Kaiweets USB soldering iron

[tony359]

Hi all,

I am testing a USB powered soldering iron. It's made by Kaiweets but I believe it's also sold under other names.

As a standard test, I checked whether there was any voltage leak from the tip - that said the thing runs on USB-C power delivery so am I correct that the whole unit is floating and not grounded? If I check, I measure 90V at the tip but I have a feeling that that is not relevant? Maybe caused by that capacitor between primary and secondary inside the PSU? I do not see current flowing at all.

In other words, do you think it's expected and safe for PCBs?

Thanks!

[stj]

it's the psu,
forget the voltage unless it's over 1000v, see if you can meter the current instead.

[tony359]

I read 0.000A so that should be safe? Even for sensitive devices?

My question is: is that unavoidable when using USB power?

[stj]

interesting question.
does the usb psu have an earth on the mains-side?

my iron with switching psu has enough leakage to make green leds blink.

[tony359]

Yes, I discovered the leak on the T3A but for me that is unacceptable. But that is a different thing as you can light up LEDs and I can measure a CURRENT. I can make a speaker sound. This USB iron won't do that.

All my three PD supplies don't have a ground pin - I guess this is common. That means - please correct me if I am mistaken - that whatever voltage I am reading at the tip won't go anywhere as it's like the iron is behind an isolation transformer: I can touch the live of MAINS after an isolation transformer and I won't get shocked.

Now, my question is: is this ok for ANY electronics? I don't care if it "should" be ok, I would like to know if it's ALWAYS ok. In other words: is this an expected behaviour when dealing with an isolated USB power supply?

[stj]

a few mA is harmless, it's no differnt to probing with a multimeter.
try grounding the tip to a wall socket.

[tony359]

I checked current between tip and ground and I am reading 0.000. I suspect I would read 0V if I grounded the tip, I'll try.

a few mA is ok. It's the 80V that kind of concerns me.

[stj]

it's the big prioblem with unearthed switching psu design.
i have seen many times a bog spark when plugging or unplugging cables between a tv and a source like a sattelite reciever.
each hase a ground voltage with upto a couple of hundred difference that has to equalise when you connect them!

[redwire]

One quick test is seeing if the tip can light up two back-back parallel (AC connected) LED's.

I believe it's bad to have HV at the tip, many mosfets are damaged by even 30V on the gate, it's a very thin layer of glass. I had a period where I kept killing parts and it was potential between the soldering iron and ESD mat.
Y-cap leakage current is AC and low for shock, you can measure it a multimeter. But a stray charge does build up, it gets rectified being on the DC bus.
That spark seen connecting gear is more than enough to damage ICs and give a tingle.

So I always hard-ground my soldering equipment. Using a two-prong power brick doesn't work unless it's high quality and uses neutral for the Y-cap connection. Most laptop bricks that are decent are three-prong with Y-caps and 1MEG to PE so you don't get the HV potential.

[tony359]

Interesting pointers, thanks. I am familiar with the spark you get when you connect equipment which is powered by non-earthed power supplies.

So if I understand this correctly, that voltage is referenced to the neutral - which in turns is referenced to ground at some point - and the culprit is the Y-cap which is in between primary and secondary. Can someone remind me why that cap is there?
If the supply was NOT referenced to anything - i.e. through an isolation transformer - I wouldn't be able to read 80V at the tip, I would read 0V - am I correct?

The fact that my multimeter CAN read 80V at the tip means that the supply is somehow referenced to the same ground - because of the Y-cap.
So there is a potential (no pun intended!) to cause damage as those 80V might end up somewhere. However, this is an inevitable by-product of using a USB soldering iron, unless the manufacturer supplied a ground-referenced power supply for the iron, which is not the case on this occasion.

I see the Pinecil iron features the same un-earthed power supply so shall I assume it's also affected? Anybody with one of those who could confirm?

Thanks for now!

[stj]

the cap layout varies, but it's about supressing RF mostly.
the real problem with not using mains earth is that everything is floating.
normally if you have the earth then negative is equal and live is 240v above it.
without earth the difference is 240v but the potential between that and ground could be thousands.