90V AC potential to ground, occasional tingling sensation

[optilude]

Hi,

I have various DC-powered devices (guitar effects pedals and related gear). They are powered with 9V, 12V or 18V centre-negative DC power supplies. I also have AC-powered amplifiers that I have built myself. Yesterday, for the first time I can recall, I felt a mild shock/tingle when touching the chassis of an amplifier (which is metal and connected directly to ground via the earth wire of its power cable) and a metal part on one of the guitar effects pedals.

I’ve been trying to work out what’s going on ever since. (Below, when I say “true earth”, I generally mean I stuck a multimeter prong into the earth prong of a wall outlet.)

I know the amp chassis is grounded properly. I get multimeter beep continuity between the amp chassis and true earth. I measure zero DC or AC potential between the chassis and true earth.

However, sometimes (but not always…) I can measure 80-100V AC potential between the amp chassis on the metal part of the (DC-powered) effects pedal. I assume this is what’s causing the tingle.

I think that, by design, the effects pedal cannot be properly grounded, because it is powered by a two-pole DC barrel plug which in turn is powered by a wall-wart power supply that has a plastic earth prong.

In fact, if I measure AC voltage potential between either prong of the DC power adapter (when plugged in) and true earth, I get 90V AC consistently. I don’t fully understand this, but I am guessing it has to do with the way the DC power adapter turns AC into DC without a reference to earth? (I have done this test - one prong in the earth of a wall outlet, the other on either pole of the 9V DC plug – in different parts of the house, and it always measured 90V AC).

Then, I had the effects pedal unplugged and I felt another tingle. So I measured, and for a time I had 90V AC between that and true earth even when it wasn’t plugged in. I can’t get my head around how that happened, and it isn’t always like that.

The only other thing I can think of perhaps being relevant, is that both the amp and the power supply for the effects pedal are plugged into the same 4-gang extension lead, which has “surge protection” and a single physical on/off switch.

I’d love some help to understand what’s actually going on here. There’s a part of me that thinks something must be wrong with the wiring in the house but there‘s no other reason to think so (and I have used wall socket tester to look for ground faults or reversed live/neutral and it comes up fine).

Thank you,
Martin

 

[RandomGrinch]

Excluding any other faults, the phenomenon is common with switched mode power supplies.

Here's an example...

 

[flameport]

Cheapo crappy power supplies.

centre-negative DC power supplies.

made by Lucifer himself.


AC-DC supplies will have a capacitor between the AC and DC sides for interference suppression.
Decent manufacturers of such things use high quality correctly rated capacitors.
Other manufacturers may not.
There is also the possibility of other faults within one or more of the supplies, and even poor design with inadequate separation between the AC and DC components.

 

[optilude]

I must admit I did not spend much money on the power supply itself.

I’ve actually tested four different AC-DC power supplies for the 90V AC potential to ground. One is a £10 9V power adapter, plastic middle prong. Then there are a 12V and an 18V plastic-middle-prong adapters that are the input to different isolated power supplies with multiple DC sockets on them (a common way to power multiple guitar pedals). All of these have the 90V AC potential.

The final one is a 12V that has a proper three-prong IEC plug going into a power brick that then outputs DC. This is physically larger of course, and it has 0V AC to ground.

The specific device that’s been giving me tingles is powered by the 18V AC-DC adapter into this thing: https://www.amazon.co.uk/dp/B0778GPWMR?psc=1&ref=ppx_yo2ov_dt_b_product_details

Are you saying I should find another 18V centre-negative AC-DC adapter that is better? I’ve tried to search though it’s not very easy to find one. I could try an adjustable adapter (e.g. https://www.amazon.co.uk/gp/product/B0BLHFB32P/ref=ox_sc_act_title_1?smid=A1DFDPCYSF0RRK&psc=1) but hard to gauge if these are good.

 

[SFK]

This is a good video showing how ac to DC adapters can create a large potential difference.

 

[SFK]

you saying I should find another 18V centre-negative AC-DC adapter that is better?

For a quality product I would be looking to get one from RS, farnel or CPC:

https://cpc.farnell.com/c/electrical-lighting/batteries-power-supplies/power-supplies/dc-power-supplies/18v-dc-power-supplies

 

[bernardgreen]

When the power module has Live and Neutral connections but no Earth then the internal circuitry will float around the midpoint voltage between Live and Neutral. Any exposed and touchable conductive surface will be pulled by capacitive coupling towards that midpoint voltage, Provided the amount of capacitive coupling is small then the amount of current that can be taken from the touchable conductive surface by a finger will be negligible and in almost all situations be harmless to a person who may feel a slight tingle.

 

[flameport]

but hard to gauge if these are good

It's actually very easy.

Is it:
1 - being sold on Amazon, ebay or some other marketplace site
2 - has a 'brand' which is just some made up nonsense, and searching for that brand reveals only marketplace selling sites - no actual manufacturer
3 - is being sold by someone you never heard of (on Amazon click on the seller name to see who they really are - most of the time it's some unknown name and holding address in China)
then it's 100% guaranteed to be shoddy, dangerous junk.

 

[JohnW2]

It's actually very easy. ... Is it:
1 - being sold on Amazon, ebay or some other marketplace site
2 - has a 'brand' which is just some made up nonsense, and searching for that brand reveals only marketplace selling sites - no actual manufacturer
3 - is being sold by someone you never heard of (on Amazon click on the seller name to see who they really are - most of the time it's some unknown name and holding address in China)
then it's 100% guaranteed to be shoddy, dangerous junk.

It would be great if it were as simple as that ...but it's not,in our non-Utopian world!

In reality, it could satisfy some or all of those criteria and not be "shoddy dangerous junk" OR it might be "shoddy dangerous junk" despite not satisfying any of those criteria. If you were objective,the most you could really say is that satisfying those criteria would make it 'more likely' that the product was "shoddy dangerous junk"

...and, in any event, as I always say, in the real world any statement containing the words "100% guaranteed" is almost certainly untrue!

Kind Regards, John

 

[hllns]

Redundant laptop PSUs from reputable brands can be a good source of half decent 18-19v PSUs. Your chances of finding one of those with the correct centre pin negative connector are more or less zero though so would require the connector to be rewired.

 

[optilude]

That’s good advice. I have plenty of polarity reversing adapter leads .

 

[SUNRAY]

That’s good advice. I have plenty of polarity reversing adapter leads .

NO NO NO

Change the plug... One day you will forget to use the adapter

 

[SUNRAY]

Redundant laptop PSUs from reputable brands can be a good source of half decent 18-19v PSUs. Your chances of finding one of those with the correct centre pin negative connector are more or less zero though so would require the connector to be rewired.

Only if they have an earth connexion, otherwise they are just as liable to float near the midpoint of the mains.

 

[JohnW2]

NO NO NO .... Change the plug... One day you will forget to use the adapter

Superglue - or even heatshrink

 

[SUNRAY]

Superglue - or even heatshrink

double the length and vulnerable and additional loose connexion point.

Been there, been there and been there