Using a scope

[clel miller]

I got a BK 2120B from a friend. It seems to be in almost new condition.
Anyway....
I have it hooked up to the 120 wall supply. The scope ground is on the power cord ground, the probe is on the hot lead, and the neutral is safed off.
I have never used a scope before, and the manual is a bit confusing for me. I understand the Volts/Div. knob, but I am confused by the pullout knob in the center of that V/D knob. It is marked "pull x5 mag". I guess it increases the V/D by 5 times, but it is also variable. So, for example, you can be looking at a peak to peak wave of 100 volts, turn that center knob, and increase or decrease the number of divisions. How am I supposed to use that knob exactly.?
Thank You

 

[Reloadron]

I don't know if you have it but this would be a good start to understand your new scope.

The X5 increases the vertical gain by a factor of 5. So if you are at the most sensitive range of 5 mV/Div it would take it to 1 mV/Div. See page 3 of the link. The variable gain takes the scope out of the calibrated gain modes (vertical) and is useful if you want for example to set a signal to a specific number of vertical divisions. There are applications where this is useful. Just remember it is an uncalibrated function.

Before I forget, scope ground is power ground. Make sure you never connect scope ground (vertical inputs) to mains power hot! Bad things will happen! Read the manual and get an understanding of the instrument, start with measuring low level signals.

Ron

 

[clel miller]

I do have that manual, thanks. But I was still having a hard time understanding that center knob.
I follow what you are saying, except for the variable function of that center knob. Where would you have it turned for "normal" operation.?
With the center knob pushed, and turned fully CW, my wall voltage has a peak value of about 165 volts. That can't be right, can it.?

 

[PatM]

With the center knob pushed, and turned fully CW, my wall voltage has a peak value of about 165 volts. That can't be right, can it.?

When you talk about line voltage, that is normally the RMS voltage, which is .707 of peak.
Check this link:
Measurements of AC magnitude - Basic AC Theory

 

[clel miller]

Sorry guys...I apologize. I am so use to looking at a digital meter. RMS of a 170 peak wave is 120 apprx. So I at least know I am on the right track. I appreciate your patience, I know this stuff gets asked A LOT.
Thank You

 

[Reloadron]

Sorry guys...I apologize. I am so use to looking at a digital meter. RMS of a 170 peak wave is 120 apprx. So I at least know I am on the right track. I appreciate your patience, I know this stuff gets asked A LOT.
Thank You

No problem at all. The scope as was mentioned displays a peak to peak voltage. Actually here in the US the voltage is 120 VAC RMS. The RMS voltage is .707 of E peak so 120 X 1.414 = 169.68 and E peak to peak is 2X that or 339.36 volts peak to peak.

Thus with the scope vertical set for 5 volts / div and using a 10X probe (50 V/Div) we get 339.36 / 50 = 6.78 divisions of vertical display. Remember US mains frequency is 60 Hz so 1/60= 16.66 mS. Setting the time base at 20 mS/Div will yield an easily viewable sine wave.

So why use the vertical in an uncalibrated mode? Let's say we have an AC signal (waveform) modulated AM. We want to know the % of modulation. The formula would be Emax - Emin / Emax + Emin X 100. Sometimes it becomes easier to count divisions rather than volts. I could use the variable volts/div to set my Emax exactly to let's say 6 divisions, then horizontally move the trace and measure Emin. For simplicity let's say Emin is 3 divisions. I now have 6 - 3 / 6 + 3 = .3333 x 100 = 33.3%. That being just a single example. It is also a very over simplified example. But you should get the idea. There are times when we can use divisions rather than voltage to get where we want to go.

Ron

 

[clel miller]

Ron -
I follow what you are saying, though I am not sure I will ever use a scope in that manner.
Believe me when I say, the education some of you guys have is not lost on me. I never went to school, but that does not mean I do not appreciate the time and effort it took for others to attend college. I am sure some of you guys have up wards of 6 years or more in school, and some of you probably have multiple degrees. My hat is off to all of you guys (and gals of course).
Thanks Again

 

[Reloadron]

Just enjoy your new addition. The more you use and understand a scope the more you will appreciate it. The scope is really a great tool.

They come in many flavors with a package of features. Some you will use and some you may never use.

Ron

 

[killivolt]

Your fine here. If you don't get it right now it's ok. I'm am probably the youngest yet oldest who doesn't understand everything.

Remember Mohamed. Either you can move the mountain all at once or you can move it bit by bit.

kv

Edit: Ron, as I remember I think you were the one helping understand my scope settings

 

[Reloadron]

For anyone who does not have a scope, the forum's tcmtech is currently offering what I see as one hell of a deal that can be found here.

Ron