Problems with galvanic isolation

Hello all,
I am working on a project consisting on controlling the light intensity of a series of light bulbs using wireless sensor nodes. To do
this, I have built an experiment with five light bulbs, each of them connected to an analog dimmer. These analog dimmers are controlled by the wireless nodes. These wireless nodes have a DAC which gives up to 3V on their output. Since the analog dimmers require an input from 0 to 10 V, I built an inverter-amplifier circuit between each node and each dimmer, using operational amplifiers.
The nodes are connected through a USB cable into USB Hubs which are in turn connected to the wall socket.
The operational amplifiers are supplied with +15/-15/mass by a regulable tension source, which is in turn also connected to a wall socket.
The idea is to control each of the light bulbs indepently, telling to each node per radio to change its DAC output. But, if I want to set a node's DAC to a determined voltage while keeping the others to zero, the voltage divides itself among all nodes.
It seems that there is no galvanic isolation between the node and its USB connector, so that a circuit is being formed through the masses.
Could anyone give me some suggestions to help solve this problem?
Thank you very much,
- J.
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On 8/22/06 8:04 AM, in article snipped-for-privacy@i3g2000cwc.googlegroups.com, " snipped-for-privacy@gmail.com"

I have no idea what this means.
Bill -- Ferme le Bush
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snipped-for-privacy@gmail.com writes:

Sounds like you have some grounding ocnnection missing somewehre in the analogue signal paths...

Is that a problem in your case ?
When you have the node powered from the same power source as the rest of thee system usually there are no problems. Each of the nodes are just connected to their USB DACs and from there to opamp + dimmer. No other connections, no other gouridng sources. Comminications data comes wirelessly.

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Tomi Engdahl (http://www.iki.fi/then /)
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Tomi Holger Engdahl wrote:

I think so. The nodes are supplied through the USB cables, which take power from the USB Hub which is connected to a wall socket, whereas the operational amplifier is being supplied from a standard adjustable power source (+15, -15 and ground). Here is a diagramm which should (hopefully) clarify things a little:
http://images1.fotopic.net/?iid=yxouom&noresize=1&nostamp=1&qualityp
The dimmer is velleman's K8064 DC-Controlled dimmer.
The nodes are moteIV's Tmote Sky nodes:
http://www.moteiv.com/products-tmotesky.php

There are actually two power sources: The USB Hubs (Supplying the nodes) and the lab power source giving +15, -15 and ground to the op amp.

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The USB cable connects the cable shield ground of your nodes and USB HUB togeteher. Most propably the wall socket does not groudn the USB hub.

Is this power suplly ground connected to mains ground or not ?

It cleared situation.

I knwo this. I have used that module also. The input of that module is floating, not connected to mains side or ground in the module in any direct way (the control voltage goes to optoisolator).

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Tomi Engdahl (http://www.iki.fi/then /)
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Hello,
Thanks for your contribution. I already solved my problem. If you are interested, you can take a look to sci.electronics.design
Cheers,
- J.
Tomi Holger Engdahl wrote:

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