Using an oscilloscope in a vehicle - Page 1
Forum section: electronics / beginners

author: jsi
author_rank: Regular Contributor
timestamp: June 07, 2016, 11:07:37 pm
content: So thanks to input from the fine folks on this board, I settled on a Rigol 1054z for my first scope. What a nice tool. Now I get to climb the various learning curves to really be able to use it fully. The curve I'm on right now involves my day job working for a school district. My team is responsible for all of the technology on the school buses, 2 way radios, GPS tracking systems, and video recording. We have 185 buses and on a fairly regular basis something will randomly fail. I'm all for blaming spikes, because we all know how dirty automotive power is. But I'd like to prove it and if possible prevent the damage before it happens.So with a fair bit of excitement I brought my 1054z to work to prove spikes are killing the equipment, but I didn't get much that looked too bad, I think. Putting the probe on the main feed from the positive terminal didn't show much at all regardless of starting the engine, turning on fans, etc. The only way I could get a trace that was interesting was to probe the switched power terminal and have all the blower fans running at the same time and then turn the bus off. I've attached screen captures for your viewing pleasure. (the two screen captures are of the same trace, just a different time scale.)What's the best way to track down these spikes, if they exist at all?

author: tautech
author_rank: Super Contributor
timestamp: June 07, 2016, 11:46:23 pm
content: You've got a tool that will show you what you want if you use (set) it correctly to do so.I doubt very much that momentary dips in the vehicles supply will cause any damage to equipment, spikes are what you need to focus on. Each piece of equipment will have a PSU whose bulk capacitance should provide reasonably stable supply over momentary dips.How to find what you seek:In a word: Capture.Required settings:Normal triggering set above typical system operating voltage.Slow timebase.SINGLE shot.Then examine captured results with faster time base and lower V/div.Addition of other channel inputs may identify where spikes originate from. eg Aircon clutch winding back EMF when turning off.EditOld electromechanical regulators can be noisy too, however you don't say if these are fitted to these vehicles. Auto sparkies can help with GP suppressors to quell most of the ordinary vehicle electrical system muck.

author: uncle_bob
author_rank: Unknown
timestamp: June 08, 2016, 12:47:33 am
content: HiEach gizmo on the platform (bus / car / truck / golf cart) has at least a B+ and a ground. Most of them have wires off to other gizmos. The most likely problem is a loose / poor / missing (yikes!) connection on one gizmo or the other. Voltages rise or drop when they are not supposed to. Current flows through places not happy to do so. The tracing process involves a *lot* of checks in a *lot* of locations.One classic example:We put 27 add-on boxes into the front left side of the (whatever). Since we could only find two ground lugs, we stacked up all 27 grounds on the one or the other lug. There's maybe 10A into each gizmo when it tries to do it's thing. Hmmmm ..... 10A x 27 items = 270A. Each lug *might* be ok for 20A ....Any bets on what the ground voltage on those gizmos looks like? Any guesses what the ground differential between the gizmos looks like?Bob

author: Brumby
author_rank: Unknown
timestamp: June 08, 2016, 01:53:54 am
content: Adding to Bob's fun example, is the myriad of places you can end up with bad/broken connections.I had one really painful exercise trying to track down a particular problem under the hood of a car. The problem was found through sheer luck. It was getting late and the light I was using fell, causing the engine bay to fall into near darkness - which is when I noticed a spark out of the corner of my left eye. It turned out that the ground strap connecting the block to the firewall had broken and wasjustmaking contactsomeof the time.Automotive chassis are the unfriendliest electrical environment for 12V (or even 24V) I've ever encountered.

author: uncle_bob
author_rank: Unknown
timestamp: June 08, 2016, 11:33:07 pm
content: Quote from: Brumby on June 08, 2016, 01:53:54 amHiIf you look at the way the manufacturer does it, they have *very* specific grounding and supply bus practices in place. They do have the luxury of being able to put a lug into the chassis where they need it. Judging from the number of places they ground stuff off .... there is a lot of work that goes into picking those locations. (Yes, I've spent some time doing EMI surveys ...).Bob