Laptop Power Supply

[IJWS15]

Not really DIY but looking for informed advice. Just been provided with a new laptop at work and it came with a 19v power supply which is earthed. IT guy said I could use it with the old style power supply (19v unearthed with double insulated symbol) but I might get a tingle off the laptop case ! ! ! ! ! !

I intend to get a second power supply to leave at home but they can only supply the old version. They have the same size plug at the 19v end.

He also said you could use the figure of 8 plug in the clover leaf socket - I would not want to push that hard to get it in but gave me an indication of his level of knowledge.

Now my basic understanding is saying it is only 19v and it is OK but is this safe and is the earth only necessary for the power supply or is the laptop itself likely to be earthed through the DC neutral?

 

[bernardgreen]

Now my basic understanding is saying it is only 19v and it is OK but is this safe and is the earth only necessary for the power supply or is the laptop itself likely to be earthed through the DC neutral?

If the unit is un-earthed and the figure 8 plug can be put in either way round. Hence the 19 volts cannot be "earthed" via the Neutral as Live and Neutral can be reversed depending on which way the plug is inserted.

19 volts difference between two wires is safe BUT if one wire is at 230 volts above ground then neither wire is safe to touch.

The lead from an un-earthed PSU will be around 115 volts above ground ( mid point between Live and Neutral ) but this induced voltage will be via a small capacitor so the amount of current that can flow is very small. hence only a tingle from the laptop and not a serious shock.

NOTEsome cheap / fake / counterfeit PSUs can fail and the output lead can become directly connected to the mains and this will make touching the laptop hazardous.

This is one example of a fake Dell PSU

http://www.electricstuff.co.uk/acadapter.html

 

[IJWS15]

The new PSU has a cloverleaf connection.

Should not be fake - the units are branded with the name of the company I work for - just a different part of the group.

 

[Astra99]

Some laptop PSUs use the earth pin of the mains lead to "leak away" any eddy currents generated in the iron core of the transformer. It is usual that there is no connection between the earth and the output of the PSU. (From what I understand this improves the PSU's efficiency).

This type of PSU is a devil when it comes to Portable Appliance Testing. You cannot get an Earth Bond connection, so as a Class 1 appliance it would fail. However, tested as Class Two (which is what it really is apart from the "leak-off" connection) it passes. I therefore test these as if they were Class Two items and append a note to the test record.

 

[plugwash]

Short answer: your IT guy is right on one point and wrong on antoher. It should be safe to use the old power supply if it met safety standards when it was made but some people can feel a slight tingle, it is NOT ok to force a figure eight connector into a cloverleaf socket. Power supplies are also an important safety device and if corners are cut in their design and/or manufacture it can be dangerous to both people and equipment.



Long answer.

Modern DC power supplies typically use a "flyback converter", If the transformer was ideal this would provide perfect isolation but in reality there is always some parasitic electrostatic interaction between the windings as well as the desired magnetic interaction. At the high frequencies a flyback converter runs at this electrostatic interaction creates a high frequency between the input and the output. If this was not suppressed then the cables from the power supply will act as a dipole producing a signficant EMI problem.

To suppress this they put low value between the input and the output of the power supply. However inevitablly these capacitors also leak some current at mains frequency.

For a class 1 (earthed) power supply this isn't too big a deal, you can arrange the caps between input->earth and ouput->earth. The high frequency suppression path between input and output is there but the output tends towards earth if left to float and any mains leakage goes to earth.

For a class 2 (unearthed) power supply they can't do that. So the capacitors have to go directly between input and output which leads to the power supply floating towards some fraction of mains and a measurable leakage current through anything that connects the output of the power supply to earth (e.g. a person). There are accepted limits for the leakage current in this scenario and there are special capacitors ("Y capacitors") designed so that a short-circuit failure is extremely unlikely.

Unfortunately some people are very sensitive to electric current and can feel current levels that are deemed safe. You can also get problems if you connect a large number of class 2 power supplies together, individually the leakage is acceptable but collectively it is not and finally you can get problems if the power supply builder cut corners and used capacitors that were either too large or the wrong type.

 

[plugwash]

Some laptop PSUs use the earth pin of the mains lead to "leak away" any eddy currents generated in the iron core of the transformer.

This is BS, eddy currents in the iron core stay in the iron core. Reasons for having earths include use of earthed metal as a safety barrier and the suppression of high frequency noise without placing unwanted (earth relative) voltage/current on the ouput.

I understand that the latter is the main reason larger switched mode power supplies are usually class 1 even when they have a "floating" ouput.

Laptop and similar power supplies do indeed pose a problem from a testing point of view, the earth must be regarded as important to safety unless the manufacturer explicitly says otherwise but due to the sealed unit nature it can't be directly tested.

If it has a detatchable power cord you can test the power cord which goes a long way to reducing the risk (a failure in a flexible cable is by far the most likely cause of a lost earth).

You could also measure the open circuit voltage and/or short circuit current betweent the output and earth while operating, not sure if pat testers can easilly do this though.

 

[Monkeh]

Some laptop PSUs use the earth pin of the mains lead to "leak away" any eddy currents generated in the iron core of the transformer.

This is BS, eddy currents in the iron core stay in the iron core.

Not to mention there is no iron core, this is not 50Hz we're dealing with.

 

[IJWS15]

So does this mean I should be OK if I use an older design class 2 psu (double insulated) in place of the class 1 psu that came with the laptop?

Both have the same brand name on them.

 

[Monkeh]

So does this mean I should be OK if I use an older design class 2 psu (double insulated) in place of the class 1 psu that came with the laptop?

Both have the same brand name on them.

You might get a bit of a tingle from time to time. It'll be fine, use whichever you want.