Gary Hucker Build Log


Here’s a tip if you want to install Waveforms on Fedora workstation. I received an error relating to libc6 v2.14 not installed although the later version of libc6 is already installed.
I got around it by using this command…
sudo rpm --install --nodeps -vh digilent.waveforms_3.7.5.x86_64-2.rpm

Linux Mint wasn’t so hard - just install Adept 2 first (found on same page as Waveform linux download).


Okay - I measured the AC power supply with the battery installed in the charger and the scope reads 12.5V (25V p-p) and the multimeter reads about 9.56V.
I was assuming the discrepancy between the two measuring devices was because the multimeter is a true RMS reading and the scope is not?
If Vrms = Vp / √2 then 12.5/√2 = 8.84.
It doesn’t = 9.56 reading from MM.

Maybe I should crank up the old HP analog scope and see what it reads…


What is the current rating of the AC adapter?

The equation Vp / √2 assumes a perfect sinusoidal waveform. If there is any distortion the meter could give a slightly different reading. Also, with the scope you are looking at one peak whereas the meter is measuring continuously. Any variation in the amplitude of the waveform over time will also result in a discrepancy between the two methods of measurement.


Thanks for the detailed prompt reply.
375mA output at 8V.
The waveform is a little distorted; your explanation makes sense and explains the discrepancy after taking RMS into account.
Correct me if I’m wrong but does this mean that any AC voltage reading taken via a scope should be converted to RMS before doing any other calculations?


Ah, 8V. Looking back at your posts I see you never said it so I must have assumed it was a 12V adapter. And in that case, with an unloaded output voltage so close to it’s rated voltage I also assumed it was a decent quality low current (100ma or so) adapter. But no, it’s just a cheapie. :wink:

No, not necessarily, it depends what you are trying to measure. If you are measuring the RMS output power of an audio amplifier, for example, then yes. In the case of your adapter, both measurements were correct they just represented two different aspects of the waveform. Knowing how they differed allow you to convert one to the other.


Thanks again @1.21Gigawatts


It’s quite old and fairly basic I would say.


My CE Header and Teensy boards turned up yesterday. Installed Arduino IDE and Teensyduino. Just waiting for some more bits and the sensor board to turn up.

In the meantime I have a project where I will be using a teensy to convert an old IBM Model F keyboard so I can use it on a modern PC.