Oscilloscope question - Page 1
Forum section: electronics / testgear

author: b.c.henry
author_rank: Newbie
timestamp: May 27, 2015, 09:04:13 pm
content: Hello!I recently became acquainted with this site and have thoroughly enjoyed it! Now for the question...I am an automotive technician and, as some of you may or may not know, oscilloscopes are becoming quite the piece of diagnostic equipment in the automotive field. Since for the most part we are checking pretty low frequency sensors and such, they are not as powerful as many of the engineering grade scopes often discussed here. Mine happens to be an OTC 3857 and my question to you fine people is this: I am getting conflicting information regarding the voltages I can safely put through the scope. The promotional material and some of the user manual says it is safe up to +/-400V! However, elsewhere in the manual, I read that max voltage is +/-70V. I have emailed their "tech support" and barely seem to know what an oscilloscope is and I receive both answers from them as well. Input impedance is 10M Ohms (confirmed with my Fluke 88) so I tend to think 400V shouldn't be a big problem for it, but I don't know how much, if any, other factors might play into that. I can always get an attenuator but I hate to spend money if I don't gave to. The reason I ask is because I often use the ignition coil to trigger events and/or simply to get an ignition pattern and on older cars access to the secondary ignition (plug wires) was easy and measured with an inductive pickup. Today, more often than not, secondary is almost unattainable but the same information can be gained from the primary side of the coil, which can reach a few hundred volts.Any information would be greatly appreciated. If there are any questions or anything else I can/should test to gain information, let me know.CHEERS!

author: R_G_B_
author_rank: Frequent Contributor
timestamp: May 27, 2015, 09:18:57 pm
content: you could try using one of these Hantek HT201 20:1or these:New HT25 8' Secondary Ignition Capacitive Auto Pickup Probe X10000or a coil on plug probe.from ebay

author: b.c.henry
author_rank: Newbie
timestamp: May 27, 2015, 09:29:12 pm
content: I have looked into the 20:1 but just haven't pulled the trigger. Partly because if it is capable of 400V I don't really need it. I have sort of made a COP probe by welding a bolt to a small metal plate that I place on top of the coil and clamp my secondary probe to it. The problem with COP probes is the waveform often isn't very good and sometimes won't even appear (I have used/seen several different ones). I also will often trigger using primary current.

author: Muxr
author_rank: Super Contributor
timestamp: May 27, 2015, 10:02:56 pm
content: What's the resistance of your probe? The resistance of your probe and the impedance of your scope form what's called a voltage divider. From that you should be able to calculate if the numbers vibe with your manual.Here is a good article about voltage dividers:https://learn.sparkfun.com/tutorials/voltage-dividers(with a calculator on the page)w2aew talks about this principle in this video, about high voltage probes:

author: b.c.henry
author_rank: Newbie
timestamp: May 28, 2015, 05:45:04 pm
content: Probe resistance is negligible, its just RG-58 coax, 0.5 Ohms total between both cables. So, if I gather correctly, there is no voltage division going on and the scope is getting the full hit of whatever voltage I put through it? I understand that probably (maybe) everybody is different, but would they sample prior to the resistor? Is it possible that a voltage divider is already inside the scope module itself?

author: Muxr
author_rank: Super Contributor
timestamp: May 28, 2015, 08:00:15 pm
content: Quote from: b.c.henry on May 28, 2015, 05:45:04 pmThe analogue front end has some voltage attenuation most likely. The brochure says +/- 400vhttp://www.otctools.com/sites/default/files/12-64.pdfIt is hard to say if that's the actual max or they are being conservative. I would perhaps be worried about spikes in the source you're trying to measure.You can pickup a 10x probe fairly cheaply.