USB charger problem - live cable

[Lee697]

G'day

I bought my son a 240V USB charger at Officeworks yesterday - branded 'comsol'. Model no WCN34BK

Last night he complained he was getting tingling off of the metallic shielding of the USB cord - it is one of the ones from Jaycar with a metallic armour around the cable (childproof!). It was late, so I unplugged it and told him to leave it.

This morning - I plugged it in and measured the voltage from earth (household socket pin) to the shielding - 100V AC.
I did the same with an Apple branded charger we have - earth to shielding - 0V.

Can anyone suggest what is going on?? Cheers....
Lee

[sleemanj]

Leakage through the noise suppression capacitor to mains earth.

There should be no current behind it.

Apple supply may have the sheilding connected to earth directly.

Read the first answer here
https://electronics.stackexchange.com/questions/216959/what-does-the-y-capacitor-in-a-smps-do

[Whales]

EDIT: "Y capacitor" == "noise suppression capacitor" in most power-supply contexts

Two-pin wall plug on it I presume?

Half of mains suggests it could just be Y capacitor tingle.  Can you safely add a 10K resistor between your two DMM probes as you measure that voltage (eg via alligator-leads)?

If that collapses the voltage down to something more sensible: it's a double-insulated design with class Y caps leaking a bit of mains.  Standard design practice, I hate it but it's how they do it.  Lots of topics on this forum about it if you search.

If the voltage doesn't collapse to something more reasonable under the load resistor: suspect it's faulty charger, indeed treat it as unsafe until you know otherwise.

The apple charger may use different size Y caps, a fully-isolated design (EDIT: wrong way around?) or be a 3-pin wall plug.  There are many ways to reduce or eliminate the tingles in such devices, but they all cost more money.

[Lee697]

Thanks chaps.... indeed there is no current behind it, else my tongue test might have turned out differently! :O

I'll try the resistor now.....

[Lee697]

Two-pin wall plug on it I presume?

Can you safely add a 10K resistor between your two DMM probes as you measure that voltage (eg via alligator-leads)?

If that collapses the voltage down to something more sensible: it's a double-insulated design with class Y caps leaking a bit of mains.  Standard design practice, I hate it but it's how they do it.  Lots of topics on this forum about it if you search.

If the voltage doesn't collapse to something more reasonable under the load resistor: suspect it's faulty charger, indeed treat it as unsafe until you know otherwise.

The apple charger may use different size Y caps, a fully-isolated design (EDIT: wrong way around?) or be a 3-pin wall plug.  There are many ways to reduce or eliminate the tingles in such devices, but they all cost more money.

I paralleled a 10K resistor across - voltage dropped from 99V AC to 2V AC.... sounds OK....
Both of these chargers are 2 pin units.....

In any case I might relegate the metallic sleeved cords to the car for the kids.....


and thanks again!

[Jeroen3]

Won't help much if the phone has any metal parts. Or headphones with metal.
And some capacitive touchscreens are inoperable with this 50 Hz hum on them.

I'd toss the charger. Such a low 5W supply shouldn't be giving a noticable tingling. Only measurable.

[Whales]

Won't help much if the phone has any metal parts. Or headphones with metal.
And some capacitive touchscreens are inoperable with this 50 Hz hum on them.

I'd toss the charger. Such a low 5W supply shouldn't be giving a noticable tingling. Only measurable.

50Hz shouldn't effect the touchscreen scanning, much higher frequencies (ie switching noise, higher-freq interference from mains dimmers) would be the culprits in that situation.

Using larger Y caps adds more tingle but is otherwise not a reliable indication of a charger being "lower" or "higher" quality.  In fact larger Y caps (more tingle) suggest there might better noise suppression, but really this depends on the whole design (not just one part). 

Lee697: The only reliable methods of determining whether or not a charger is a good one are not are (1) extensive testing and (2) disassembly to inspect the design.  There are lots of forum topics, blog posts and videos on the latter (inc by Dave).  Again: tingling from Y-caps is not an indication of charger quality or safety.

Car analogy: should you buy the car with the louder/quieter horn because it's safer?  No, you need to consider the whole car when judging safety, not just the horn.  A loud or quiet horn may be annoying, but it's not an overall indicator of quality or safety.

[wraper]

Won't help much if the phone has any metal parts. Or headphones with metal.
And some capacitive touchscreens are inoperable with this 50 Hz hum on them.

I'd toss the charger. Such a low 5W supply shouldn't be giving a noticable tingling. Only measurable.

Really? 0.2mA is completely acceptable (calculated from voltage drop over 10k resistor). And these days chargers have way more than 5W rating. One in question has 17W rating.

[tooki]

Won't help much if the phone has any metal parts. Or headphones with metal.
And some capacitive touchscreens are inoperable with this 50 Hz hum on them.

I'd toss the charger. Such a low 5W supply shouldn't be giving a noticable tingling. Only measurable.

Then you’re limiting yourself to cheap, dangerous, non-compliant Chinese crapola chargers. The suppression caps in any properly made charger are enough to be tingly sometimes.

[wraper]

Then you’re limiting yourself to cheap, dangerous, non-compliant Chinese crapola chargers. The suppression caps in any properly made charger are enough to be tingly sometimes.

Nokia chargers from the past did dot have this noise suppression cap. IIRC they used shielding between windings. And it worked fine even with phones having capacities touchscreens. If regular transformer is used without Y cap between primary and secondary sides, capacitive touchscreens of connected devices will simply stop working or will have very erratic behavior.