Power supply help needed

[Emanresu]

I can find plenty of these but the polarity is always positive outer/negative inner, when I need it the other way round.

That's a strange setup. Suggests if you hold onto an earth and touch the outer, you'll get 24v/2A when you touch it.

If you are doing any rewiring check with an ohmmeter first. Or use an expendable volunteer.

 

[lazybloke]

That's a strange setup. Suggests if you hold onto an earth and touch the outer, you'll get 24v/2A when you touch it.

These power supplies only output direct current, so O Level physics tells us that i=V/R
With the human body having a resistance of thousands, if not 10s of thousands of ohms, a current 2 Amps is just not possible from 24 volts.
I doubt you'd even feel the current unless you applied the voltage directly to your tongue (you've licked PP3 batteries, right?).

Alternating current might be a very different matter - I'd need someone else to explain why.

 

[presta]

Suggests if you hold onto an earth and touch the outer, you'll get 24v/2A when you touch it.

The output is floating, it's not connected to earth.

These power supplies only output direct current, so O Level physics tells us that i=V/R
With the human body having a resistance of thousands, if not 10s of thousands of ohms, a current 2 Amps is just not possible from 24 volts.
I doubt you'd even feel the current unless you applied the voltage directly to your tongue (you've licked PP3 batteries, right?).

Alternating current might be a very different matter - I'd need someone else to explain why.

Body resistance varies wildly, both from one individual to another and depending on how wet the skin is, it's also non-linear, so it becomes much lower as the voltage increases, but yes, you won't get anything like 2A from 24V.

Not that it needs to be, 500mA is enough. The lowest recorded fatal voltage was 42V AC (and the highest survived 230kV), but the body is about 3-4 times more sensitive to AC than DC, Wikipedia doesn't say why. I've survived a 240V AC shock from arm to arm across the chest.

When I bought my HiFi I could feel the leakage current from the front panel buzzing on my fingertips even though it was well within spec, but I can't remember what that limit was now.

 

[slowmotion]

Rummage around here....

https://www.ebay.co.uk/sch/i.html?_...1313&_nkw=polarity+reversing+DC+plug&_sacat=0

 

[lazybloke]

@presta, With AC you have to consider impedance rather than resistance. Impedance is a complex number, it is both 'real' and 'imaginary'!!!!!

To put it another ways, AC doesn't experience resistance in the same manner as DC.

 

[presta]

@presta, With AC you have to consider impedance rather than resistance. Impedance is a complex number, it is both 'real' and 'imaginary'!!!!!

To put it another ways, AC doesn't experience resistance in the same manner as DC.

It's true that the ratio of voltage to current in an AC circuit is the impedance, but since the reactance of an object the size of a human is going to be negligible at 50Hz (several orders of magnitude higher and lower than the resistance), the impedance is solely resistive. You could also include the skin-effect, but as the skin depth's tens of kilometres, that's also negligible.

 

[Regular.Cyclist]

The output is floating, it's not connected to earth.

Body resistance varies wildly, both from one individual to another and depending on how wet the skin is, it's also non-linear, so it becomes much lower as the voltage increases, but yes, you won't get anything like 2A from 24V.

Not that it needs to be, 500mA is enough. The lowest recorded fatal voltage was 42V AC (and the highest survived 230kV), but the body is about 3-4 times more sensitive to AC than DC, Wikipedia doesn't say why. I've survived a 240V AC shock from arm to arm across the chest.

When I bought my HiFi I could feel the leakage current from the front panel buzzing on my fingertips even though it was well within spec, but I can't remember what that limit was now.

There have been much lower voltage fatal shocks that that. Look into micro shocks. If skin resistant is breached by an injury, or during surgery for example, then internal resistance can be as low as 300 ohms and anything over 20 microamps can cause fatal heart arrhythmias.

It’s one on the reasons why medical equipment need to comply with IEC 60601 (particularly IEC 69601-1 for electrical safety) in terms of design and manufacture and must meet the very low electrical safety requirements in IEC 62353 is routine testing.

 

[glasgowcyclist]

Rummage around here....

https://www.ebay.co.uk/sch/i.html?_...1313&_nkw=polarity+reversing+DC+plug&_sacat=0

Thanks. My brother found me a power supply with an internal polarity switch that provides the correct polarity automagically.

 

[lazybloke]

Ithe skin depth's tens of kilometres

Thick-skinned!