Prop Plug trips GFI

Gregg Harrington

Hello everyone,

I have had a bit of a problem I am having a difficult time understanding. I have designed a board with a Propeller, EEPROM, XBee with an AC power supply with an on board AC/DC power converter. When I power the board from my 3.3v test point, the board programs via a prop plug correctly and works everything works as expected. However, when I power the board in via AC, it also works correctly (the Prop code runs correctly), EXCEPT that when I plugin in the prop plug GFI I am plugging my board into trips.

I am a little confused as to why this might be. Do I need to isolate my AC and DC sides better? Is there a diode I need somewhere to limit the connection to GND?

Power Schematic

Programming Header Schematic

Any help or suggests on how to address this problem is greatly appreciated.

Cheers,

Gregg

Sapphire

Your AC/DC supply is not isolated. When you connect the programming header, the board ground is connected to your computer ground, which is a path for current from the AC mains to earth. If you measure the potential from your board ground to your programming connector (the one attached to your computer) while it is disconnected, you will see voltage. Possibly high voltage. This is a dangerous situation as your board ground could be floating at 50+ volts relative to earth. Use an isolation transformer on the AC input or a galvanic isolated AC/DC converter.

localroger

I see this often in industry with serial ports. Both your AC/DC supply and your computer are non-isolated. You can check this by unplugging and checking the resistance between logic ground and the power neutral (wide spade) prong of your power plug. You will most likely find they are shorted together. This is a common design and is typical of desktop computers. It can cause all kinds of prolems with serial ports for the same reason.

Isolating either power supply should stop the GFI tripping. Switching to a laptop with a brick switching power supply will probably do the trick, as would putting either device on an isolation transformer.

Gregg Harrington

Wow, great feedback and thanks. I don't have the parts needed to test this right now, but I feel an online order coming on

I will try both of these for learning, I will isolated circuit via a transformer and order a galvanic isolated AC/DC converter to play with.

Thanks!

Gregg

Gregg Harrington

As a follow up question, is the high floating voltage in my design a likely reason for the overload and vaporization of my VSS line on the prop plugin when I plugged into my computer without the GFI previously?

Thanks again,

Gregg

Sapphire

Yes it is.

Heater.

On the other hand I have seen the opposite problem a few times.

Like two pieces of equipment both with isolated supplies connected via RS485/422 that immediately fails blowing the driver chips. And that's two units within the same cabinet connected to the same mains power point. Turns out that ground on one end is flying up and down -60v to +60v at mains frequency w.r.t. the other.

Quite how that happens I never figured out.

Gregg Harrington

While I am not sure that this all makes perfect sense, I do now understand isolation better and why it's important. Turns out building the circuit I want in an isolated manner is cheaper and safer. So it's a 100% win for me!

Thanks for the explanation guys! Helped so much!

Cheers,

Gregg

LoopyByteloose

For a long time, the isolation transformer was a standard safety feature in consumer electrionics, but as production moved overseas and the desire for smaller lighter devices, the shift of wall warts without any isolation transform has crept it. I suspect there are electronic engineers in large portions of the world that design power supplies these days without any concept of what an isolation transformer is or does.

The manufacturers have justified not using isolation transformer by building in plastic cases that are suppose to completely isolate the user from any shock hazard. But the hobbyist just jumps in with bare boards and not much knowledge of sane safety proceedure.

When in doubt, it just might be wisest to power bench work from a battery and not have anything connected to the house mains.

localroger

Actually, Loopy, most wall warts and switching bricks are isolated. Even the non-isolated PC power supplies have a transformer, usually used to step mains voltage down to 40VDC or so for the bridge rectifier and switching circuitry. PC's generally aren't isolated because they have metal cases, and metal cased devices can't be considered "double-insulated." It's considered better design to ground them even though there is a transformer that could> isolate them, because if the isolation fails there's no second layer of protection.

LoopyByteloose

@localroger
Yes, in the USA that is true. UL approval is generally observed.

But in Asia, we get the other wallwarts -- those that never were allowed import to the USA or EU. And some of these may be making it to the USA and EU via EBay sales. China produces a lot of electronics for export that go to other destination where safety compliances are not so rigorous.

I just know that an isolation transformer is a very good thing, regardless of other safety proceedures. I have just been zapped too often from the lack of isolation. Everyone working with bare board projects that are powered by AC mains should acquire some awareness of safety proceedures.

If you are relying on a grounded metal case for 'double insulation' safety protection, you should generally not poke around inside it when the power is on.

+++++++++++
A lot of the 'culture of safety' is taken for granted by western cultures. But people over here are still catching up.

Recently we had a crane collapse of a 300 ton lift in Taipei and complete crush a passing car. It was obvious that the crane operator had inadequate training and there was no plan for traffic control. Electrical house wiring here is nowhere near US standards of safety.

If you are going to globally source cheap electronics components, it helps to be aware of holes in safety complicances.