Oscilloscope question ...AC or DC input? - Page 1
Forum section: electronics / beginners

author: Robert Smith Eco Warrior
author_rank: Regular Contributor
timestamp: May 13, 2021, 02:08:41 pm
content: Hi All,I got my first oscilloscope yesterday, a Hameg 303-6.It seems fine. I was experimenting a bit with it last night. The x1 / x10 probe that came with it was no where near calibrated so followed the instructions in the Hameg instructions. I then tested a bundle of components using the component tester. That seems to work fine. I was then getting a reading off myself. I was obviously picking up a 50Hz signal from myself, presumably I am acting as an aerial and picking up some interference from the computer, oscilloscope itself, fridge or something else nearby. It was clearly a 50Hz signal so I decided it must originate from the 240v supply somehow.I have convinced myself that it is working pretty well so want to use it for the things I bought it to do.The first thing I want to investigate is the ripple on our off grid battery bank. I have convinced myself that the grounding issue is not an issue. The battery negative is connected to the inverter output 240v ac neutral and also earth so ground on the scope should also be the battery negative / ground there too. I want to measure the ripple that is on the battery voltage of around 58v dc, so I am only wanting to investigate the few volts at the top of that 58v dc not squint at a thin squigly line near the top of the oscilloscope screen. If I select ac input instead of dc will that settle with the average input voltage mid screen on the oscilloscope? I am just a little confused by the user manual so thought I would ask.

author: radiolistener
author_rank: Super Contributor
timestamp: May 13, 2021, 02:21:11 pm
content: Quote from: Robert Smith Eco Warrior on May 13, 2021, 02:08:41 pmThis is because your body works like a plate of a capacitor. Another capacitor plate is a mains wires, computer or some metal things which is connected to the mains. Air between you and mains works like capacitor insulator. So, actually you're connected your probe to the mains or to some interference source through a large capacitor. As you know, AC current can flow through capacitor. And this is why you can see this interference.Quote from: Robert Smith Eco Warrior on May 13, 2021, 02:08:41 pmyes, it will happens in analog circuit. AC mode get input signal through a capacitor, which blocks DC offset and pass AC wave only.You can imagine AC mode like DC mode with automatic DC offset tuning in such way to get average middle of the wave at zero level.

author: Robert Smith Eco Warrior
author_rank: Regular Contributor
timestamp: May 13, 2021, 02:27:08 pm
content: Excellent. I shall give it a try.

author: fordem
author_rank: Regular Contributor
timestamp: May 13, 2021, 02:48:59 pm
content: Unlike a volt meter where the AC/DC setting is specific to the measurement, the AC/DC on a scope input is not related to the signal being measured, but to the method of "coupling" the signal to the scope input.I find thinking of the DC setting on the scope as "directly connected" makes it easier to understand, the AC coupling blocks any DC (direct current) voltage component of the signal, so that the AC (alternating current) voltage component can be viewed.

author: Robert Smith Eco Warrior
author_rank: Regular Contributor
timestamp: May 13, 2021, 02:51:47 pm
content: Thanks for the repliesThe next thing I want to try after this is to work out the fuel injection duration on my old car engine. I have a complete Volvo 940 and also another volvo 940 engine that I will be using, running on woodgas to power a three phase generator. The engine will be started on petrol until the wood gassifier is up to temperature. I want to make up a circuit that fires the original fuel injectors to start and run the engine. It is simplified by the engine running at a fixed 1200 ish rpm and injector timing is not an issue as it just squirts the fuel into the manifold ready to be slurped in at a very approximate timing.I want to connect up the oscilloscope to one of the injectors so I can work out how many milliseconds the injectors are open for. The engine does not run perfectly at one speed so the injector opening frequency will be up and down a bit and timing may be a bit irregular. Will the oscilloscope automatic or manual triggering level bring each injector pulse to the left of the screen if I get the timebase similar to the injector opening frequency? ... or maybe a multiple of it?

author: TimFox
author_rank: Super Contributor
timestamp: May 13, 2021, 03:07:55 pm
content: One trick I learned in the late '60s is how to measure the DC component (mean value) of a periodic waveform:Adjust the V/div to keep the waveform on the screen, then switch from DC coupling (no input capacitor) to AC coupling (with input coupling capacitor). The vertical shift of the waveform equals the DC component (x -1 for this order of operation).

author: rstofer
author_rank: Super Contributor
timestamp: May 13, 2021, 03:22:09 pm
content: Quote from: Robert Smith Eco Warrior on May 13, 2021, 02:51:47 pmUsually, the default trigger point will be in the middle of the screen. This allows you to view the signal pre-event and post-event. You can move the point left and right with the Horizontal Position knob. There is usually a little triangle at the top of the grid that indicates where the trigger occurs.

author: Caliaxy
author_rank: Frequent Contributor
timestamp: May 13, 2021, 04:20:59 pm
content: I believe OP’s oscilloscope is analog, so he won’t be able to see what happens before triggering.If this is the case, the pulse will be on the left side the screen, unless you move it to the middle, but the scope won’t display anything to the left of the triggering point. Make sure you trigger on the left edge of the pulse (probably the falling edge). By adjusting the time base you’ll be able to enlarge the representation of the pulse (so you can measure its duration more accurately) or have multiple pulses on screen (to measure the repetition rate).Just be careful, because you are probing an inductor and you might have high voltage spikes that can damage your oscilloscope (people use attenuators to prevent that from happening).

author: Robert Smith Eco Warrior
author_rank: Regular Contributor
timestamp: May 13, 2021, 10:10:15 pm
content: Quote from: Caliaxy on May 13, 2021, 04:20:59 pmYes it is an old analogue oscilloscope.Good point about it being an inductor. Would adding in a flyback diode help? I could maybe use the probe on its x10 setting and maybe sneak up on it using a potential divider with a very large voltage reduction and then probe that, then reduce the potential divider ratio and probe again and continue until I am sure there are no spikes? Would the oscilloscope show an inductor spike? I don't know the typical duration of a spike? If the potential divider changed the signal amplitude I am not bothered about that. I only need to know the duration.

author: Robert Smith Eco Warrior
author_rank: Regular Contributor
timestamp: May 14, 2021, 04:50:58 pm
content: Hi Folk,I have tried the oscilloscope on the dc input to my inverter. It is a sort of overcast day so the solar panels are providing charge but not a great amount.I am getting a 100Hz repeating wiggly waveform of about 1 volt peak to peak. I think the 100 Hz thing is the inverter pulling power every half cycle of its 50Hz output. The overlaid wiggly bit might be the charge controller.I am going to wait until it is dark and then have another go to rule out the charge controller effect.I will then try repeat the testing tomorrow if the sun is bright to then spot the difference to figure out the effect the charge controller has.No bangs or flashes so that is good