Thanks Bill. Am I correct in assuming that if terminal 85 and the RF module minus terminal are both connected to the battery negative, they don't need to be connected to one another as I have drawn?

That would be correct as long as they share a common ground bond.

Ron

 

Thanks Bill. Am I correct in assuming that if terminal 85 and the RF module minus terminal are both connected to the battery negative, they don't need to be connected to one another as I have drawn?

True. They both need to connect to the battery negative, and probably separate wires is the better choice.

 

Okay great, I think I've got it all figured out for now. Thanks all for your help.

 

But add a 1 amp fuse in the other feed from the battery to the RF relay section

I was just wondering about this 1 amp fuse. I trust your advice, I'd just like to understand the mechanics behind this so that I can learn from it (then hopefully be able to work more things like this out on my own in the future and not have to ask for so much help on forums!).

I guess my question is, why 1 amp? The RF module is rated at 10a (though I understand it probably wouldn't be a good idea to put 10A through it), why not a 3, or 5 amp fuse? My guess would be you're implying that if 1A of continuous current is being drawn then it means there's a problem, and a 1A fuse is going to pick that up sooner than a 3/5A fuse. And if the application shouldn't draw more than 1A there's no benefit in going for the higher 3/5A fuse. Is that correct? Or is there another reason?

 

The RF modules contacts are rated for 10A.

A little aside. There are relays rated for DC and relays rated for AC. Some have a minimum load, a minimum switched load, an inductive load rating. This is all in the boring details.

Motors are inductive. For a short time they draw a lot of current. The EE rule for an inductor, is that the currnet cannpt change instantaneously. When you size a generator for an inductive load, the VA or product of Volts and Amps needs to handle that startup current. 5x FLA (Full Load Amps) might be a decent rule of thumb. VA is not exactly Watts, it's reactive power/
A fuse or circuit breaker has a delay curve and typically slow-blow fuses are used for motors. Frequent start and stops draw lots of current and may overload a breaker.

Your main objective is to protect the wiring. In a US house you have 20A or 15A circuits. The 20A circuits get 12 AWG and the the 15A get 14AWG wire. The length matters. You want less than 4% Voltage drop. The fuse is designed to blow when the short circuit current is exceeded. Other rules include "continuous loads". They cannot exceed 80% of the rated capacity,

So, wire selection or AWG is based on 2x the length and the tolerated voltage drop for first order issues. if the wire touces the chassi you want the fuse to blow in a controlled place. You don't want the wire to melt. Exceptions, a car starter. The battery actually limits the current.

So you have protection for the low voltage module (1 Amp) and protections for the wiring to the motor 30A.

 

The one amp fuse is to protect the low current wiring that powers the RF module and the relay coil. The 30 amp rating is only for the pwer contacts of the one relay, and the load. The rest of the system that only uses a bit of power and is wired with much thinner wire will not be protected by a 30 amp fuse, if there is a failure or a short circuit. So the one amp fuse is to protect the low power wires.