Make: Electronics (19 page)
Authors: Charles Platt
Figure 2-57.
Four assorted 12-volt relays, shown with and without their packages. The automotive relay (far left) is the simplest and easiest to understand, because it is designed without much concern for the size of the package. Smaller relays are more ingeniously designed, more complex, and more difficult to figure out. Usually, but not always, a smaller relay is designed to switch less current than a larger one.
Fundamentals
Inside a relay
A relay contains a coil of wire wrapped around an iron core. When electricity runs through the coil, the iron core exerts a magnetic force, which pulls a lever, which pushes or pulls a springy strip of metal, closing two contacts. So as long as electricity runs through the coil, the relay is “energized” and its contacts remain closed.
When the power stops passing through the coil, the relay lets go and the springy strip of metal snaps back into its original position, opening the contacts. (The exception to this rule is a latching relay, which requires a second pulse through a separate coil to flip it back to its original position; but we won’t be using latching relays until later in the book.)
Relays are categorized like switches. Thus, you have SPST relays, DPST, SPDT, and so on.
Compare the schematics in Figure 2-58 with the schematics of switches in Figure 2-38. The main difference is that the relay has a coil that activates the switch. The switch is shown in its “relaxed” mode, when no power flows through the coil.
Figure 2-58.
Various ways to show a relay in a schematic. Top left: SPST. Top right and bottom left: SPDT. Bottom right: DPDT. The styles at bottom-left and bottom-right will be used in this book.
The contacts are shown as little triangles. When there are two poles instead of one, the coil activates both switches simultaneously.
Most relays are nonpolarized, meaning that you can run electricity through the coil in either direction, and the relay doesn’t care. You should check the data sheet to make sure, though. Some relay coils work on AC voltage, but almost all low-voltage relays use direct current—a steady flow of electricity, such as you would get from a battery. We’ll be using DC relays in this book.
Relays suffer from the same limitations as switches: their contacts will be eroded by sparking if you try to switch too much voltage. It’s not worth saving a few dollars by using a relay that is rated for less current or voltage than your application requires. The relay will fail you when you need it most, and may be inconvenient to replace.
Because there are so many different types of relays, read the specifications carefully before you buy one. Look for these basics:
Coil voltage
The voltage that the relay is supposed to receive when you energize it.
Set voltage
The minimum voltage that the relay needs to close its switch. This will be a bit less than the ideal coil voltage.
Operating current
The power consumption of the coil, usually in milliamps, when the relay is energized. Sometimes the power is expressed in milliwatts.
Switching capacity
The maximum amount of current that you can switch with contacts inside the relay. Usually this is for a “resistive load,” meaning a passive device such as light bulb. When you use a relay to switch on a motor, the motor takes a big initial surge of current before it gets up to speed. In this case, you should choose a relay rated for double the current that the motor draws when it is running.
Procedure
Turn the relay with its legs in the air and attach wires and LEDs as shown in Figure 2-59, with a 680Ω resistor (a 1K resistor will be OK if you don’t have the correct value). Also attach a pushbutton switch. (Your pushbutton switch may look different from the one shown, but as long as it is a SPST pushbutton with two contacts at the bottom, it will work the same way.) When you press the pushbutton, the relay will make the first LED go out and the second LED light up. When you release the pushbutton, the first LED lights up and the second one goes out.
Figure 2-59.
As before, you can use patch cords, if you have them, instead of some of the wired connections shown here.
How It Works
Check the schematic in Figure 2-60 and compare it with Figure 2-59. Also see Figure 2-62, which shows how the pins outside the relay make connections inside the relay when its coil is energized, and when it is not energized.
Figure 2-60.
Same circuit, shown in schematic form.
This is a DPDT relay, but we are only using one pole and ignoring the other. Why not buy a SPDT relay? Because I want the pins to be spaced the way they are when you will upgrade this circuit by transferring it onto a breadboard, which will happen very shortly.
On the schematic, I have shown the switch inside the relay in its relaxed state. When the coil is energized, the switch flips upward, which seems counterintuitive, but just happens to be the way that this particular relay is made.
When you’re sure you understand how the circuit works, it’s time to move on to the next step: making a small modification to get the relay to switch itself on and off, as we’ll do in
Experiment 8
.
Figure 2-61.
The layout of the pins of the relay, superimposed on a grid of 1/10-inch squares.This is the type of relay that you will need in
Experiment 8
.
Figure 2-62.
How the relay connects the pins, when it is not energized (left) and when it is energized (right).
Experiment 8: A Relay Oscillator
You will need:
- AC adapter, breadboard, wire, wire cutters and strippers.
- DPDT relay. Quantity: 1.
- LEDs. Quantity: 2.
- Pushbutton, SPST. Quantity: 1.
- Alligator clips. Quantity: 8.
- Resistor, approximately 680Ω. Quantity: 1.
- Capacitor, electrolytic, 1,000 μF. Quantity: 1.
Look at the revised drawing in Figure 2-63 and the revised schematic in Figure 2-64 and compare them with the previous ones. Originally, there was a direct connection from the pushbutton to the coil. In the new version, the power gets to the coil by going, first, through the contacts of the relay.
Figure 2-63.
A small revision to the previous circuit causes the relay to start oscillating when power is applied.
