My project is to build a car controlled by Bluetooth. first I'm building a H-bridge to control the motors. I'm using a 3v DC motor that needs a current of, 0.26A(no load) - 0.69A(with load) and I'm using 2N2222A transistor here is the data sheet (https://www.onsemi.com/pub/Collateral/P2N2222A-D.PDF) connected to a 9v battery datasheet

eventually the transistors base will be powered by a arduino or other micro controller but for now im just connecting the transistors to the battery.

The H-bridge is connected properly, when i test with an LED it works fine. however when i connect to my DC motor the voltage collapse. the transistors can handle the current. the battery can supply the current (worked when directly connected to power supply) required.

what i think could be the problem is not correctly understanding gain of my transistors. but i have tried different resistors in hopes of getting it right and tried to understand and use the formulas to calculate the correct base resistor values (which may have not been correct) but i am still having no luck with it.

if it helps i was following this Instructables tutorial: H Bridge on a Breadboard

enter image description here

I tried to make the schematic as neat as possible, i just wanted to show how every thing is wired exactly on my bread board.

  • \$\begingroup\$ UPDATE: i just randomly put a 100 ohm resistor on the base of the transistors and now the motor spins. (very slowly) however i still don't understand what happened. \$\endgroup\$ – ElQwerto Oct 1 '17 at 7:54
  • \$\begingroup\$ You know, H-Bridge is called this way for a reason. Because it looks like H. What you draw is not just rough, it's rude. Why would any normal person even try to understand it? \$\endgroup\$ – Gregory Kornblum Oct 1 '17 at 8:02
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    \$\begingroup\$ Are both transistors NPN or is it just bad design? Please draw exactly what you have.. \$\endgroup\$ – Deep Oct 1 '17 at 8:03
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    \$\begingroup\$ You know it's a H bridge so there is no excuse for such a bad diagram - draw it like a H bridge is my recommendation. \$\endgroup\$ – Andy aka Oct 1 '17 at 9:11

Issue #1 you have no freewheel diodes to facilitate the motor stator current to continue flowing when you stop enabling the BJT's

Issue #2 "rough design" needs to reflect EXACTLY what you have

Issue #3 you have NPN's for the top but no means to source their base (ie their base needs to be higher than 9V)

Issue #4 your NPN are the wrong way round if this is really what you have


simulate this circuit – Schematic created using CircuitLab

Your circuit redrawn CLEARER

  • \$\begingroup\$ oh ya i was gonna place diodes after i got the H-bride working, by rough design i just ment it looked bad \$\endgroup\$ – ElQwerto Oct 1 '17 at 7:57
  • \$\begingroup\$ nonono, they go down immediately. \$\endgroup\$ – JonRB Oct 1 '17 at 7:57
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    \$\begingroup\$ preferably with mosfets and their drivers :) \$\endgroup\$ – Gregory Kornblum Oct 1 '17 at 8:01
  • \$\begingroup\$ 2 N channel and 2 P channel MOSFETs to add into Gregory's comment ^^" \$\endgroup\$ – Deep Oct 1 '17 at 8:11
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    \$\begingroup\$ Q1 and Q3 don't reflect what the op has drawn BTW. He has drawn them upside down and that would have been clear should he have taken the time to draw the circuit correctly in the first place! The transistor AMR is 600 mA and not suitable for his load also. \$\endgroup\$ – Andy aka Oct 1 '17 at 9:30

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