Scope shows half the expected voltage?? ....user error? - Page 1
Forum section: electronics / beginners

author: Bzonic
author_rank: Contributor
timestamp: February 10, 2015, 12:34:11 pm
content: Hello all, First time poster hoping for a little guidance regarding an oscilloscope reading. I’m fairly certain the root of my issue is user error lol but googling has not uncovered an answer. I’m trying to do a simple voltage validation on a oscilloscope (actually I have two scopes and I’m getting the same result for both). I’ve got a 6v DC lantern battery hooked up to the probe and the reading on the scope only shows a movement of 3 minor divisions up from the ground baseline. I’ve set the ground baseline using the ground coupling and then I switch to DC coupling and get the jump up of 3 minor divisions. Shouldn’t it jump up 6 minor divisions?I’m left to conclude that you must double the reading? I did verify that the battery reads a full 6v on a digital multimeter before I did all this. Thanks for some help!

author: GreyWoolfe
author_rank: Unknown
timestamp: February 10, 2015, 12:51:03 pm
content: Have you verified that the scopes are set to 1 volt per division on the time base? Are you set to 1X or 10X on your probes? Are you sure that the battery is putting out 6V-as in checked with a DMM?

author: Bzonic
author_rank: Contributor
timestamp: February 10, 2015, 12:54:33 pm
content: I used a DMM to verify that the battery is a full 6v. The volts per division are at 1v/div. The probe does not have a 1x /10x switch on it. It is an Iwatsu probe with just a little screw adjustment on the base.

author: GreyWoolfe
author_rank: Unknown
timestamp: February 10, 2015, 12:58:33 pm
content: If there is no switch then they are probably 10X probes which would attenuate the signal 10X not half. I will have to rig up batteries to get 6V and see if I can duplicate that on my scopes.

author: mij59
author_rank: Frequent Contributor
timestamp: February 10, 2015, 01:06:42 pm
content: What scope are you using ?Channel attenuator (VAR knob) set to calibrated ?Same for other channels ?

author: Bzonic
author_rank: Contributor
timestamp: February 10, 2015, 01:09:06 pm
content: What really confuses me is that I measured the 6v on both my Tek 465 and an Iwatsu 5702 with nearly identical results. I'm assuming it has to do with the probes or me not interpreting the results correctly.

author: mij59
author_rank: Frequent Contributor
timestamp: February 10, 2015, 01:13:35 pm
content: Must be the probe, or the way you are measuring the voltage.What probe do you use ?

author: Bzonic
author_rank: Contributor
timestamp: February 10, 2015, 01:20:20 pm
content: I used the Iwatsu probe first then a Tek probe after... with the same result. I'm attaching the main probe lead to the positive battery terminal and the ground clip to the negative battery terminal.

author: KedasProbe
author_rank: Frequent Contributor
timestamp: February 10, 2015, 01:22:39 pm
content: Just use a cable connection not a probe.Put your DMM also on it and then make a picture for us.

author: mij59
author_rank: Frequent Contributor
timestamp: February 10, 2015, 01:28:04 pm
content: Or hook the dmm up to the probe at the same time.

author: Bzonic
author_rank: Contributor
timestamp: February 10, 2015, 01:29:52 pm
content: Ok will do. Thanks for the feedback all. I will get some pics later today when I'm back home and go from there.

author: electr_peter
author_rank: Unknown
timestamp: February 10, 2015, 01:32:09 pm
content: Maybe Bzonic is using 50 ohm probe cable on 50 Ohm termination -> 1:2 attenuation

author: mij59
author_rank: Frequent Contributor
timestamp: February 10, 2015, 01:43:13 pm
content: Quote from: electr_peter on February 10, 2015, 01:32:09 pmGood one !Only this must be an external terminator, both scopes don't have internal 50 Ohm termination.

author: Bzonic
author_rank: Contributor
timestamp: February 10, 2015, 01:54:50 pm
content: Interesting - although it went over my head a little bit. If I were to exclude probes from the equation by using just a cable should I expect a full 6v reading? Maybe take something like speaker wire and place it on the battery leads and the correct scope input areas?

author: electr_peter
author_rank: Unknown
timestamp: February 10, 2015, 02:28:26 pm
content: Bzonic, do you have a DMM? Measure resistance of scope input connector and resistance of a cable you are probing with just to be sure.

author: Bzonic
author_rank: Contributor
timestamp: February 10, 2015, 02:51:22 pm
content: electr_peter - I do and I will check that once I'm home. Thanks!

author: Bzonic
author_rank: Contributor
timestamp: February 10, 2015, 07:39:22 pm
content: After letting all this roll around in my noggin for a few hours I think I’m just reading the scope incorrectly. I’m almost certain that I’m using 10x probes so keeping in mind that I’ve got it set to 1 volt per division and there are 5 minor divisions within a major division I’m left with each minor division really representing 2v. So what I originally perceived as half the voltage (3 volts) is really the 6v I’m after once I calculate the 2v per minor division that it is. Does that sound like I’m understanding the 10x attenuation correctly given my 1v per division setup? Thanks for any confirmations or corrections to my logic!

author: vk6zgo
author_rank: Super Contributor
timestamp: February 11, 2015, 01:01:18 am
content: Quote from: Bzonic on February 10, 2015, 07:39:22 pmYep! You've got it!!

author: Bzonic
author_rank: Contributor
timestamp: February 11, 2015, 11:57:36 am
content: :)Thanks to everyone who provided comments by the way. Hopefully this noob misunderstanding will help future noobs when they start dealing with 10x attenuation and how it must be handled in conjunction with the selected volts per division.