Relay Connection Figure 2.2 in error ?

[Lloyd Burns]

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? :cool:

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.

[Joe Johnson]

If I see a message from Mr. Burns, I give it a look see.

Thanks for yet another of your frequent and valuable contributions to these forums.

Hats off to Lloyd.

Joe J.

[Lloyd Burns]

Thank you, Dr J

I always look first to see what you have to say.

[kiracofe8]

Look *very* carefully at the diagram. It says "Ground (Blue Relay Module) OR +12VDC (Red Relay Module)" (emphasis added) What they meant was exactly what you suggested. You hook both devices up to ground, not one up to ground and one up to +12V (providing you have a Blue module. Otherwise hook up both to +12V.) In no case should you hook up one to Gnd and one to +12 for the reasons you mentioned. They probably could have come up with a clearer way of presenting this.

Also, for the Red Spike where you are hooking both up to +12V, nothing says that this is supposed to be the unfused terminal block. I'm sure that FIRST meant for you to hook to the fuses, not straight to the battery, they just didn't present the idea too clearly...

[Lloyd Burns]

Originally posted by kiracofe8
Look *very* carefully at the diagram. It says "Ground (Blue Relay Module) OR +12VDC (Red Relay Module)" (emphasis added) What they meant was exactly what you suggested. You hook both devices up to ground, not one up to ground and one up to +12V (providing you have a Blue module. Otherwise hook up both to +12V.) In no case should you hook up one to Gnd and one to +12 for the reasons you mentioned. They probably could have come up with a clearer way of presenting this.

Also, for the Red Spike where you are hooking both up to +12V, nothing says that this is supposed to be the unfused terminal block. I'm sure that FIRST meant for you to hook to the fuses, not straight to the battery, they just didn't present the idea too clearly...

Perhaps you have a different picture than appears in my copy of the hardcopy manual. In mine, the left ends of the devices go to different connections on the fuse block.

The notation you to which refer appears on the block to the right of the devices, and refers (IMO) to the action of Blue and Red Spikes.

As stated before, the operation of this connection is counter-intuitive (0 = on, 1 = off), and could become dangerous in the (perhaps unlikely) event that the wires to the valve get shorted.

[kiracofe8]

Originally posted by Lloyd Burns


Perhaps you have a different picture than appears in my copy of the hardcopy manual. In mine, the left ends of the devices go to different connections on the fuse block.

The notation you to which refer appears on the block to the right of the devices, and refers (IMO) to the action of Blue and Red Spikes.

As stated before, the operation of this connection is counter-intuitive (0 = on, 1 = off), and could become dangerous in the (perhaps unlikely) event that the wires to the valve get shorted.

The way I interpret this, the left end is the Relay and the right end is the fuse block. I read it this way because the left end simply says "+" and "-", which correspond to the "M+" and "M-" outputs on the relay (this labels on this should probably be changed to "M+" and "M-") whereas the other side says Ground OR +12V, which implies that it is the fuse block, because neither of the relay outputs are labeled Ground or +12V. This interpretation is supported by looking at Figure 2.3, where they use the same labels "+" and "-". In this case, those labels must refer to the relay outputs, because hooking those diodes straight up to the battery would make absolutely no sense whatsoever...

Bottom line: Regardless of FIRST's lack of effective visual communication skills, hook up your relays the way you first suggested (one side of each device to Ground, the other side to M+/M-) and you won't have problems...

[Lloyd Burns]

And it would be hard to connect to the +12V before a 20 or 30 A circuit breaker.:o