I am a little confused as to how I need to wire up the Open Collector Drivers to operate my relays  (http://uk.rs-online.com/web/search/searchBrowseAction.html?method=searchProducts&searchTerm=511-1032)

Can anyone offer a diagram  or explanation of how to use the Open Collector connections. I have done some reading on OCs but AIUI I need to provide a 12v supply i.e. I can use the brick PSU - then where do I go - through the relay and then into the OC connector as if it was ground/negative?

Leon.

You are correct:

Just connect the relay between the 12V supply and the Open Collector output.  

The reason for the Open Collector output is two fold: i) higher current capable. ii) The supply voltage does not have to come from the same supply, as long as it is not greater than the webbrick supply and the two supplies have there common connected (labelled 0V or G typically). So if you want to switch something that takes only 9V that can also be done common the 0V of the two supplies, connect the device to 9V and the other end to the open collector.

Technical comment: 

The reason the supply cannot be higher than the webbrick supply is because the circuit is protected from back Emf by a diode that clamps the PIN to the webbrick powqer rail.
 

Hi,

 Thanks for clarifying that for me, I was somewhat reticent to send 12v into a brick connector without checking it first!

 This is an excellent capability I shall hook up my 2 heating mats and boiler with this arrangement. Would you recommend getting a dedicated 12v supply and connecting the grounds or can I use the brick supply? I did read somewhere in the manual that the brick supply is slighty higher than required to prevent the analogue outputs send out slightly less than the 10V so i would not want to interfere with that.

Leon.

Unless the relays require large current loads then just use the existing 12V supply.

Also remember that the webbrick has 4 mains  circuits on board.