Checkpoint LAB USB-RS232 Instrukcja Użytkownika Pobierz pdf (Strona 10)

H BRIDGE

How H Bridges work

The switches are turned on in pairs, either

high left and lower right, or lower left and

high right, but never both switches on the

same "side" of the bridge. If both switches

on one side of a bridge are turned on it

creates a short circuit between the

battery plus and battery minus terminals.

Anything that can carry a current will

work, from four SPST switches, one DPDT

switch, relays, transistors, MOSFETs.

Brake mode. Upper or lower switches

closed.

H BRIDGE

SUPPRESSION DIODES AND ARCING

Why? Protect the four transistors.

How do Suppression Diodes help? First, note these diodes are

reversed from normal power supply so no current will flow when

the motor is off or when the motor is on.

The issue is when a motor switches off. Since internal electrical

circuitry of a motor is a coil of wire (i.e. inductor), current in an

inductor needs to continue to flow. When the diagonal

transistor pair switches off and thus causing I

go to zero, the

motor coils (i.e. inductors) are still charged with current and

must flow through the internal inductors in the motor. This is

called Back EMF. Without suppression diodes, the current must

flow through the transistors which are switched off. Since the

transistors are off, the Back EMF will go in 100’s of voltages to

push the current through the motor’s inductor circuitry. This

Back EMG voltage is high enough to damage transistors. So

the current path when the transistor pair switches off is through

the motor (Back EMF), through the suppression diode to V+,

through the power supply, through the Gnd, through the lower

suppression diode and then back into the motor. This current

surge will end without damaging the electronics. Okay when

current flows into the positive lead of the power supply, the

voltage level will dip or perhaps even go negative for a short

period of time. You can trigger your scope to see this.

Electronics such as a microcontroller can easily reset itself when

this happens. Some times at random.

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Checkpoint LAB USB-RS232 Instrukcja Użytkownika Strona 10