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Here's the H bridge I'm building

  • Vcc = 9v
  • 10 k resistors for the pullup
  • 1k resistor for the transistor
  • 2 PNP BC557
  • 2 NPN BC547
  • 2 push buttons

It should have a DC motor but I'm using LED and a resistor instead for safety purposes.

schematic

(original)

enter image description here

so I'm connecting everything exactly like the sheet, but still not working.

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    \$\begingroup\$ One of your blue ground lines is going to the power rail, that might be part of it. Keep in mind that the LEDs are one way, so it will not turn on if you press one combination of the buttons. Also, check the datasheet on your BJT and make sure the pins are right. What's your power supply? Also, consider making your breadboard clear, it's super hard to track down problems when it looks like this. I don't agree with the answers saying 'don't use a breadboard.' I've done this with a breadboard before, and it was a find test. \$\endgroup\$ Commented May 3, 2020 at 19:38
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    \$\begingroup\$ This design wouldn't be very useful under the best of circumstances anyway. There is very little reason to build a bridge of discrete parts today and none to do it with small bipolar junction transistors. \$\endgroup\$ Commented May 3, 2020 at 19:54
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    \$\begingroup\$ If it's an assignment, then, by all means, tidy up your board! This is a mess, and I really can't make out what you've connected to what because it's such a tangle of unshortened wires and resistors going in random directions, in combination with a schematic that doesn't label the parts with names (R1, R2, ...) \$\endgroup\$ Commented May 3, 2020 at 19:57
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    \$\begingroup\$ @SillyInventor not saying it can't work; it's just that you just need a single bad connection for it to not work, and the way this is executed that is extremely likely. \$\endgroup\$ Commented May 3, 2020 at 19:59
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    \$\begingroup\$ Also worth noting, push buttons rarely make proper contact with breadboards - their leads are too short, so your buttons may not actually be doing anything at all. \$\endgroup\$ Commented May 3, 2020 at 20:40

4 Answers 4

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The 10k resistors are doing not a lot because they are weak compared to the 1k resistors. So, when both switches are open the junctions of the 1k resistors are held at mid-supply, all the transistors are on and you have a shoot through situation.

Now close a switch and that side of the motor will be pulled to Vcc but both the transistors are still switched on on the other side of the bridge.

This circuit uses emitter followers instead of common emitter configuration in order to avoid shoot through.

H-Bridge

Disadvantage of the transistors being driven in emitter follower mode is that there is more voltage dropped across them than in common emitter mode necessitating more heat sinking.

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A few comments on breadboarding & prototyping.

I agree that having neat schematics & breadboards are helpful, particularly when you are first starting out.

However, some of the very best analog designers eschewed neatness. Robert Pease, the renowned analog designer (died in a car accident in 2011) from National Semiconductor and Philbrick, tended to have breadboards that looked more like this:

Photo of hand-made breadboard

Image source: Electronic Design magazine - What's All This Analog Engineering Stuff, Anyhow?

Jim Williams who worked for Linear Technology Corporation was another analog guru known for his rats-nest bread boards.

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I was considering making this a comment, but I think this works as an engineering insightful answer:

Sometimes a schematic can be as right as possible, the physical implementation can just be too far from reproducing the "idealizations" of a schematic that the resulting device just doesn't work.

In the case of cheap solder-free breadboard like yours: That idealization is probably the assumption that a connection on the breadboard is close enough to an ideal conductor, which the lines on your schematic are.

Breadboard contacting is notoriously unreliable. And your plugging and wiring looks especially flimsy and unreliable. I don't think, for example, that your switches and your resistors make reliable contact.

Get out a soldering iron and build this on perfboard. If you don't have perfboard at hand, build it free-flying. Still better than this breadboard – not even your fault, these things are just nightmares.

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To drive the motor (in the one or the other direction), you have to turn on the pair of p-n-p and n-p-n transistors in the corresponding diagonal.

To do that, connect a network of 1 k resistor and push button in series between the bases of each diagonal pair of transistors. The resistor will serve as a common base resistor when you press the button.

Also, connect a 10 k resistor in parallel to each base-emitter junction to reliably cut off it when the base is floating.

EDIT: Only do not press both buttons simultaneously to avoid the short connection.

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