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I was watching this video tutorial on H-bridges.

The video presenter said that when one input is HIGH and the other LOW, the output of the H-bridge is Vcc/2, while in the other case, it is -Vcc/2.

In other sources, I saw that the outputs are Vcc and -Vcc.

What exactly is the case?

ocrdu
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user1584421
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2 Answers2

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The video is using a confusing example. There's a battery connected to the supply rails of the half-bridge but there is no connection to ground. It's implied that circuit ground is somehow connected to the "middle" of the battery. The circuit in the video cannot work as drawn since there is no return path. This is a simplified version of what the presenter in the video was trying to get at.

schematic

simulate this circuit – Schematic created using CircuitLab

vir
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  • Thanks! But are the outputs `Vcc and -Vcc`? Or `Vcc/2 and -Vcc/2`? – user1584421 Oct 18 '22 at 21:45
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    The output across the motor in the schematic I posted is +9 or -9V. In the video, the two batteries are (over)simplified to one battery and ground is implied to be in the "middle" of it, which would make it Vcc/2. They're defining their source in a confusing manner, I wouldn't get too worried about it. – vir Oct 18 '22 at 21:48
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The video is talking about a 1/2 bridge which is different then a full H bridge. When looking at voltage you need to define your reference point which you forgot to do. By default this is considered ground therefore +VCC and -VCC would be correct. If you use the common point (ground) where the two batteries are connected in series at the output you will have Plus if the upper switch is closed, minus if the bottom switch is closed and a fault (short) if both switches are closed. If you use another reference point the things will definitely change. Using the minus to the plus you will get the sum of both batteries, the converse is also true.

Gil
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  • Thank you! But are you sure about 1/2 H-Bridge being that the voltage output is half the original voltage? The answer here: https://electronics.stackexchange.com/questions/566546/difference-between-h-bridge-dual-half-bridge-and-quad-half-bridge states that an H-Bridge can be used to switch on or off current passing a DC motor, but only in one direction. – user1584421 Oct 19 '22 at 08:22
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    You missed the **reference point**. The way your link showed was a switch to + and a switch to - with the motor connected in between. The motor common was the reference point. Use your multimeter and connected to the junction point of two batteries in series then meas the voltage at the ends of the batteries you will get + and -. For a DC motor that they are referencing to reverse direction the polarity has to be reversed, it does not care how that is done. – Gil Oct 19 '22 at 16:31