Diodes hurt my brain

[joe61]

I'm trying to understand transistors. I've put together an h-bridge on a breadboard that works, but I'm not clear about why the diodes work the way they do.

Given the setup on http://www.robotroom.com/BipolarHBridge2.html for an example. It looks to me like the green wires next to the pager motor provide more than one path back to the transistors. What would stop current from flowing back along on of the non-diode protected paths?

Thanks

Joe

[Soeren]

Hi,

Given the setup on http://www.robotroom.com/BipolarHBridge2.html for an example. It looks to me like the green wires next to the pager motor provide more than one path back to the transistors. What would stop current from flowing back along on of the non-diode protected paths?

Yeah, too bad that Dave Crook cannot draw a regular schematic, or else it would be clearer to you, I'm sure.
Each side of the motor goes to a node common to both transistors in that side plus to 2 diodes which is reverse biased under normal conditions (they're there to catch inductive fly-back from the motor).
It's a very rudimentary H-bridge, as there's no control at all, just the 4 bare transistor switches with base resistors.

Anyway... Your subject line indicates something different from the content of your post, so I'm not sure whether this answers your doubts.

[MikeK]

I think what he's confused about is how the diodes do what they're supposed to do in that circuit...?

Diodes conduct electricity from anode to cathode.  When a motor is stopped the collapsing field induces a current to run backwards to the way it was running.  The diodes are there to direct the current back to the battery.  If the diodes wheren't there the "back" current would shoot through one or more transistors and destroy them.

[joe61]

I think what he's confused about is how the diodes do what they're supposed to do in that circuit...?

Diodes conduct electricity from anode to cathode.  When a motor is stopped the collapsing field induces a current to run backwards to the way it was running.  The diodes are there to direct the current back to the battery.  If the diodes wheren't there the "back" current would shoot through one or more transistors and destroy them.

Yes, it's just that there is more than one branch that the back current from the motor can take. I'm not saying it well because I'm confused, sorry.

The way it looks to me is that back current would come from the leads attached to the motor, and go up the green wire immediately above it. From there, it could go through the diode, or it could take a right/left and head back to the transistor. What makes the current go up through the diode?

Thanks

Joe

[joe61]

Yeah, too bad that Dave Crook cannot draw a regular schematic, or else it would be clearer to you, I'm sure.

I'm afraid you're giving me too much credit :-).

The h-bridge I actually built was (mostly) taken from http://www.mcmanis.com/chuck/robotics/tutorial/h-bridge/bjt-impl.html, but it took me a long time to figure out how to get from the schematic to the breadboard. So while Cook's example might not be the best, it has the virtue of being easier for me to find my way around.

Thanks

Joe

[joe61]

I think the light just went on. If the motor is stopped then the transistor is off, so the diode is the path of least resistance.

So the next question is why a diode is needed for this? Wouldn't a low value resistor work too?

Joe

[MikeK]

A diode conducts in one direction only, the resistor conducts both.

[TheBadger]

It's a very rudimentary H-bridge, as there's no control at all, just the 4 bare transistor switches with base resistors.

Anyway... Your subject line indicates something different from the content of your post, so I'm not sure whether this answers your doubts.

Hi, This is the only kind of H bridge I have ever used, so I was wondering what makes up a more advanced H bridge if this is only rudimentary?

[Soeren]

Hi,

I was wondering what makes up a more advanced H bridge if this is only rudimentary?

Just a quick reply (going to visit my grandson - and his parents of course )
Bob Blick has got an H-bridge, a Servo Pulse to Dual H-bridge and a Servo Pulse to PWM that you may like to study.

[Billy]

When a motor is stopped the collapsing field induces a current to run backwards to the way it was running.

This is a bit of a nit-pick but in the interest of truth and understanding:
In inductors (such as motors), the collapsing field will cause the current to continue in the same direction it was flowing. That current attempting to flow in the same direction is what causes a high reverse voltage (high reverse voltage being required to stop the current). The diodes we have been talking about allow the current to continue to flow through the motor and die out due to resistive losses in the motor as well as back EMF effects.