Understanding an oscilliscope screen - Page 1
Forum section: electronics / beginners

author: Hextejas
author_rank: Frequent Contributor
timestamp: January 24, 2018, 02:42:21 pm
content: I have these readings on my little, hand made scope and I have some questions.The signal is approx 11v AC, 60hz, from a wall wart.Do any of the Vertical or horizontal gradients?, on the display somehow show the 11v, 60hz ? If so, how ?Another test signal that I tried was 3.5v AC, 1khz, and at a .5volt setup, the wave form rose about 6+ gradients. That made sense for 3.5v. However I tried all kinds of different switch settings to try to get something that showed a picture of 1khz.The same with the 60hz.If not for the display of the readings I would not have been able to figure out the 11v, 60hz.Thanks

author: Andy Watson
author_rank: Super Contributor
timestamp: January 24, 2018, 02:52:07 pm
content: The quoted 11V must be theRMSoutput of the wall-wart. Multiply by \$\sqrt{2}\$ to find the peak value - which your scope is showing - give or take a bit.

author: Benta
author_rank: Super Contributor
timestamp: January 24, 2018, 03:26:06 pm
content: Look at the bottom of the screen.To the left it says 5V. That's means there's 5V between each horizontal line on your screen.In the middle it says 5ms. That means 5ms between each vertical line.The trace is perfectly consistent with an 11 V, 60 Hz sine wave.

author: Hextejas
author_rank: Frequent Contributor
timestamp: January 24, 2018, 03:37:59 pm
content: Quote from: Benta on January 24, 2018, 03:26:06 pmHelp me see thus Benta. I see about 6 horizontal lines between the bottom and the top of the wave. Using the 5, how do you get 11 volts out of that.

author: Paul Rose
author_rank: Regular Contributor
timestamp: January 24, 2018, 03:50:34 pm
content: I see 5.6 divisions peak to peak ( .2 short on the top and bottom). 5.6 divisions x 5 volts per division = 28 volts28 volts peak-to-peak, divided by 2 gives 14 volts center-to-peak.14 volts divided by square root of 2 ( which is 1.414 ) gives 9.9 volts RMS.This is in the same ballpark as the 11v rating of the transformer. It is very close to the displayed 10.27 RMS. Either the transformer rating is off a bit, or the scope is off a bit.The period should be 1000 milliseconds / 60 Hz = 16.7 milliseconds, or 3.3 divisions horizontally ( each division is 5 milliseconds ), between peaks, which looks pretty close

author: rstofer
author_rank: Super Contributor
timestamp: January 24, 2018, 03:51:20 pm
content: To get VPEAKmultiply VRMSby square root of two. So, 11V * 1.414 = 15.54 That is the Peak voltage - the voltage between 0V and the top of the waveform. To get VP-Pmultiply by 2 and you get about 31.108V. Then divide by 5V per division and you get a little over 6 divisions.But the scope is already telling you the VP-Pis 28.8V so maybe your 11V number is a bit off.

author: Hextejas
author_rank: Frequent Contributor
timestamp: January 24, 2018, 03:55:20 pm
content: Quote from: Paul Rose on January 24, 2018, 03:50:34 pmThank you Paul. That explains it perfectly. As an aside, I independently figured out the freq, but the voltage was still a puzzle.Success, and I am proud of this little scope.

author: stj
author_rank: Super Contributor
timestamp: January 24, 2018, 05:57:20 pm
content: transformers will give a much higher output when they have no load,if you meter a transformer, connect it to a lamp or something first.