe
Will a pair of 100K resistors as voltage divider work (with input set to 100x on the scope)?
Not very well. In input impedance on that range is 100K, which will change the divider ratio significantly.
Will I need to attach any capacitors?
I doubt if you would see any difference
Is this XXXx multiplier with reference to the first op amp input voltage (somewhere around 0-1V)?
Can't answer this one.
That would mean my scope already has the 1x, 10x, and 100x probe circuits built in, right?
One way of looking at it.
Would that mean using an additional 10x passive divider give me grossly innacurate results?
Using 100K resistors ... yes

Your last question. The TL087 will hit the supply rails (+/-5V) when you stuff more than +/-5V / 100000 Ohms = +/- 20pA into it.
Is 200pA the maximum INPUT current allowed? If so, than you could stuff +/-50V before the part fails. The keyword here is how much current can any input substrate clamp diodes handle. If the TL087 have them.

Suggests than one should always start with the highest attenuation setting and then work down to get a reasonable almost full-screen display.

My 2 cents worth as I am not an oscilloscope guru.
davef
Posts: 78
Joined: Sat Jun 06, 2015 10:01 am

Re: Probe circuit for measuring higher voltage on oscillosco

Post by davef »

On second thought ... on the 10mV range the input impedance of the system is very high so maybe an option is to make your own external divider, say 20M Ohms with a 20K shunt and probably a trimmer cap to give you a "10V" range.

Effectively making your own 10X probe.
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