When the coils on "Relay 1" and "Relay 4" are pulled high (electricity is flowing through them), then the motor will spin forwards (see "Image 1"). The only thing that will slow down the spin of the motor is if you have a controller that can modulate the 9VDC the power signal BEFORE it enters the H-bridge. What this means for you is that the motor will spin as fast as it can in one direction and then when reversed, spin as fast as it can in the other direction. The power must be decreased to slow down the motor before it goes through the H-bridge). the H-bridge itself cannot modulate the power source to down the motor. An H-bridge made using relays (like the one we are making) cannot easily be used to change a motor's speed (unless, of course, the motor voltage is being supplied from a power source capable of being modulated. The best type of H-bridge is made with transistors since these can easily be used to control a motor's speed (by using the smaller voltage to modulate the much larger motor voltage). H-bridges can be made with either 4 relays or 4 transistors. These two separate voltage sources are kept isolated from one another. The other set of switches allows electricity to flow in the opposite direction.Īnother important characteristic of an H-bridge is that it typically can use a smaller voltage (5VDC from a micro controller, for instance) to control a much larger voltage (12VDC used to power a motor). One set of switches when closed allows electricity to flow one way. An H-bridge is a series of four controllable switches in which there are two sets of two switches.
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