I have a 5V and a 12V power supply. Both have a N, L, Ground
connections for the AC side. Both also have a V- and V+ connection.
Measuring between V- and V+ I get the correct voltages, i.e. 5V and
12V. Measuring between ground (AC side) and V- on both the 5V and 12V
gives 0V. Measuring between ground (AC side) and V+ on the 5V supply
gives 0V and on the 12V supply 12V.

Why does the 5V power supply not show 5V from ground to V+? Note that
the 5V can deliver up to 6A and it is under a 0.6A load when I measure
the voltage.

Thanks

<hufaunder@yahoo.com>
>
>I have a 5V and a 12V power supply. Both have a N, L, Ground
> connections for the AC side. Both also have a V- and V+ connection.
> Measuring between V- and V+ I get the correct voltages, i.e. 5V and
> 12V. Measuring between ground (AC side) and V- on both the 5V and 12V
> gives 0V.


** Use a * ohm * meter to check for continuity.

Bet the 5 volt one has no connection from AC ground to the DC output.

Most simple PSUs have the DC output "floating" ie not connected to safety
ground.




......... Phil

On Aug 15, 9:33 pm, "Phil Allison" <philalli...@tpg.com.au> wrote:
> <hufaun...@yahoo.com>
>
> >I have a 5V and a 12V power supply. Both have a N, L, Ground
> > connections for the AC side. Both also have a V- and V+ connection.
> > Measuring between V- and V+ I get the correct voltages, i.e. 5V and
> > 12V. Measuring between ground (AC side) and V- on both the 5V and 12V
> > gives 0V.
>
> ** Use a * ohm * meter to check for continuity.
>
> Bet the 5 volt one has no connection from AC ground to the DC output.
>
> Most simple PSUs have the DC output "floating" ie not connected to safety
> ground.

So if both the 12V and 5V need the same reference then could I connect
V- on the 5V PS and the V- on the 12V PS and the ground on the AC side
of the 12V PS all together? Are there any safety issues with this?

Thanks

<hufaunder@yahoo.com> wrote in message
news:1187248169.669459.247130@i13g2000prf.googlegroups.com...
> On Aug 15, 9:33 pm, "Phil Allison" <philalli...@tpg.com.au> wrote:
>> <hufaun...@yahoo.com>
>>
>> >I have a 5V and a 12V power supply. Both have a N, L, Ground
>> > connections for the AC side. Both also have a V- and V+ connection.
>> > Measuring between V- and V+ I get the correct voltages, i.e. 5V and
>> > 12V. Measuring between ground (AC side) and V- on both the 5V and 12V
>> > gives 0V.
>>
>> ** Use a * ohm * meter to check for continuity.
>>
>> Bet the 5 volt one has no connection from AC ground to the DC output.
>>
>> Most simple PSUs have the DC output "floating" ie not connected to
>> safety
>> ground.
>
> So if both the 12V and 5V need the same reference then could I connect
> V- on the 5V PS and the V- on the 12V PS and the ground on the AC side
> of the 12V PS all together? Are there any safety issues with this?
>
> Thanks
>
There should not be any issues with this - safety or otherwise. As Phil
says, output ground is often floating, but just as commonly, if the power
supply has a metal enclosure, it will have an AC-side safety ground, and the
output ground will likely be tied to it.

Arfa

On Aug 16, 3:09 am, "hufaun...@yahoo.com" <hufaun...@yahoo.com> wrote:

> So if both the 12V and 5V need the same reference then could I connect
> V- on the 5V PS and the V- on the 12V PS and the ground on the AC side
> of the 12V PS all together? Are there any safety issues with this?

You have already stated that with the 12V supply, V- and AC-ground are
connected? That is, you did what Phil suggested and looked for
continuity between the -12V and AC-ground?

Keep in mind that some (cheap) power-supplies use autotransformers,
such that the LVAC side is still directly connected to the AC mains...
meaning that the DC side also remains connected to the AC mains.
Others use step-down transfomers with the secondary isolated from the
primary. You need to determine which is which in your case. More work
with the VOM, but easily determined.

Unless both supplies are isolation-type and writing for myself, I
would not connnect the two together without further testing for
potential between the two, or between the workpiece (what you are
powering) and the two if connected together. If they are isolation-
type, then there should be no safety issues.

But test for AC and DC potential between the two supplies anyway
before making any common connections.

Next question: Does the 12V supply have enough capacity that you could
use it to provide 12V and 5V via a divider?

Peter Wieck
Wyncote, PA