[MadeInTheShed]

Ive spent the last few evenings trying to set up a relay to control my router and vacumm on/off's.
Here is the set up........

Signal from Hobbycnc board (via lpt1) turns a 5v solid state relay off and on. Works fine.
As this relay cannot deal with the current that the motor and vaccumm draw, I have this 5v relay switching a 12 v supply , works fine.
Connect this 12v to my 30A relay (which will switch the AC to the router) and it will turn on via Mach 3 control, but will not turn off. If I disconnect the relay, it switches off, reconnect it and it stays off, till mach 3 tells it to turn on again.
If I use a meter to measure the voltage, instead of connecting the relay, it works fine - Mach 3 ON, 12v ....... Mach3 OFF 0v.

Any ideas why this is happening?

5v and 12v supplies are being sourced from a spare connector from the Mach3 computer.

Ive included the schematic to try and make it clearer..