1. Divide your length of wire by 10. (Make sure that you measure the length in feet.)
2. Use the result to multiply the number in the table.
The table also arbitrarily assumes that you have a 12-volt supply. Again, you will have to make allowances if you are using a different voltage. So, to use the table:
1. Divide 12 by the actual voltage of your power supply.
2. Use the result to multiply the number in the table.
I can summarize those two steps like this:
Percent voltage lost = P × (12 / V) × (L / 10)
where P is the number from the table, V is your power-supply voltage, and L is the length of your wire.
Figure 5-104. The voltage drop imposed by wiring will depend on the current and the resistance in the circuit. The drop will be greatest when the resistance of the circuit is low and the amperage is high.
Theory
Calculating voltage drop (continued)
This table shows the percent voltage lost in a circuit with 10-foot wire at 12 volts.
Wire Gauge
Amperes
1
2
3
4
5
6
7
8
9
10
10
0.08
0.17
0.25
0.33
0.42
0.50
0.58
0.67
0.75
0.83
12
0.13
0.27
0.40
0.53
0.66
0.80
0.93
1.1
1.2
1.3
14
0.21
0.42
0.63
0.84
1.1
1.3
1.4
1.5
1.9
2.1
16
0.33
0.67
1.0
1.3
1.7
2.0
2.3
2.7
3.0
3.4
18
0.53
1.1
1.6
2.1
2.7
3.2
3.7
4.3
4.8
5.3
20
0.85
1.7
2.6
3.4
4.3
5.1
6.0
6.8
7.7
8.5
22
1.3
2.7
4.0
5.4
6.7
8.1
9.4
11
12
13
24
2.1
4.3
6.4
8.6
11
13
15
17
19
21
26
3.4
6.8
10
14
17
20
24
27
31
34
28
5.4
11
16
22
27
32
38
43
49
54
30
8.6
17
26
34
43
52
60
69
77
86
Remember, though, that the wire resistance will be higher if you are using stranded copper wire or tinned copper wire, and this will increase the percentage of voltage lost.
Experiment 33: Moving in Steps
Time now to build something more sophisticated: a cart that orients itself toward a light source. I’m going to tell you all you need to get started on this project, but this time I won’t go all the way to the end in exhaustive detail. I want you to get into the habit of figuring out the details, improving on plans, and eventually inventing things for yourself.
You will need:
555 timers. Quantity: 8.
Trimmer potentiometer, 2K linear. Quantity: 2.
LEDs. Quantity: 4. If you get tired of using series resistors to protect LEDs in a 12-volt circuit, consider buying 12-volt LEDs such as Chicago Miniature 606-4302H1-12V, which contain their own resistors built in. However, the schematic in Figure 5-108 assumes that you will use regular 2V or 2.5V LEDs.
Stepper motor: Unipolar, four-phase, 12-volt. Parallax 27964 or similar, consuming 100mA maximum. Quantity: 2.
Photoresistors, ideally 500 to 3,000Ω range. Quantity: 2.
ULN2001A or ULN2003A Darlington arrays by STMicroelectronics. Quantity: 2.
CMOS octal or decade counter. Quantity: 2.
Various resistors and capacitors.
Exploring Your Motor
I’ve specified a unipolar, four-phase, 12-volt motor because this is a very common type. A typical sample is shown in Figure 5-105. If you can’t easily find the one that I’ve listed, you should feel safe in buying any other that has the same generic description. “Unipolar” means that you don’t have to switch the power supply from positive to negative and back to positive again, to run the motor. Four-phase means that the pulses that run the motor must be applied in sequence to four separate wires. Because you will be running your motor directly from 555 timers, the lower its power consumption, the better.
Figure 5-105. A typical stepper motor. The shaft rotates in steps when negative pulses are applied to four of the wires in sequence, the fifth wire being common-positive.
First, though, we can apply voltage to the motor without using any other components at all. Most likely it will have five wires already attached, with the ends stripped and tinned, so that you can easily insert them into holes in a breadboard, as shown in Figure 5-106. Check the data sheet for your motor; you should find that four of the wires are used to energize the motor and turn it in steps, while the fifth is the common connection. In many cases, the common connection should be hooked to the positive side of your power supply, while you apply negative voltage to the other four wires in sequence, one step at a time.
Figure 5-106. The simplest test of a stepper motor is to apply voltage manually to each of its four control wires, while a piece of duct tape, attached to the output shaft, makes it easy to see how the motor responds.
The data sheet will tell you in what sequence to apply power to the wires. You can figure this out by trial and error if necessary. One thing to bear in mind: a stepper motor is very tolerant. As long as you apply the correct voltage to it, you can’t burn it out.
To see exactly what the motor is doing, stick a piece of duct tape to the end of the shaft. Then apply voltage to wires, one at a time, by moving your negative power connection from one to the next. You should see the shaft turning in little steps.
Inside the motor are coils and magnets, but they function differently from those in a DC motor. You can begin by imagining the configuration as being like the diagram in Figure 5-107. Each time you apply voltage to a different coil, the black quadrant of the shaft turns to face that coil. In reality, of course, the motor turns less than 90° from one coil to the next, but this simplified model is a good way to get a rough idea of what’s happening. For a more precise explanation, see the upcoming section “Theory: Inside a stepper motor.”
Bear in mind that as long as any of the wires of the motor are connected, it is constantly drawing power, even while sitting and doing nothing. Unlike a regular DC motor, a stepper motor is designed to do nothing for much of the time. When you apply voltage to a different wire, it steps to that position and then resumes doing nothing.
The coil inside the motor is holding the shaft in position, and the power that the motor draws will be dissipated as heat. It’s quite normal for the motor to get warm while you’re using it. The trouble is, if you use a battery to power it, and you forget that you have it connected, the battery will not hold its charge for long.
Figure 5-107. This greatly simplified diagram helps in visualizing the way in which a stepper
