- Using the right tools gets the job done right the first time, but can save many hours in troubleshooting a bad wiring job.
Always remember the adage, "You don't buy tools, you invest in them". Proper tools make the job easier and produce better results.
- 1 Overview
- 2 Basics
- 3 Tool Safety
- 4 Electrical/Electronic Tools
- 4.1 Digital Multimeters
- 4.2 Accurate Measurements
- 4.3 Metering Safety
- 4.4 Simple Troubleshooting with a MultiMeter
- 5 Cheap Electrical Short Circuit Detector
- 6 Bi-Color LED's as Track Indicators
- The safe use of your tools is in your hands. Always use the proper tools and proper safety equipment. Check with the manufacturer for more info.
- Remember that sharp tools are safer to use than dull ones.
First things first. You'll need some basic tools...
- Short circuit detector (A home made one can be made for a few dollars using readily available parts - see below)
- Wire strippers. They should be adjusted to avoid nicking the wire. Using a knife to strip wire usually results in a nicked conductor, which ultimately breaks at the nicked point.
- Electrical Tape, various colours are available which aid in identification of wires and circuits.
- Diagonals (sometimes called side cutters or wire cutters). Use tools designed for the metal you plan to cut.
- Linesman's Pliers
- Screwdrivers, various sizes and types to fit the fasteners to be used. Remember to use the correct size blade for the screw. Common bits are Robertson, Philips, and the common slot.
- Hammer, if you are also using staples.
- Stapler: With the proper attachments and staples, they can be very useful for fixing wires in place.
- Safety glasses and gloves are not a bad idea.
- Crimpers. If you wish to terminate your wiring with crimp on connectors, you need a pair of crimpers. Don't waste your time with the one that comes in the box as part of a kit. A proper crimping tool not only works better, but is easier to use.
A drill and proper sized bits may be needed to make holes for wires to pass through. Hand drills will work, but an electric variable speed drill is faster. As always, sharp bits are important as the drill bit should cut on its own with just light pressure. A VSR drill (Variable Speed Reversible) can also be used to drive screws with appropriate bits.
- As with all hand tools, they should be inspected to determine they are in good condition. Screwdrivers in particular: Worn, rounded, bent or chipped blades can be dangerous to use. Same applies to dull knives: They are more likely to cut or injure you than a sharp one. Dull bits in your drill mean excessive pressure is needed to force the bit through, which can result in damage or injury should the drill suddenly punch through the material, or jam and cause the entire drill (and you) to violently twist. Loose material should be properly clamped to avoid it suddenly spinning if the bit grabs.
Remember, the fastest way to dull a drill bit is to turn it in reverse.
A Multimeter is a very useful tool. It is useful enough that should you consider getting one, get a good one. Those cheap analog ones (the meter with the needle) are not a good choice. The best choice is a digital multimeter (DMM). They are almost fool-proof.
They come in handy checking for shorts, opens, or continuity. They are also useful for figuring out the wiring on a locomotive or if your power supply is working. They have other applications like check automotive light bulbs, presence of voltage on a fixture, etc. Many household applications too.
They are not really good at measuring the digital waveform used with DCC voltages. A DMM is designed with pure 50 or 60 Hz sine waves in mind, not a high frequency digital waveform. That is where a RRAmpmeter comes into play.
The RRAmpmeter is a DMM designed from the start for DC and DCC applications. It can measure voltage on the track and current draw. It is a useful tool if you have problems later on. It can also be installed in line to measure current and voltage on an operating layout.
Digital Multimeters (DMM) range from cheap to expensive. As a general rule, a low cost DMM may be adequate for low voltages. If you plan to use your DMM for more than just track work, buy a good one. Cheap ones can be a safety hazard when used on high voltage (more than 48V) circuits. As with all tools, cheap tools can be more dangerous than the proper one for the application. A higher quality DMM will not only last longer, but is more accurate as well. Avoid the older analog meters, the ones with the needle that indicates the quantity being measured. The low cost analog meters introduce their own problems into the equation, making it difficult to diagnose an issue accurately. A DMM appears almost as an open circuit during voltage measurements, where a cheap analog meter can appear as a 20,000 ohm load in parallel with the circuit being tested. Parallax error is another issue that will come with an analog meter.
True RMS versus Average Readings
One important aspect of a digital multimeter is the ability to make accurate RMS measurements. Many of the better quality instruments will incorporate true RMS measurements, where lesser instruments will use average values. The difference is that a true RMS meter will give you an accurate indication of the DC heating value of an AC waveform. It will take into account any distortion, harmonics and DC content in the waveform to arrive at the value.
- Many True RMS (TRMS) meters have significant errors when measuring a square wave signal, such as that of a DCC Waveform. The error can be as much as 10% or more.
Cheaper digital multimeters do not provide true RMS measurements, they measure the average value of an AC voltage, and are calibrated to correct the result to the RMS value for a sine wave. Under normal usage this works well, but errors occur if harmonics are present, causing the error to become progressively worse as the harmonic content increases. In the case of a square waveform, the resulting error can be more than 11%, and the duty cycle of the waveform will introduce even more errors into your reading. If DC is present, the error will be even greater.
With many DMMs, the maximum frequency is 1kHz, after that the accuracy is no longer guaranteed. With the DCC waveform being in the 8 to 10 kHz range, you cannot expect much accuracy even with a good DMM.
Why is this important?
Unless you use the same meter every time, it is not possible to compare previous and future readings with what you see today. Also comparing readings with other modellers using different meters will not provide any useful information, as they may be using a meter with better (or worse) specifications than yours. If comparisons are made using quality meters with similar specifications, the results are much more useful.
Understanding DMM Specifications
A Basic Digital Multimeter will have the following capabilities:
- Current, AC/DC
- Voltage, AC/DC
A more sophisticated model may offer additional functions, such as:
- Continuity Mode (Beeps when the circuit is complete)
- Transistor testing
- Additional features do not mean the device is more accurate!
Periodic recalibration is also recommended. See the manual. All DMMs drift over time.
- Many DMMs will automatically select the best range for the input being measured. Some may also offer the user the ability to manually select a range.
- The DMM will display the polarity of the reading automatically with respect to the common lead. You don't have to worry about connecting the leads to the positive and negative points, the meter will indicate whether the positive (red) lead is positive or negative with respect to the common (black) lead.
The only true way to get accurate measurements of a DCC signal is with an oscilloscope. The next best thing is the RRampmeter, which is specifically designed for the DCC waveform. A good high quality DMM will cost more than a RRampmeter. But it is useful for other troubleshooting and maintenance work you will have.
Meter loading is when the meter alters the circuit parameters and the resulting readings. The DMM excels here, having an impedance of megohms per volt. Cheap analog meters (the ones with the needle moving across a scale) can have impedances as low as 1000 ohms per volt. That can cause problems by putting a low resistance in parallel with an equal or higher resistor, resulting in misleading readings.
Another issue is safety. High quality electrical measuring devices are designed for safe usage on high voltages. They will be marked as to their limits. Cheap meters can be dangerous at high voltages (which is defined as any voltage over 48V).
Always ensure that you have the correct connections and range selected before connecting the meter. Many have fuses on the Amps circuit in case of overloading. Do not attempt to measure voltage with the leads on the current terminals. Do not attempt to measure resistance on an energized circuit.
Simple Troubleshooting with a MultiMeter
Measuring Current (Ammeter)
Verify the connections and settings of your DMM. Often you much move a lead from one connection to another for current measurements. Note the maximum current. Many DMMs have a fuse to limit it, and will blow if overloaded.
Once the meter is configured for use as an ammeter, connect it in series with the load. This means breaking the circuit and inserting your ammeter in it to complete the circuit, so all the current flows through it. Connecting the meter in parallel with the source or load will not be good, and may damage your meter.
Note on Meter Fuses
If you blow the current protection fuse, always replace it with an exact replacement fuse. This is for safety. In some meters, the fuse is used as a shunt, and the improper replacement will degrade performance.
Your voltmeter can be used to measure voltage (potential) across the terminals of a power supply, or across the rails. The voltmeter goes in parallel with the load.
The voltmeter measures the potential difference between two points. So if you place a probe on either side of a gap in the rails, it will indicate if there is a difference across the gap. A voltage would indicate that the rails are of opposite polarity. No voltage would indicate an in-sync condition, since there is no difference between either side of the gap. It could also indicate there is no connection to a power source on one side...
Example: If you connect a voltmeter across the terminals of an electrical switch:
- With the switch open you should see full voltage.
- If the switch is closed (circuit is completed) there will be no voltage.
Connecting a voltmeter across a resistor will measure the voltage drop in accordance with Ohm's Law. Current flowing through the resistor causes a voltage across it proportional to the resistance. If full voltage is appearing, the resistor is bad as there is little to no voltage drop. Resistors tend to fail high, so the current flow/voltage drop will be minimal.
- Important: An Ohmmeter is never used on an energized circuit!
The Ohms (Ω) function measures resistance. It does that by supplying a small current from the internal battery to the probes, and measuring the amount of current flowing. Lots of current, low resistance. No current, the resistance is infinity (an open circuit.) Which is why the scale on an analog ohmmeter is in reverse (zero is at the right, or "full scale deflection".)
Before using your ohmmeter, touch the two probes together and look at the display. The meter should indicate a value close to zero ohms. It will never be perfect, as there is resistance in the leads. This check verifies the meter is working correctly. If using an analog meter, adjust the zero control potentiometer to bring the needle to zero.
When the probes are not touching, the display should indicate infinity. Read the manual that came with your meter to verify how the open circuit or infinity value is displayed. Many DMMs will display a "1" at the left side of the display.
The ohmmeter feature can be used to check fuses for continuity, measure resistors, verify the anode of a diode, and other uses. For checking track wiring, you can use it to verify Rail A is connected to the correct bus wire, since it should have zero resistance between the bus connection and the rail. Rail B should be an open circuit, if not you have a wire crossed somewhere.
Your ohmmeter is also used to check motor wiring, verifying that the motor isn't connected to the frame. Between the frame and the motor's connections, the meter should indicate infinity. Other values indicate there is a connection somewhere. Once the motor's brushes are completely isolated, the readings should be infinity between the frame and the brush connections.
Cheap Electrical Short Circuit Detector
IMPORTANT - Build a Short Detecting Beeper BEFORE You Start ANY DCC Track Wiring!
When doing any wiring on a track segment, you should connect a buzzing short circuit detector to the track block you're working on. This will help you detect shorts before soldering a connection, unless you really like having to cut newly connected wires until the short is found! You can build one of these using some cheap Radio Shack parts.
- 18-22 gauge wire (just a few feet)
- 2 Alligator Clips
- 9 volt battery
To use, you simply attach an allligator clip to each rail you are wiring. Make sure you disconnect your booster - it may show up as a short and/or fry the buzzer. If there is a short between the rails, the buzzer will go off so you'll quickly know there is a problem.
If you have several boosters (or power districts), you will need to move the beeper to each district as you wire. If there are several teams wiring different districts at once, just make them each a buzzer - they're cheap insurance!
Bi-Color LED's as Track Indicators
A bi-colour LED can be hooked up as indicators around the layout is a convenient way to see the power status of sections of the layout. The LED indicates whether a track section is powered up, if "Zero-stretching" Analog mode is being used and its local direction, and can even be used to indicate if the GAPS in a reverse section are matched or not.
You will need:
- One 2 lead bi-colour LED (Any cheap 2 lead bi-colour LED will work)
- One 1K ohm, 1/4 watt resistor or larger
Connect the 1K resistor in series with either one of the LED leads to make a "ballasted" LED (current limited). With the 2 leaded bi-color LED there is no strict polarity to observe, the emitted color depends on the direction the LED leads are connected to the track. If you plan on using zero-stretching, it's recommended that you connect neighboring sections with the same polarity so the LED will be of consistant color.
Simply connect the "ballasted" LED across the track to indicate the track is powered. If you connect a "ballasted" LED across one of the double gaps of a reverse section the LED will be OFF (unlit) when the gap polarity is matched.
- Wiring - Primary wiring article.
External Link: Check points for corrrect crimping