USB Oscilloscope

Mike only

Dear Sir,

I'm learning how to measure using oscilloscope, and the Parallax USB osciloscope has become my choice. Now I have a plan to build a DC power supply with a variable voltage from 0 to 12V.·May·I use this oscilloscope to check the·output DC voltage if the current is 10A? I know only the limitation of 20 Vpp maximum input for ch1 and ch2, but how about the limitation of maximum current for both channels? The other thing is, may I check the voltage directly on the transformer to confirm the correct voltage (12V)?
Thanks for your information.

Mike only·

allanlane5

Unless you're building a "constant current" power supply (and most people don't) what you'll be doing is creating a voltage supply, that has a certain current limitation. What that means is, you apply your 12 volts across some resistance, and V = IR, V/R = I, some I == Current will flow.

So, what you're really asking is, if you put 12 volts across the o'scope inputs, what's going to happen? And what happens depends on the "input impedance" (aka "input resistance") of the o'scope probes. Which usually is quite high -- 1 MegOhm to 10 MegOhms. 12 volts divided by 10 MegOhms doesn't result in much current.

So yes, if the probes can withstand 20 volts peak-to-peak, they can certainly withstand 12 volts DC.

Mike only

Thanks for your reply.

How about if the power supply (ps)·is running and it's connected to the 12V instrument, which maybe need 10A? May I·still check the output voltage on the ps using USB scope? Because I'm sure that the high current (10A) will flow into the scope's input

Mike only.

allanlane5

The o'scope probe won't see that current. Ohm's law applies in this case -- V = IR, or Voltage = Current times Resistance. The current in a 'branch' of a circuit is equal to the voltage across that 'branch' divided by the resistance of that branch. I = V/R.

If your circuit is pulling 10 amps out of a 12 volt supply, then V/I = R, or 12/10 == 1.2 ohms. Your 12 volt instrument is acting like a 1.2 ohm load. If you now connect a volt-meter, or an o'scope probe, between the 12 volt supply line and ground, you've now created another 'branch' in your circuit. The current in that 'branch' will ONLY depend on the voltage across that branch, and the resistance of THAT BRANCH. In the case of a voltmeter or an o'scope probe, the resistance of the probe is around 10 MegOhms -- so 12 volts / 10e6 Ohms == 1.2e-6 Amps, or 1.2 micro-amperes.

In other words, a voltage probe like a voltmeter or o'scope probe won't see the 10 amps 'flying by' to your 12 volt instrument. It's the voltage limitation (that 20 volts peak-to-peak) that matters, and 12 volts is less than that, so you're ok.

Now, you MUST NOT hang your o'scope probe off of 115 VAC lines. Not because of the current, but because the 115 volt voltage will destroy the probe, because that IS above the 20 volt peak-to-peak limit.

Unsoundcode

allanlane5 said...
Now, you MUST NOT hang your o'scope probe off of 115 VAC lines. Not because of the current, but because the 115 volt voltage will destroy the probe, because that IS above the 20 volt peak-to-peak limit.

What Allan says is perhaps the one thing that is most likely to catch you out and counts for AC voltages a lot less than 110v. The 20vpp rating of your oscilloscope actually equates to about 7v RMS. So connecting directly to the output of a transformer of a higher RMS voltage could likely cause damage to the scope. Most digital volt meters give the RMS reading and if you want to verify the output voltage of your transformer that would be the instrument to use.

Jeff T.

Chris Savage

The usable range of input voltages on the scope is -10V to +10V. Take care.

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Chris Savage
Parallax Tech Support

allanlane5

Oy. So, DO NOT connect it to a 12 volt DC line. Sorry about that -- apparently +10 volts is the MOST it can take.

Mike only

Thank you to you all for the answer.
That has increased my confidence-level on how to use this scope in my other projects
Best Regards,

Mike only

Post Edited (Mike only) : 12/10/2007 4:40:47 AM GMT

Chris Savage

Fortunately the 12V would not likely hurt the scope if it was regulated. However, 12V from an unregulated supply or automotive system would be a problem. Take care.

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Chris Savage
Parallax Tech Support

crgwbr

If my thinking is correctly, you could run the voltage through an op-amp thats set to cut the voltage in half. So, for a +12 vin signal, you would get 6v going to the scope.

regards,
Craig

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Chris Savage

An even simpler method would be to use a voltage divider. Two resistors could accomplish the same task. Take care.

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Chris Savage
Parallax Tech Support