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List:       owfs-developers
Subject:    Re: [Owfs-developers] Adding an external power supply
From:       Jan Kandziora <jjj () gmx ! de>
Date:       2011-10-07 10:58:39
Message-ID: 201110071258.40086.jjj () gmx ! de
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Am Freitag, 7. Oktober 2011, 10:24:13 schrieb Pascal Brugier:
> 
> I want to add an external power supply for the bus using an ATX power.
> I read that there would be some troubles, data lost on sensor, with the
> ground of the ATX power supply when connected or not connected with the
> groun of the bus comming thru the USB master.
> 
USB GND is put through filter coils inside the computer and inside the onewire 
host adaptor. If you are doin'it'wrong either 

* the amount of power through these coils is too limited for your needs
* you bypass the coils and lose the filtering, allowing noise on the line

But usually the built-in coils are big enough to power a lot of onewire 
devices. If not, the correct way is to do it is to cut the original USB GND 
line and use an additional one with a filter coil in it which is sufficient 
for the needed power.

Also note that the power through the GND line will cause a voltage drop in 
that line. Chips on the far end may not see the required GND potential of less 
than 0,7V and so, fail to operate properly. If you plan to deliver 1A or more 
over 10m or more, don't use telephone wires for GND.

Kind regards

	Jan

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