PSU issues with LPC17, USB, Macbook ...

[rvlt]

Hey,

 

today I accidentally destroyed two MCP3208 while working on my midi-keyboard. At first I thought some electrostatic voltage was the reason for this, but then I investigated further and realized that I had a strange +28V voltage measured between GND and the metal case of my controller.

 

Ok, here is the setting:

 

 

- I have a keyboard controller case with a metal top. It is powered by an internal PSU which gives me a clean +5V for the LPC17. On the mains connector the two AC lines and the GND are connnected to the PSU, and the GND is also connected to the metal case.

 

-  the PSU output is GND and +5V and is connected to J2 of the LPC17. The USB power option jumper and J1 are not even stuffed on the board. As mentioned on ucapps I made a bridge between the outer pins of the missing 7805. The LPC17 and all connected boards, pots, displays etc. run just fine.

 

- When I connect the LPC17 to a (battery powered) macbook via USB everything is still running fine.

 

- But as soon as I connect my apple power supply to my macbook, I measure +28V between GND on the LPC17 (or Ainser64) and the metal keyboard case. I just realized that because a GND wire coming from the AinSer64 was touching the case and destroyed the MCP3208 on the AinSer board, two times ...

When I deconnect the macbook's PSU the voltage is gone.

 

-> Where is my error? Has anyone an explanation for this? Is the keyboard PSU faulty, or the apple PSU, or my macbook?

 

The solution for me is pretty simple atm, because I don't need the USB connector on that keyboard, so I could simply desolder it, although it is handy sometimes to quickly connect it to a computer.

But If anyone could shed some light into this, please let me know.

 

 

 

[TK.]

:shocked:

 

When you are googling for "switched psu ground earth", you will find many reports about similar issues.

I assume (but I'm not an expert!) that the earth connection to the metal case is problematic.

 

I would use a PSU with integrated mains connector ("protection class II", see also this wikipedia article) so that the metal case can be earthed with the ground output of the PSU.

 

Best Regards, Thorsten.

[novski]

Is your Schematic correct? I see the same symbol after the PSU as before.

To Test you can measure GND (after PSU) to Earth. How much Ohms do you measure between those 2 Pins? 

[rvlt]

Thank you guys,

 

novski, there is no connection between these two points, they are isolated.

 

I tried a different approach today: I disconnected the switching PSU and instead used a simple transformer in conjuntion with the LPC17 onboard PSU (I put in a 7805...). But the result was the same: As soon as I connect my macbook PSU, there is a 28V difference between "LPC17-Ground" and "Earth/Chassis Ground".

 

I guess most people don't run into this problem because they either use USB power or external wallwarts.

To make this clear again: As long as these two grounds are isolated, everything is working fine. Maybe this is normal? Don't know, but it still sounds weird to me...

 

Anyway I won't use the direct USB connection from now on.

[TK.]

Beware: without the USB connection you won't get debugging messages in MIOS Terminal, it could be very difficult troubleshoot a non-working configuration.

 

This issue could be related to the electrical installations in your flat/house. E.g. a problem with earth.

Here for example some postings from somebody who has a similar issue: http://www.eevblog.com/forum/beginners/connecting-system-ground-to-mains-earth/

 

And just to confirm: I tested this with my own Mac PSU today and haven't measured a voltage between earth and USB ground of my MBP.

 

I would still go for a PSU with integrated mains connector - such a PSU can be built into your metal case without the need for connecting the metal of the case to ground.

 

Btw.: you will face the same issue once you are connecting a MIDI cable from your Mac to the core module, because the cable shield is connected to ground (at both sides) as well.

 

Best Regards, Thorsten.

[rvlt]

Hey,

 

I did a few more tests:

 

1. I used a different macbook with a different PSU: same problem

2. I used an extension chord from another room (different fuse): same problem

3. I used our studio mac (mac pro desktop): all fine !

 

hmm … this made me wonder if this whole issue was related to grounding my macbook. So I tried the extension chord from my apple PSU, the one with three prongs (AC and Ground), and voila: When I use this I don't have any more problems.

 

Seems that this is a common problem:

 

http://www.kellerbude.de/threads/1728-MacBook-Pro-Gehäuse-steht-unter-Strom-Spannung

 

TK, could you tell me which cable you used when you tested this? The three-pin (schuko) cable or the small two pin adapter?

 

I really wonder how manufactures do it with devices which have a grounded case and a USB connector.

 

Anyway, I think I should start looking for a 5V PSU Class II like you suggested. 

 

====

 

regarding the midi connection: I also thought about this, but when I tried it I didn't have that issue. On (I guess) most devices the midi socket's outer ring is connected to ground, but the midi cables did not have the ring connected on both sides. I tried that with three different (non-selfmade) midi cables: they all had the 5 pins connected, but the outer ring was unconnected. The "real ground" on midi cables (middle pin 2) is normally connect  to ground on midi out sockets, but "should not" be connected on midi in sockets. That's why I didn't have that issue with midi, only with a USB connection.