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Unread 30-01-2002, 20:11
Lloyd Burns Lloyd Burns is offline
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FRC #1246 (Agincourt Robotics)
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Join Date: Jun 2001
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Relay Connection Figure 2.2 in error ?

In THE ROBOT, page 10, in 2.1.3 Relay Modules, Figure 2.2 purports to show the connection for "independent control of two devices from a single relay module", where the obviuos candidates for 'device' being pneumatics valves.

The manual shows one device connected between + (presumably +12V) and one of the relay terminals (say M+). The other device is connected to - (presumably the - battery terminal (or 0V).

When the power goes on, with no relay active, device one is ON, since both M terminals are at 0V (in a Blue Spike). If the 'fwd' of this realy is activated, M+ will go to +12V and the valve will go OFF ! Not intuitive !

Notice that the +12V input to the relay is the fused line, and has a fuse block breaker in deries with it. When Device 1 is ON, the only breaker in the circuit is rated at 60A. The wire to the device, if injured by grabbing or collision, is only 16 ga. wire. Incandescense at 200A, anyone?

As for the other device, it will be between +12V through the M- terminal and OV via the return on the fuse block, when the 'rev' signal is sent. This is reasonable operation: '1' for ON, '0' for OFF.

SOLUTION

If both valves were returned to ground, then one valve would come on when 'fwd' = 1, and the other would come on when 'rev' = 1, which would be a saner way to run a robot.

Another big problem is that connecting to the fuse block, according to the checklist of Update 4, is a No No.

A couple of urgent changes ?

A complication: the Red Spikes are arranged to rest on +12V, not 0V. the circuit would have to be re-arranged for them, with coils returns going to the fuse block breakered + terminal.

Even in the circuit from the book, though, neither device would be so grossly unprotected as with a Blue Spike.

I wrote to FRCtech2002 but it may take some time there to go through.
 


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