Adding GROUND to an AC converter: is it safe to do?

[tony359]

Hi all,

I've got an AC to AC converter to use Japanese items outside of Japan. It allows 100-240V input and outputs 105V, 55Hz. It works but I'm powering a Hakko de-soldering gun and the converter doesn't have ground connected inside. So the Hakko tip is floating.



Now, I'd imagine this converter acts as an isolation transformer so ground from a "safety standpoint" is not really required. But from an ESD standpoint, I'd like my Hakko tip grounded, am I correct?

I've opened the converter and of course it's not a linear transformer design. I'd imagine it might rectify AC to DC and then generate AC back. That explains how it can turn my 50Hz to 55Hz

Now, Ground is indeed unused internally but it's accessible on the PCB. And all sockets have the ground lug available, there is just nothing soldered on it.

Can I safely wire a ground lead from the inlet socket to the outlet sockets? Do you see any issues in doing that?

Yes, I am comfortable working inside this item and I take full responsibility. I have fixed a variety of SMPS in the past, I (should) know what to do and what not to do

[Benta]

By "GROUND" I presume you mean PE or "protective earth"?
There's no reason for the power supply to provide this, floating is much more flexible.
If you have ESD concerns, a better solution would be to connect your soldering iron to your work area directly. Eg, with a clip on the heater body and a wire to your working surface.

[tony359]

In the UK we have Live, Neutral and Ground. I'd imagine Ground can also be called PE, yes.

Thanks for the idea. But if I wanted to go with the more practical way of having PE going to the iron via its cord, do you see any safety or technical concerns if I just wired those 100V sockets to PE coming from my mains socket?

To make things clearer I made a drawing. I would like to add the connection shown by the arrow

[Benta]

I see no problem in that, provided the input/output connectors support it.
My harping on the "ground" naming is, that it is meaningless, because it can be used at will.
PE is the only internationally accepted name/term. Just for info.

[tony359]

understood on "ground".

Yes, it's pretty simple and safe to run a wire from the PE input of the converter to the PE output of the Japanese 100V socket. It's all there, just waiting for a wire to be soldered on.

On a slightly separate topic, am I correct in thinking that I won't be reading 105V in between the output of the converter and PE? Also, if I wire a light bulb between the Live of the 105V output and PE, it won't turn on?

[Gyro]

That looks a nicely constructed converter. Plenty of glass fibre insulated sleeves etc bode well for the quality of other parts. A low voltage AC-DC SMPS would, of course be isolated between input and output. A mains AC-AC converter - I really wouldn't make that assumption.

I can understand the wish to carry through the protective earth from input to output. ESD leads (should) contain a series 1M Ohm resistor. Any high frequency noise could be imposed across this resistor, possibly damaging ESD sensitive parts. Additionally, If the Hakko, or any other equipment that you intend to plug into it, has a 3-pin plug, it must absolutely be mains earthed, no quesiton - It is not good that the 3-pin output sockets are not earthed by default.

In terms of reservation, the only one I would have is implementation. If your earth wire becomes detached from the input socket and contacts something internal, you have the worst possible scenario. Make sure your connections are mechanically secure before soldering, use heatshrink sleeving at the ends and take a leaf from the manufacturer's book and use heat resistant sleeving to protect it from live parts, heatsinks, transisor tabs, etc.

[tony359]

Thanks @Gyro

Yes, the Hakko has a 3-pin plug and the earth pin shows continuity with the metal parts.

100% I will make sure the ground wire is double insulated and properly soldered so it doesn't come off.

And yes, I agree the converter looks nicely done. There are temperature sensors, there is a fuse and it can be easily opened: there are 6 screws under removable rubber feet.

[Benta]

On a slightly separate topic, am I correct in thinking that I won't be reading 105V in between the output of the converter and PE? Also, if I wire a light bulb between the Live of the 105V output and PE, it won't turn on?

It will slam the GFCI, which is as it should be, otherwise it needs to be checked.
Neutral and PE mostly have the same potential, but there's no guarantee on that. There's a reason that PE is a separate wire.

[tony359]

Just for records, the GFCI is called RCD here

If I wire a light bulb between Live and Earth(PE) of my mains 240V socket, yes, the RCD/GFCI will trip.
But assuming the 100V after the converter is isolated from mains 240V, I don't think the light bulb will light up hence the RCD/GFCI will trip. Maybe the RCD/GFCI might still trip as I'd imagine there might still be a little current flowing from Live to Earth(PE) and I know RCD/GFCI are quite sensitive. Would you agree?

[Ian.M]

Looking at the back of the PCB, there doesn't appear to be any isolation between the mains input and the output H-bridge and connections to the mains output sockets.   It does have an isolated section on the right with what looks like a pair of double USB A sockets, with a transformer feeding it, presumably for low voltage USB charging, but there's nothing to indicate the 105V ac output is isolated.

[tony359]

Thanks @Ian.M

I probably lack some knowledge here to fully understand what "isolation" mean.

I have an isolation transformer, it's a linear thing. It's a 1:1 transformer so it gets 240VAC in and 240VAC out. The 240V out is not referenced to earth so if I touch the Live conductor, I don't get shocked - not that I want to try that!

I never considered this feature for an SMPS because they usually generate low voltages where electrocution is not an issue. But in this case I have 240V in and 105V out - will the switching supply provide that kind of isolation? And does my logic apply to this situation where I don't have AC -> DC but AC -> AC?

Thanks for your help!

[tony359]

I've added ground and nothing exploded 🙂  Thanks all for the help.