Using signal gen or PSU when scoping/probing - Page 1
Forum section: electronics / beginners

author: RiRaRi
author_rank: Regular Contributor
timestamp: September 10, 2023, 04:36:56 pm
content: I am very new to frequency and using scope. As i have learn, circuit has to be powered when using scope for checking component and general probing/trying to find problem. So, one way to power the circuit is by pluging it in wall. If that can not be done, next thing could be low noise PSU. But also we have signal generator which is powering the circuit with precise wave. So what to use to power the circuit when probing? I would really be thankful if someone could explain this to me and also for others who are learning how to use scope.

author: tautech
author_rank: Super Contributor
timestamp: September 11, 2023, 01:20:46 am
content: Typically the primary precaution is to avoid mains ground loops.While now old Daves video will point you in the right direction:

author: RiRaRi
author_rank: Regular Contributor
timestamp: September 11, 2023, 02:54:36 pm
content: okay but my question is: when to use signal generator to power the circuit when probing with a scope and when to use low noise PSU to power the circuit when probing and tracing with the scope?

author: u666sa
author_rank: Frequent Contributor
timestamp: September 11, 2023, 05:42:14 pm
content: PSU definitely, so you can see amps used. It should always be possible. But if not possible, yes, the original wall charger. Then you smoke datasheets and check with either oscilloscope or multimeter for conditions to be present. Good channel for practical repair is this --https://www.youtube.com/@NorthridgeFixNotice he almost never uses oscilloscope.Which is fine. Until you get to new gen intel chips where they have conditions for power mosfets to start. With his methods you wouldn't be able to diagnose new asus notebooks. So in comes this channel --I know it's a simple micro replace with some programming, but you first have to make sure because it's not cheap.

author: BrokenYugo
author_rank: Super Contributor
timestamp: September 11, 2023, 07:21:01 pm
content: A signal generator is generally a signal source, not a power source.If you're working on something where the power supply is noisy enough you can't get good measurements, that's probably the cause of whatever the fault is.This is one of those hard to answer questions without a real problem at hand. Additional test gear used all depends on what you're working on and what you're trying to see with the scope.

author: MathWizard
author_rank: Super Contributor
timestamp: September 11, 2023, 07:38:37 pm
content: If you have the money, a bench top adjustable PSU is the way to go. I have a Siglent PSU, with 3 output channels. So far it's usually been enough for my needs, but I'll get a 2nd 1 some day.Before that I used various AC-DC SMPS wall adapters, like 12V cablebox adapters. All modern ones (in my country anyways) are isolated from the 120VAC by a transformer/coupled inductor. So you can attach your mains-Earth ground connections from your scope and signal generator to anywhere on the 12V circuit. However if you have more than 1 M.E. GND attached, you always put them all on same place in the circuit, because anything connected to those GND clips, will be shorted together.For a simple adjustable PSU, I used a 12V PSU, and a circuit with a common LM317, which is a 3-terminal adjustable voltage regulator, like a 7805/7812, except adjustable. I never even had the LM317 on a heatsink, so it was just for small circuits on a breadboard.I you want to build your own linear PSU, from old junk, old stereo's are a good place to find transformers. I've rescued a few nice transformers from stereo's people were throwing out.

author: armandine2
author_rank: Frequent Contributor
timestamp: September 11, 2023, 07:39:52 pm
content: Quote from: RiRaRi on September 11, 2023, 02:54:36 pmMy guess (FWIW) is you are not going to get these "top down" rules in a  electronics hobby forum- the best we can hope for is genuine test set ups

author: MathWizard
author_rank: Super Contributor
timestamp: September 11, 2023, 07:49:18 pm
content: Some signal generators have a DC mode, and can power small circuits. Some circuits don't need DC, so if you wanted to make an AC voltage divider with a cap and resistor, than you would use a signal gen to provide an input signal.Some passive circuits, so just RLC components, still might want DC biasing, as well as an AC signal. Most modern signal generators can have a DC offset, but you may also want a different DC voltage applied to another point, besides the AC input. So you'd want a PSU and signal gen.For basic stuff, at lower frequencies, those $20 ebay signal gen's at worth it.

author: TimFox
author_rank: Super Contributor
timestamp: September 11, 2023, 08:51:30 pm
content: Normally, a simple circuit has one set of connections for DC power (e.g., +5V and common) and another for signal input (e.g., IN and Low).In such a circuit, the voltage applied to the signal input is quite a bit lower than the DC voltage applied to the rails.If your circuit normally does not use DC power (such as a passive R-L-C filter circuit without semiconductors), then applying the signal generator to the input and connecting the output to a 'scope suffices.