0
\$\begingroup\$

I have AC/DC 3/6/12V out universal adapter with two pins (without earth connector) and would like to use it for powering my arduino project and playing with oscilloscope. Knowing that my oscilloscope will make big bang if power supply is connected to main earth ground, I would like to test somehow if some of the AC 220V wires are connected to main earth ground. I know that my AC/DC is safe because of plug type but how to know that somewhere in the system main earth ground is not connected to phase or neutral.

\$\endgroup\$
1
\$\begingroup\$

The ground of your oscilloscope will be connected to mains earth assuming you're using a socket that has an eart connection. For safety, it should have this !

If the mains adapter you're using to feed your arduino board has a main earth connection then the same will aply tho this adapter.

Most mains adapter however do not have a mains earth connection and these will be floating with respect to earth.

For both adapter cases it is safe to connect the arduino's ground to the scope's ground.

To safely measure mains voltage on a scope you need to use an isolated probe or measure through a transformer for isolation.

Some (battery operated) portable scopes have an isolated ground and this in combination with appropriate high voltage rated inputs is enough to measure mains directly. But most bench scopes are unsuited for this.

I also highly recommend this video by Dave from the EEVBLOG !

\$\endgroup\$
  • \$\begingroup\$ +1 The Tek TPS2024 is great for this- mains or battery power and the four channels are individually isolated (good for hundreds of volts) so you can look at a 3-phase power line etc. \$\endgroup\$ – Spehro Pefhany Jan 15 '16 at 21:52
  • \$\begingroup\$ Looking at that Tek TPS2024 I instantly notice how it's BNC plugs look different (black instead of metal) because they're shielded and isolated. Of course you have to use them with isolated probes so you cannot touch the shielding. \$\endgroup\$ – Bimpelrekkie Jan 16 '16 at 11:55
  • \$\begingroup\$ Good observation - they're plastic and probably a bit fragile (Tek includes a spare set- I've not broken one yet). The legal warning stickers say not to use the included probes (which are properly insulated but not CATxx rated) beyond 30V RMS and the firmware forces you to agree every time it powers up! \$\endgroup\$ – Spehro Pefhany Jan 16 '16 at 16:56
0
\$\begingroup\$

If you use your AC/DC adapter to power your arduino board, and use the oscilloscope just to look at signals on the board then there will be no issue with your oscilloscope blowing up. The 3V/6V/12V power output from the adapter is not connected to ground in any way, it is isolated or "floating". The problem can arise if you use your oscilloscope to connect to the 220V AC voltage. Then you can expect a big bang. There are ways around this too, but it's safer not to do it!

\$\endgroup\$
0
\$\begingroup\$

schematic

simulate this circuit – Schematic created using CircuitLab

Figure 1. Isolated power supply. Earthed oscilloscope.

This simplified circuit shows that the low-voltage DC supply is electrically isolated from the mains. You can safely connect either V+ or V- (but not both together) to the oscilloscope ground.

\$\endgroup\$
  • \$\begingroup\$ This isn't true, the scope has dual power supply and the center tap is connected to the earth. \$\endgroup\$ – Marko Buršič Jan 16 '16 at 13:12
  • \$\begingroup\$ Thanks, @Marko. I don't see any reference to the scope power supply or make or model in the OP's question. I read his question as to whether the DC outputs of his PSU were referenced to the mains L or N. What do you think? \$\endgroup\$ – Transistor Jan 16 '16 at 13:22
0
\$\begingroup\$

My old analog scope has a transformer to supply CRT and all analog circuits, I know beacause I got a schematics with it. So, whenever I need to measure like you, I do connect a mains plug that has the earth pin removed, now I get a floating case and scope's ground. It shall be noted that I use gloves and watch the other people if they move around the scope, since it can be lethaly dangerous, also it is advisable to know or measure a priori, the signal that will be connected to chasis with probe's croco clips, like ground, neutral, earth, not a live wire or +DC bus.

\$\endgroup\$
  • \$\begingroup\$ Mad and bad. Not such a problem with handheld scopes such as Fluke's Scopemeters which have fully isolated cases and test lead connnectors. \$\endgroup\$ – Transistor Jan 16 '16 at 11:40
  • \$\begingroup\$ I don't have a handheld Fluke scope, if you give me one... \$\endgroup\$ – Marko Buršič Jan 16 '16 at 13:01
  • \$\begingroup\$ @transistor Also if you look at EEVblog proposal: link from FakeMoustache answer, my solution is exactly the same and I was not the one who invented it, it was proposed to me by others, so don't be such critic, you don't have the knowledge of the entire science. \$\endgroup\$ – Marko Buršič Jan 16 '16 at 13:08
  • \$\begingroup\$ Steady-on, Marko! I'm well aware of the problem, the 'floating scope' solution and the hazards involved, hence my comment. :^) \$\endgroup\$ – Transistor Jan 16 '16 at 13:18
  • \$\begingroup\$ I had a look at the EEVblog video, thanks. At 7:40 he shows isolation as I have sketched it in my answer. At 23:00 he discusses isolation transformers for both circuit under test and oscilloscope and advises against floating the scope for safety reasons. However, I understand that people do it. \$\endgroup\$ – Transistor Jan 16 '16 at 15:30

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.