Before you can plan an installation you need to know the operating voltage and the approximate current drawn by each of the loads. The operating voltage is usually specified, e.g. a 12v motor, 3v bulbs etc., while the current is seldom mentioned. You can measure the current drawn by any load by connecting it to a battery of the correct voltage and connecting an ammeter in series with it, please see Figure 13
. You need then to switch on the circuit and read off the current. For a motor you will need to simulate the load it will be working, so a main drive motor should be fitted into the model and running with its propeller in the water when you measure the current. Most digital multimeters have a scale which will read up to 10 Amps, while I came across an analogue panel ammeter on the Internet which measures 0 to 15A and cost less than £5, Photo 11
. I have found that a cheap digital multimeter like the 318A from Rapid Electronics suits all of my requirements and there’s no need to spend more than around £20 while you can get a very basic type for less than £5, Photo 12
. Alternatively you can obtain a special Wattmeter for higher currents. This clever device will also indicate maximum current, voltage and thus power consumed (volts x amps). They can even be fitted into the model if you wish, and cost around £15 to £40 from specialist model suppliers. Examples are the JP EnErG Power Battery Analyser, Photo 13
, and the PP-Wattmeter Budget, Photo 14
, but other types are available.
Once you know the voltages and currents you can go ahead and plan the installation. For a simple single-motor model with just a speed controller and a steering servo that’s an easy task, especially if your speed controller has BEC (Battery Elimination Circuit). This is a little circuit built into the speed controller which reduces the voltage from the main motor battery and passes it down the 3-wire connection from the speed controller to supply power to the receiver, Figure 14
. When a speed controller doesn’t have a BEC then you can either fit a remote one or power the receiver from a separate 4-cell NiMH pack.
For more complicated installations you might well consider using a power distribution board such as the ACTion P92, Photo 15
. This takes the power from the main battery and directs it through fuses to five pairs of terminals, all at the same voltage as the supply battery. It also has a reduced-voltage fly-lead with a plug to power the receiver i.e. the BEC device is fitted onto the power board and not inside the speed controller, Figure 15
. A similar fused power board, albeit with switches and without the BEC output, is available from Hunter Systems while there is a very sophisticated version from Harbor Models of San Diego which includes voltage reduction for the outputs as well. With this you can use just one 12v battery to power two 12v circuits, two 6v circuits, two 3v circuits and the 4.8v receiver! It does however come with a hefty price tag and because of its size is really not suitable for small models. Note that where you have fitted a BEC speed controller you must NOT also use a separate power supply to the receiver such as a battery pack or the flylead on the ACTion P92. Further, if your model has two or more BEC-equipped speed controllers then you must disable the positive wire to the receiver on all but one of them, Figure 16
Where multiple voltages are required, a little bit of planning can often resolve the matter down to using one battery. For example, if your main power supply is 12 volts but you want to run a 6v lighting circuit from it as well, then simply have two 6v bulbs in series as one circuit (total supply voltage = 12v). You could incorporate a resistor instead of one of the bulbs, but it will usually need to be of a very low resistance and high power capacity in order to 'absorb' the unwanted power. It will, of course, also get pretty hot in operation. In such circumstances it’s easiest to hide the second bulb internally if you can’t make use of it elsewhere on the model. If you do find yourself needing a different voltage for just one circuit then it’s often the best advice to use a separate battery to supply that circuit rather than mess around with resistors etc.
If you really do have problems with planning circuits then you could take a look at the ACTion website, which includes over 170 full-colour wiring diagrams for all sorts of model boats from simple launches to multiple screw warships. See www.action-electronics.co.uk
. It is naturally biased towards ACTion’s own products. but the principles of installation apply equally to most electronic gizmos.