STM32F0 Breakout Board Weird Bug

[bdohler]

Hi All,

First post, new to the forum. Hope I'm in the right place.

Recently did a STM32F0 breakout board. Curious thing is, touching a scope probe ground to ground on the board can cause the PS I'm using to go into current limit. I have no idea what this could be doing, other than adding an improper current return path, or acting as an antennae and injecting noise into ground. The only other real IC on the board is a UART/CAN transceiver. The micro has no bootloader.

Thoughts?

Thanks!

[Jeroen3]

You problably have earth leakage. Make sure ground is ground and not 80 Vac-ish.
Spakring ground to ground with a large potential difference can cause latchup and other funny business.

http://www.marcspages.co.uk/pq/3333.htm

[wraper]

This is weird and likely not in the breakout board itself. Sounds almost as if your power supply have positive output grounded. Is the scope properly grounded in the mains socket? Isn't something else connected to another channel? You should describe your setup with more details.

Spakring ground to ground with a large potential difference can cause latchup and other funny business.

Double insulated psu (and 95+% of small PSUs are double insulated) shouldn't cause the issue. But of course should be aware of this while connecting something while device is powered to not damage something in the process.

[bdohler]

Interesting guys, thanks.

For my setup, I have a power supply on my desk. The breakout board is connected to line and neutral (not ground). Both the powersupply and Agilent scope are connected to the same power bar on my desk, and I have a single passive scope probe. The PS is a BK Precision 1758B, which does appear to have line, neutral and ground on the

Whenever I touch the probe "ground" to the breakout board, it'll go into current limit at 0.5 amps. I should point out that removing the probe can also cause it. Also, if it's already in 0.5 amp current limit, I've never seen it recover by doing anything, including removing the probe. It's also inconsistent. Scraping the probe ground against the mounting hole seems to be worse for it. But, limiting the current to 0.1 amp causes the issue to disappear. This does sound like latch-up, as apparently there is a minimum current threshold over which latchup is self sustaining.

How can I test if my power supply is leaking from positive to ground? Or would it be leaking from line to ground on the mains side? I don't see why the scope and power supply wouldn't have their grounds together? Prehaps I will use some spade terminals to tie them together to ensure that they are properly grounded. I have definitely observed the issue with a handheld DMM.

Regardless, I've already learned things and I much appreciate the help.

A colleague has also observed the bug with a different set up.

[Jeroen3]

Can you try to measure some things?

Measure AC volts between the grounds.
Measure AC milli/micro amps between the grounds.
Measure AC volts between supply ground and mains earth.
Measure AC milli/micro amps between supply ground ans mains earth.

[wraper]

Hmm, does the IC(s) on the board start to heat up? Probably this is some latch-up issue on your board. Probably ground on the board is not solid enough.

[bdohler]

Thanks so much guys.

It looks like it is earth leakage. The (-) out on my PS to GND out sees about 53 VAC. Therefore, shorting (-) to ground (through the probe, through the scope, through the power bar, through the PS, causes (-) to temporarily sit at 48 or so volts above my (+). Therefore, looks like latch-up. As soon as I tie (-) to GND BEFORE firing up the PS, I'm fine. This seems like a major fault of the PS? Or is that pretty standard for some reason?

The weird thing is that the (-) on my floating DMM can do it as well. But I guess it's got enough potential inertial to move the (-) just enough to throw it into latch-up? I would love confirmation on this.

The IC's on the board do get hot. I have two N-Channel FETS as polarity protection that got super hot. Other things seem to heat up as well, but not really the micro. To be honest the ground plane on the back of the board gets hottest.

I learned something new today. A real trap for young players.

Thanks again

[wraper]

Measure the leakage current, should be small. It's not a fault. Most likely it means breakout board is not well designed or PSU outputs a voltage spike when you do this. Could also be lack of capacitors on the board. Try attaching some electrolytic capacitor, something like 100-470uf across the power. If there are some voltage regulator on the board, measure their output voltage after latch-up occurs.

[bdohler]

There is a +5V rail from the power supply with approx. 110uF on it, as well as a +3V3 with the same. The +3V3 stays up during latch-up. I'm reasonably sure that the board is well designed, as it was done by more experienced folks than myself.

I'm much more inclined to blame the power supply. Leakage current from where? How exactly should I go about measuring it?

[Jeroen3]

Leakage current from the mains input filter. Should be less then 0.1mA.
Every device has this, except when they are properly grounded.