Oscilloscope Probes.... What the hell am I doing wrong? - Page 1
Forum section: electronics / beginners

author: ptp170
author_rank: Contributor
timestamp: September 24, 2013, 04:26:10 pm
content: Ok, been trying to figure out why my voltage readings on screen with a new set of eBay probes is so wrong. Have watched Dave's EEV Blogg on YouTube which describes the extra inline resistance in the coax which was a help but even so I'm at a bit of a loss so I decided to take some resistance measurements of certain parts of the probe.It's a x1/x10 switchable probe:From probe tip to BNC tip settles at 8.97Mohms on X10 and 250Kohms on X1.From probe tip to the point where the coaxial cable enters the small circuit trimmer board is also 8.97Mohms on X10 and 121kohms on X1.The only visible SMD resistor is on the trim board and is 130ohms....Shouldn't these values be 1Mohm and 10Mohm?If I'm simply being a idiot over this then please don't hold back!TIAChris

author: c4757p
author_rank: Super Contributor
timestamp: September 24, 2013, 04:30:08 pm
content: Quote from: ptp170 on September 24, 2013, 04:26:10 pmCareful what you wish for.....Your resistance measurements look good. 121k is a bit high IMHO, but to be honest I've never measured the resistance of a switchable probe on X1 mode. 8.97Meg is spot on, it should be 9Meg*. Are you sure it's the probe and not the scope? Stick a wire gently straight into the BNC jack and feed a voltage in that, see if it displays right.*Consider that the scope has a 1M input impedance, so probe + scope = 10M. If you apply 10V to that system, 1uA will flow, giving 1V across the 1M resistor.

author: ptp170
author_rank: Contributor
timestamp: September 24, 2013, 04:40:58 pm
content: Ok... I've fed a 10v DC feed directly into the BNC. And the scope is displaying circa 20v....

author: ptp170
author_rank: Contributor
timestamp: September 24, 2013, 04:45:55 pm
content: If I add a 1Mohm resistor to the end I get just a smidge under 10v displayed.....

author: c4757p
author_rank: Super Contributor
timestamp: September 24, 2013, 04:46:24 pm
content: Wow. That's impressively bad. Have you double-checked every single control on the scope? What kind of scope is it?

author: Andy Watson
author_rank: Super Contributor
timestamp: September 24, 2013, 04:51:20 pm
content: Quote from: ptp170 on September 24, 2013, 04:45:55 pmHave to ask, ... have you tweaked the compensation to match the probes to your scope?

author: ptp170
author_rank: Contributor
timestamp: September 24, 2013, 04:52:29 pm
content: It's an ISR-Tech ISR622 20MHz dual channel scope. V/Div set to 1V. Voltage displays 4 'boxes' high ( 20 divs high). There is also a VAR. Control per channel which alters from 1 to 2.5x.... Currently it is left turned down to '1'...

author: ptp170
author_rank: Contributor
timestamp: September 24, 2013, 04:54:22 pm
content: Quote from: Andy Watson on September 24, 2013, 04:51:20 pmYes. In x10 mode I compensated the probes using the inbuilt 1KHz 2Vp-p square wave...

author: c4757p
author_rank: Super Contributor
timestamp: September 24, 2013, 04:58:37 pm
content: Quote from: ptp170 on September 24, 2013, 04:52:29 pmEach "box" is a "div" - not the little tick marks. You're counting them wrong.

author: ptp170
author_rank: Contributor
timestamp: September 24, 2013, 05:02:14 pm
content: Quote from: c4757p on September 24, 2013, 04:58:37 pmNoobie mistake number 1.... In that case then it's displaying 4v for a 10v DC feed?

author: c4757p
author_rank: Super Contributor
timestamp: September 24, 2013, 05:04:46 pm
content: It would appear so. That's bizarre as well, but a bit more easily explained perhaps, since it's readinglow... are you sure that the VAR control is set correctly? Consider that the knob on the front could be misaligned with the actual pot and switch inside - actually rotate it.

author: grumpydoc
author_rank: Super Contributor
timestamp: September 24, 2013, 05:10:06 pm
content: Quote from: ptp170 on September 24, 2013, 04:52:29 pmOK, there's one thing - "1 Div" on the 'scope is always 1cm so at 1V/div with a 10V input really the trace should be off screen - i.e it's still wrong but the other way.Are you sure the VAR control is set to the CAL mark, since it appears to be under-reading by exactly 2.5x which is the maximum value for the VAR control.

author: ptp170
author_rank: Contributor
timestamp: September 24, 2013, 05:21:51 pm
content: Partial declaration of stupidityI've had the scope placed on the bench at such an angle that the 'CAL' markings aren't visibleThis coupled with the misinterpretation of actual 'divides' has led me on a self fuelled mission of idiocySo.... it appears that the VAR. controls attenuate by >x2.5 and not the other way around??

author: grumpydoc
author_rank: Super Contributor
timestamp: September 24, 2013, 05:24:27 pm
content: QuoteYes, the vertical VAR controls always attenuate the signal, never amplify it.

author: ptp170
author_rank: Contributor
timestamp: September 24, 2013, 05:27:12 pm
content: Quote from: grumpydoc on September 24, 2013, 05:24:27 pmNoobie mistake number 2 then....I'll just pop in to a field and shoot myselfAnd nobody called me an idiot.... most disappointingThanks for pointing the Ape towards the stone tools

author: Fraser
author_rank: Super Contributor
timestamp: September 27, 2013, 08:46:13 am
content: Everyone has to learn by their odd mistake or twoAt least you knew something was wrong ! The scary people are those who use test equipment without questioning results, considering the equipment infallible. As in this case, always have an idea on what you expect the test equipment to indicate in terms of measured values before trusting such.You have done well to work through this problem using a known input voltage. Some newbies might have just dived into their project and become totally confused by strange readings.