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Posted by Phil on Saturday, 4 July 2009, at 10:07 a.m.,
in response to Re: Loss of temp sensors due lighting., posted by Bob

Sorry for the lack of response... it's been a busy week for me, I'm running an Underwater Robotiocs camp for Middle Schoolers....

OK.

First thing to note about the WEL vs a PC based monitoring system:

All Desktop PCs have their 0V supplies connected to chassis ground (electrical earth), so if an attached signal cable is raised to a high potential, there is a current path to ground, so part of any protection system is series-resistors to limit the current flow, and any high voltage protection needs to be referenced to chassis ground. This is what all the comments that refer to the DS9097U are about.... the DS9097U ties the 1-wire bus ground to chassis ground and thereby to mains earth. It needs special protection.

The WEL doesn't have this problem, because it's 0V is not connected to Earth or any external potential (the plug pack is isolated through the transformer, and the Ethernet is isolated through transformers.) The only possible ground is if you use the serial port to talk to your computer). So you can ignore any references to schemes to protect the DS9097U.

The second thing to know is that the WEL already uses a bidirectional Zenner protection device to kill high voltage surges. I use the P6KE7V5CA which has a 7.5 V threshold and a 600W rating. I used to use the 1.5KW rating device but it became un-available.

This device it located right at the output terminals on the WEL board and has large traces to protect the board.

The problem is that the individual sensors don't have surge supression. If you take great pains to protect the WEL (by installing in-line resistors) the sensors are left high and dry.

Since you may have many feet of wiring, even if you snub the voltage at the WEL, sensors at the other end of a line are left somewhat unprotected.

My only thought was to put the surge supressors at the end of each bus run to snub voltages at both ends.

The other thing to note is that twisted pair wire also minimized the amount of differential voltage induced by lightning. If you have any stretches of bus wiring that uses non-twisted cable (like thermostat wire) then you will be more prone to differential voltages.

As for the Thermostat Monitor board being a some form of factor... I'm not sure why this would be the case. The thermostat inputs might get induced spikes on them, but they are optically isolated from the WEL bus. Plus, this board ALSO has a transient protector on it, so it's actually boosting the protection on the bus.

Perhaps the thing to be drawn from the way the sensors failed here is that the Thermostat board was actually acting as an end-of-the-line snubber for the "inside" portion of the bus. That portion that was "beyond" the thermostat board was only snubbed at one end, so it was more suceptable. Another argument for placing snubbers at the end of the bus.

I will try adding P6KE7V5CA devies along my bus to see if they degrade performance in any way.

Responses

• Re: Loss of temp sensors due lighting. - [Web Energy Logger (WEL)] - Bob -- 4 July 2009, at 2:38 p.m.
  • Re: Loss of temp sensors due lighting. - [Web Energy Logger (WEL)] - Bill -- 4 July 2009, at 6:15 p.m.

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