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Good and Bad Wiring Techniques for Sensitive Electronics CruzPro Ltd.
207 Whau Valley Road
Whangarei 0112
New Zealand
Tel: 64-9-459-1922

Internet: http://www.cruzpro.com

Good and Bad Wiring Techniques for Electronics

There are some simple wiring techniques that can help prevent serious problems when connecting sensitive electronics into the electrical system of your boat or vehicle, especially if electro-mechanical systems such as motors, solenoids or clutches are involved.

While many "sparkies" (New Zealand slang for electrical installer) know what they're doing, there are some cowboys (New Zealand slang for bad sparky) that think that ground is ground and a power line is just another power line. Nothing could be further from the truth! This is where professionals that specialize in installation of electronic instruments are often worth the extra cost over a general electrical installer. In addition to potentially saving you a lot of trouble of having to deal with unreliable operation of your electronics, they may save you a lot of money if the damage caused by transients is not covered by your equipment warranty.

The way that power is distributed between your electrical systems and your electronics systems can be critical to the proper operation of your electronic gear. Not only is wire size important to prevent voltage drops or voltage "brown-outs' but the way the various instruments and electrics are connected and the connection paths themselves can be critical. This is true of both the lines carrying the power and the lines connecting the power ground or "return" path.

Problems can occur when a piece of electronics and an electrical device such as a motor or solenoid share a common electrical path (power, ground or both). A common problem is with the sizing of an electrical wire. You may think that this is easy to calculate because voltage drops vs wire sizes are readily available and you just select the size wire corresponding to the current your electrics and electronics draw. Wrong.

While an electric motor or light bulb might draw 4 amps when it's running it can draw much more current when starting. A 4 amp 50W halogen bulb for instance can easily draw 40 amps when cold! The wire that is sized to only drop from 12V to 10V volts at 4 amps could momentarily drop to just 3 or 4 volts when a halogen bulb or bilge water pump is turned on. This voltage drop or "brown out" might last only a few thousandths, hundredths, or tenths of a second and you might not even notice it. While your light bulb or bilge pump might not be too worried about this momentary voltage drop your electronics might think otherwise. When your electronics equipment manufacture specifies a minimum and a maximum operating voltage they mean it. A 12V instrument that is specified and tested to operate reliably over a range of 9.5 to 33.0 VDC might do all kinds of strange things when presented with a voltage of just 6V - even if for just a fraction of a second.

The second big killer of electronics is voltage spikes or "transients". Transients can occur when you turn OFF electro-mechanical devices such as motors and solenoids. Energy stored in magnetic fields of these devices can create voltage spikes of hundreds of volts when the power is suddenly removed as the magnetic energy is instantly converted back to electrical energy by the collapsing magnetic field. These spikes occur when the motor or solenoid is turned OFF not ON. These short lived voltage spikes usually do no damage to electrical devices but can destroy electronics or cause them to do things you don't expect or want. Data corruption of memories or corruption of the software being run in microcomputers and even destruction of parts of the electronics are often the result of poorly wired circuits.

Lets look at five different ways of wiring a simple circuit with a battery, a motor and an electronic instrument. In order to focus on the issues I have purposely left out switches, circuit breakers, etc. Electrically all five of the connections in Figures 1 to 5 below are identical in the sense that each piece of gear has power and ground connected to it. A cowboy sparky will have done their job and happily collect payment and go their way. Electronically, however, three of these connection methods can cause your electronic instrument to fail or work unreliably and are labeled "BAD" or "WORST". One method is labeled "GOOD" and the connection method of Figure 5 is "BEST".

Common Power or Ground Leads

The problem with connection methods 1 and 2 is that a common wire is shared between the electronic instrument and the motor. The current that flows in the motor will cause a voltage drop to the instrument. This can happen not only in the power lead as in Figure 1 but a voltage drop can also occur in the common ground wire of Figure 2.

Common Power and Ground Leads

Figure 3 is the worst of all in that voltage drops in both the power lead and the ground wires will cause voltage drops to the instrument. Turning OFF the power to the motor can also introduce a voltage spike in the common wires carrying power to the instrument and motor with circuit #3 being the worst offender. At DC bigger wires will reduce the voltage drop problem but voltage spikes are not a DC phenomena but occur at very high frequencies. The resistance of a wire (called impedance) increases with frequency so a wire that looks "heavy" at DC will not look so heavy at the higher frequencies of a voltage spike.

No Common Power or Ground Leads

The circuits in Figure 4 and 5 are much better in that the wires supplying power to the electronic instrument is not carrying any of the current being drawn by the motor. There is no common current path hence no additional voltage drop to the electronic instrument. The only change that the instrument sees is the slight drop that will occur when power is drawn from the battery as opposed to the extra voltage drop due to the resistance of any common wires.

Good batteries have a low internal resistance (or impedance) and work very well to "snub" voltage spikes caused by an electro-magnetic device. By having the wires that power any electronic instruments run all the way back to the battery on a dedicated line without sharing them with any electromechanical devices you will almost eliminate the chances of voltage transients caused by motors, etc. from reaching the electronic instruments. There is still a chance that voltage spikes can be induced to the wires carrying power to the instruments by electro magnetic emissions ("radio" waves) but this is usually much less of a problem and can be reduced or eliminated by routing the wires so that they don't run parallel or near each other.

The circuit of Figure 5 follows the old adage of "an ounce of prevention is better than a pound of cure" by placing a transient absorbing diode across the power leads right at the motor. A 1.5KE18A for 12V systems or a 1.5KE33A for 24V systems are 1500 watt transient diodes and should handle most small electrical motors. Using a 1500 watts transient rejection diode across an engine starter motor or start solenoid will probably not add much protection and may even cause the diode to fail. If all motor manufacturers put these inside the motors we would all experience fewer problems with electronics but since this would increase the cost of the motor, few do. Most all electronics manufacturers put some form of voltage transient protection inside their instruments but this is prevention rather than a cure and only provides protection to a certain level against voltage drop outs and voltage transients.

As shown above, it is not difficult to wire your electronics so that transients and voltage drop outs are virtually eliminated so why not do it right the first time? It is not suggested or required to have every circuit on your boat return all the way to the battery. One motor is unlikely to be damaged by a transient from another motor or solenoid on the same circuit but isolating your electronics on one separate circuit would be a very good idea. Some people even use a complete separate battery for their electronics to isolate problems. Ask your sparky if he or she knows the difference between the five circuits above.

Here is a list of some electromagnetic and electrical devices commonly found on boats that can cause problems to electronics wired to the same circuit:

  • Starter motors/starter solenoids
  • Bilge pumps
  • Compressors
  • Windshield wiper motors
  • Autopilot motors
  • Radar scanners
  • Relays
  • Solenoids
  • Electric cluches and winches
  • Electric windlasses
  • Electric fans
  • Halogen light bulbs

Bert P. van den Berg

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