Earth ground resistance through AC/DC adapter - Page 1
Forum section: electronics / beginners

author: ElectricPower
author_rank: Frequent Contributor
timestamp: March 05, 2021, 04:53:58 am
content: I'm totally newbie when it comes to using a Oscilloscope, but i have read that it's safe to probe an let's say Breadboard circuit powered by an 5V AC/DC adapter with an 2-pin AC connector (without ground pin). Like the adaptors that came with laptops etc.I tried to measure the resistance with multimeter between ground on the DC output on the 5V AC/DC adapter and the ground on my Oscilloscope and ground in my AC main power outlet. The multimeter show that the resistance is around 1M ohm. (mega ohm).Why do i read that there is an connection there when the adapter is not earth grounded at all?

author: BrokenYugo
author_rank: Super Contributor
timestamp: March 05, 2021, 05:39:49 am
content: Hard to understand what exactly you measured or were trying to measure, what was connected to what while taking these measurements? There's generally one megaohm inside every scope between input and ground, you can measure this if the coupling is set to DC. I've also seen a high value (only passes a very low, safe current) resistor in 2 pin isolated power supplies that bridges the mains and isolated side a couple times, but those were several megs.

author: ElectricPower
author_rank: Frequent Contributor
timestamp: March 05, 2021, 05:57:28 am
content: Quote from: BrokenYugo on March 05, 2021, 05:39:49 amNothing was connected. The 5V AC/DC adapter has an 2-pin AC plug that was connected to the wall socket (AC). But nothing was connected to the DC end of that adapter when i measured. One probe on the DC connector ground on the AC/DC adapter and the other probe to oscilloscope ground. The oscilloscope was turned on when measured.The multimeter showed me around 1 mega ohm resistance.

author: Kibabalu
author_rank: Regular Contributor
timestamp: March 05, 2021, 06:21:00 am
content: In general you shouldn't measure resisistance or conductance inside an active circuit under voltage!What do you measure with the ac plug not plugged into the wall socket? Is this also not "OL"?

author: ElectricPower
author_rank: Frequent Contributor
timestamp: March 05, 2021, 06:23:02 am
content: Quote from: Kibabalu on March 05, 2021, 06:21:00 amI have not tried that. I will try it later to day and give feedback

author: salbayeng
author_rank: Frequent Contributor
timestamp: March 05, 2021, 07:47:09 am
content: If you touch the meter terminals with your fingers, you can get around 1Megohm. ??

author: BrokenYugo
author_rank: Super Contributor
timestamp: March 05, 2021, 04:18:28 pm
content: Since the meter measures resistance by injecting a current limited voltage and seeing what makes it to the other probe, you can't measure energized circuitry and get any sort of sensible resistance reading.

author: ElectricPower
author_rank: Frequent Contributor
timestamp: March 06, 2021, 02:53:34 am
content: I have measured again now (without touching with my fingers) and with my 2-pin not grounded 5V AC/DC supply connected to AC outlet i get 118k ohm resistance when i measued betveen ground on the DC jack and ground on schannel 1 and Channel 2 on scope whith the scope turned off but plugged in.When i unconnect the AC outlet on the AC/DC adapter and do the same measurement, i got "OL" on the scope.I just can't understand why there is an connection to the earth ground at all with the adapter plugged in since the adapter just have an 2-pin connector.

author: Kibabalu
author_rank: Regular Contributor
timestamp: March 06, 2021, 07:26:04 am
content: What does your multimeter show with plugged AC/DC adapter and unplugged scope? I bet, you get still your 118K. Isn't it?Due to the resistance measurement approach, you cannot measure inside setups that are powered up!

author: ElectricPower
author_rank: Frequent Contributor
timestamp: March 06, 2021, 01:41:56 pm
content: Quote from: Kibabalu on March 06, 2021, 07:26:04 amWith AC/DC adapter connected and osilocope not plugged in i get OL on the scope. With osilloscope plugged inn (but not powered up) i get 118k ohm resistance.

author: Alti
author_rank: Frequent Contributor
timestamp: March 06, 2021, 02:16:46 pm
content: This is a class II power supply so it does not have direct electrical connection in between DC side and AC side. However, this is a switching supply and there is a capacitive coupling in between primary and secondary windings of the transformer. So they must have included Y2 capacitor in between primary(DC) and secondary(mains). This capacitor conducts current at 50Hz (<mA range) and it is going to fool your DMM on ohms range. Take an AC current meter and measure the current.As for safety - yes, it is safe to ground class II power supply via scope's probes. But do not try that with class I !! Also, this power supply does not look like a laboratory grade equipment and it might give some unexpected results like excessive ripple, overshoot, serious short circuit currents etc.

author: ElectricPower
author_rank: Frequent Contributor
timestamp: March 06, 2021, 02:24:59 pm
content: Quote from: Alti on March 06, 2021, 02:16:46 pmThanksSo it's generally safe to probe circuits that i'm powering with that adapter?No, this is probably not a laboratory grade adapter. I baugt it from Aliexpress.I have just testet this adapter to give it 5V and 8 amp load and it's get hot, but survived one hour with that load. It's good enough for me anyways

author: Alti
author_rank: Frequent Contributor
timestamp: March 06, 2021, 03:41:23 pm
content: Quote from: ElectricPower on March 06, 2021, 02:24:59 pmAre you concerned about your personal safety or the scope's?Laboratory equipment is designed to survive abuse, backfeeding, overloads, etc. This does not necessarily apply to Aliexpress $4 firestarters. Cannot you afford a DC supply from reputable manufacturer? Get some used HP or IBM notebook supply if you need to experiment with this kind of gear on a wooden bench..

author: Simon
author_rank: Unknown
timestamp: March 07, 2021, 09:32:48 am
content: The only sensible measurement you can take with an ohmmeter is between the outputs and the mains plug when not powered. The ground of your scope is the scopes metal chassis and therefor for safety reasons is directly connected to mains earth so that if the ground of what you are probing is connected to any other voltage that is not actual earth it will hold the chassis at earth potential rather than allow the floating voltage to energise the metal chassis of the scope that you can touch with more than 50 V. If it is simply a floating voltage it will accept being pinned to earth (providing the isolation it is connected through can withstand it which it should). If it is not it will cause some sort or overload and hopefully trigger something to cut out rather than you touch a scope chassis that is now no longer at 0 V relative to your body.

author: Simon
author_rank: Unknown
timestamp: March 07, 2021, 04:05:01 pm
content: everything is mains earth referenced? Computer power supplies like scopes have their metal chassis earthed for the same reasons as the scope.