4 transistor problem

[Andrrrei]

Hello,
I`m just a beginner in electronics. I recently saw a question in a quiz and I couldn`t figure out the answer altough it should be easy for you guys.
The question is - Which transistors have to conduct so that both LED`s are ON ? Explain.

 

[BobK]

Both LEDs cannot be on since they are connected to the same nodes with opposite polarity.

But, possibly you misquoted the question. If it is which transistors must conduct for EACH LED to be on, consider this: The voltage on the anode must be higher than the voltage on the cathode. Which transistors must conduct for that to be true for LED1, then for LED2?

By the way, without a current limiting resistor, the LEDs will not be on for long.

Bob

 

[Andrrrei]

Thanks for answering Bob !
Well my newbie intuition will be that Q1-Q4 for LED1 and Q3-Q2 for LED2.
Am I right ?

 

[BobK]

Correct.

This circuit is called an H-bridge, and it is usually used to allow you to run a DC motor in either direction by reversing the polarity.

Bob

 

[Andrrrei]

Well, speaking of bidirectional motor, there was another question in the quiz :

What output driver is used for powering a bidirectional motor ?

What exactly does an output driver means ?

 

[BobK]

Output driver would be the circuit that supplies the voltages to the device. An H-bridge is a driver that can control a bidirectional motor from a single supply voltage. A half-bridge can do that same from a bipolar supply. When I first started out I was very confused by H-bridge thinking it was an abbreviation for half-bridge. It is not. the H refers to the shape of the circuit which you can see in your OP.

Bob