emergency Shutdown and fuse ratings for circuit

Hi,

So I am making a robot, and I have an 11 volt lipo battery supplying my 12v motors through the roboclaw 30a. I have the logic for the RClaw and an Arduino from a separate 7v battery (to prevent the effects of brown outs and other stuff). What I want is to make all this as safe as possible, so I have an emergency shutoff switch.My problem is I want to connect that emergency shutoff to all power supply of both batteries. How can I have one switch controlling 2 power sources at different power voltage ratings ? Do I have to have the switch connected to relays and the switch turns off both relays when pressed, so the 7 volt has a relay on the +ve, and the 11v has a relay on the +ve. The emergency switch feeds both relays so they remain open. Then someone hits the ESw and the relays drop out and the power is cut. This isn't really an emergency shut off IMO,an EShutoff is turning all the power off instantly to no other components (in case they fail etc). Can anyone help me ?

Also what ratings of fuse do I need for both of these curcuits ? The motors have 20 amp stall rating, but have 2 amp rated-load current. So do I let the current go up to 20Amp (so 20a fuse), or do I blow a fuse at say 5amp ? The Arduino and roboclaw should I put that 7volt circuit on a 1 amp fuse ? This is just the logi of the roboclaw obviously not the motor circuit on the roboclaw.

I got kids and they really love to grab hold of the thing, so I'm trying to make it safe, any other safety advice would also be appreciated !

Thanks

PS my web site is here (being built ... )

http://www.roboticsfordreamers.com

Marcus

You need a DPST switch.

Fuses:

Important questions would be how many motors would be running at once, how many can be stalled, and for how long?

In general, most fuses are sure to blow if twice their rated capacity passes through them. Thus, if you have a 5A fuse, it will need to be replaced the first time a motor stalls and tries to draw 20A.

Another point about fuses is that you need to see what the circuit would look like during a 'fuse blow' event. It there is enough resistance, it is possible to short something out and never reach a fuse blow current.

A 'slow blow' fuse is called that since it will take longer for a fault current to blow the fuse. Look at the fuse blow time vs. current charts.

Emergency Power Off (Cut-throat):

You will need to be very clear exactly what is expected by sending the command, and why. Unless there is a way to reset the relays, setting latching relays to 'off' will prevent the project from turning back on! Likewise, if momentary relays are used, then turning off everything will result in just resetting the project and having it turn back on.

Think about alternates to having 'emergency shutdown' turn off the computer. Perhaps the motor section can require a relay turned on to work, and the shutdown just resets the computer? I don't know the project, so this is something you will have to look at.