Galvanic isolation in wireless sensor networks

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.

Reply to
poener
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In message , dated Tue, 22 Aug 2006, snipped-for-privacy@gmail.com writes

You write good English, but you are using some French words. Voltage, not tension. Earth (British) or ground (US), not mass. You will be better understood if you use those words.

I think your problem is that all your wireless receivers are on the same frequency. So when you operate the transmitter, they all receive the same instruction. Do you know for certain that your radio system will support ten independent channels, because that is what you need? Do you have ten transmitters, or one transmitter that has ten transmitting channels?

Another possibility is that you DO have a ten-channel system, but your transmitter is so close to the receivers that it is breaking through on those receivers that you are not intending to send to.

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Reply to
John Woodgate

Hello,

Thanks for the corrections. I am actually Spanish, but I am living in Germany at the moment, so it is sometimes complicated to find the proper word, especially when talking about engineering, since one has to use a very specific vocabulary. I will try to pay more attention to it the next time I post. :)

You are right when you say that all receivers are on the same frequency. But I am using a communications protocol which uses addressing, meaning that I can send a message to a specific node, and even though all nodes will "see" the message, only the node to which the message is addressed to will "open" it.

Another thing is that if I have 5 nodes (with its 5 amplifiers and 5 dimmers) and I send the command to set one of them at 5 V, what I get is approx. 1 V in each (Voltage/Number of nodes), so the problem seems to lie on how the experiment has been built...

- J.

Reply to
poener

So your problem isn't galvanic isolation in a wireless network (which should be automatic), but galvanic isolation in a (wired) USB network -- yes?

Without a wiring diagram it's hard to see what you've done. I assume that the DACs are powered off of USB, and puts out it's voltage independently of the others. You _should_ have two wires going from each DAC to it's amplifier -- the signal and it's ground. Do you perhaps just have the signal wire connected from each DAC? This could cause the effect you're talking about.

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Tim Wescott
Wescott Design Services
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Reply to
Tim Wescott

In message , dated Tue, 22 Aug 2006, poener writes

Yes, but without a lot more information about the circuit, it isn't possible to say what is wrong. If you disconnect the supply voltages from all but one receiver, do you then get 5 V or still 1 V?

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Reply to
John Woodgate

Hello,

I realise now I didn't make myself clear at all when I exposed my problem. I've drawn a small diagramm to illustrate what I am talking about. Here is the image:

formatting link

A few comments to it:

The dimmers are Velleman's K8064 DC Controlled Dimmers. Link:

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They take the AC Current from a wall socket and receive a control voltage from the wireless mote through the amplifier.

The nodes are MoteIV's Wireless Tmote Nodes. Link:

formatting link

They generate the control voltage. Their DAC and GND pins are connected to the amplifier circuit. They also have a USB connector from which they are connected to a USB Hub, which is connected to the wall socket. Eventually the USB Hub will be connected to a computer, to be able to communicate with the nodes via USB from the computer.

The colours on the diagramm (red, yellow, blue) represent the different voltages (ground, +15, -15) which are generated by a voltage generator, also connected to a wall socket.

If I work with one node only, the voltages are correct.

These nodes have the possibility of working with 2 AA Batteries instead of USB power supply. If I disconnect all USB cables and put batteries on each node, they also work properly. It is when they are all connected per USB that the circuit closes and the faulty behaviour I describe happens.

Reply to
poener

In message , dated Wed, 23 Aug 2006, poener writes

The way you have your amplifiers configured, the RED wire from your 'voltage generator' must be GROUND. It seems that you may have the BLUE wire as GROUND (and connected to mains earth), so that when you connect everything else, half your supply voltage is short-circuited.

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Reply to
John Woodgate

Sorry if it looks otherwise, but red is ground; yellow and blue are

+15V and -15V.

The voltage generator is also connected to the walls sockets; this means that the USB Hub and the voltage generator are sharing the same ground.

Reply to
poener

In message , dated Wed, 23 Aug 2006, poener writes

OK, then it looks like the red connection at the node is not at the same potential as the ground of the USB connector on the node.

Disconnect all your USB cables except one. Unplug that from the node. Everything works? Now measure to see if there is a DC voltage between the shell of the USB connector and the red connection on the node.

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Reply to
John Woodgate

Exactly. And since there is no galvanic isolation between the USB connector and the output pins the circuit closes.

I suspected the problem could be solved if the power generator and the USB Hub were plugged in to wall sockets which had isolated grounds from each other, but I tried that and it still doesn't work. Perhaps the circuit is closing in the USB Hub?

- J.

Reply to
poener

In message , dated Wed, 23 Aug 2006, poener writes

A closed circuit doesn't close in one specific place.

Where are you taking the 5 V output from? The mote data seems a bit coy about outputs.

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Reply to
John Woodgate

What I meant is that that the group formed by Node 1 - Amp 1 - Dimmer 1 and Node 2 - Amp 2 - Dimmer 2 should be independent from each other (electrically isolated from each other), but if giving voltage to Node

1 causes Node 1 and Node 2 to have 1/2 of that voltage each, then they are connected with each other...

The mote gives 0V to 3V (The difference between the DAC0 and the GND pins). The Dimmer needs from 0V to 10V, that's why I'm using an amplifier.

- J.

Reply to
poener

In message , dated Wed, 23 Aug 2006, poener writes

With one node only connected to the hub, and no amplifier connected to DAC0 and GND, address the node, so that the output on the red and black wires is 3 V, and measure the voltage between the USB connector shell and DAC0. If it's 3 V, connect the DAC0 and GND wires from all nodes directly to all dimmers. Do they then work with the limitation imposed by the 3 V output?

If they do, then your amplifiers are probably the cause of the trouble. Have you put 100nF capacitors from each supply pin to ground on each op-amp? Which type number op-amp are you using?

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Reply to
John Woodgate

I will check this when I get to the lab. But like I said, if I power the nodes with batteries and disconnect the USB wires, it all works. The problem is when I connect the USB wires to the nodes.

No, I haven't. I am using a TI op-amp, with part number OP07CP.

- J.

PS: Thanks for your continous help!

Reply to
poener

Hi,

I finally solved my problem.

Apparently, what has happening was this:

The ground of the voltage generator and the ground of the usb hub are the same. Since I had changed the wiring on the node output to give to the amplifier a negative input (because this is an inverting amplifier), I was introducing voltage in the ground, which was being derived to the other nodes.

The solution was very simple: I uncrossed the wires between the node's output and the inverter's input and crossed them between the inverter's output and the dimmer's input.

Now everything is working as it should.

Thanks for the help!

Reply to
poener

yeah... it sounds like an isolation problem, try running one of the nodes from a separate supply and see if that helps.

I'm guessing you'll want to power each node individually in the end anyway...

Bye. Jasen

Reply to
jasen

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