2 transistor circuit help

Comments

• brucee Posts: 239

  2013-07-17 08:40 edited 2013-07-17 08:40

  The preferred current flow through a transistor is in the direction of the arrow. I say preferred because transistors will work when backwards, but only very poorly. As you drew it the current flowing through the motor has to go backwards through one of the transistors.
  
  If all you need is to turn on the motor, then all you need is 1 transistor. Remove Q1, connect the motor to the + side of the battery. Turn Q2 around so that the emitter (arrow side) is connected to the - side of the battery.
  
  You should also protect the transistor from current spikes when it turns off. The magnetic field in the motor is energy that has to go somewhere when the transistor turns off. The easiest is to take a diode from the Q2 collector to the + side of the battery.
• abukharmeh Posts: 10

  2013-07-17 08:54 edited 2013-07-17 08:54

  i need to build sth like h bridge so i need two more transistor but i was trying at 2 transistor to see how it will work
• brucee Posts: 239

  2013-07-17 09:18 edited 2013-07-17 09:18

  An H bridge allows you to reverse direction of the motor. To build that you need to use complimentary transistors, PNP and NPN
  
  For examples of the circuit, this page is good
  
  http://www.talkingelectronics.com/projects/H-Bridge/H-Bridge-1.html
• Beau Schwabe Posts: 6,570

  2013-07-17 09:56 edited 2013-07-17 09:56

  That link is good, but you have to take the commentary with a grain of salt.... for example when comparing Design 1 versus Design 2, he says .... "It does not have any advantages over the circuit above and simply uses extra components" .... It does actually, in circuit 1, both of the PNP transistors are ON when the input signals are LOW. When one of the input signals becomes HIGH, the NPN transistor will briefly need to source enough current to overdrive the PNP that is already ON so that it will turn OFF. With Design #2 you don't have that issue.
• tonyp12 Posts: 1,951

  2013-07-17 10:18 edited 2013-07-17 10:18

  Here is a 1.6A H bridge on SIP, so you could use breadboard for experimenting
  With it's low input threshold of 1.3v, can control it directly from MCU
  http://www.mouser.com/ProductDetail/ON-Semiconductor/LB1641-E/?qs=sGAEpiMZZMtKB4wrjsn3lSzQNSn3XsIpA3TUIJmnxwk%3d
  
  this one is good too, but 3.5V threshold so maybe better for 5v mcu's or manual button presses.
  http://www.mouser.com/ProductDetail/Toshiba/TA7291SGOJ/?qs=sGAEpiMZZMtKB4wrjsn3lbY2wcnw7iEm1M%252bpQk0xjM8%3d
• LoopyByteloose Posts: 12,537

  2013-07-17 11:09 edited 2013-07-17 11:09

  Well, this is a big jump from a few days ago when you asked which transistor would control a 500ma motor.
  
  The 2N2222 is still the right NPN transistor, but you need a 2N2907 for a PNP to go with it if you want an H-bridge.
  
  http://www.futurlec.com/Transistors/2N2907.shtml
• abukharmeh Posts: 10

  2013-07-17 15:08 edited 2013-07-17 15:08

  why do most circuit of h bridges use 2 pnp and 2 npn
  why not using 4 npn what is the usage of the pnp transistor
• tonyp12 Posts: 1,951

  2013-07-17 15:35 edited 2013-07-17 15:35

  NPN on the high side of a load would require a voltage+4v, so if you have 12v going in for the motor you would have to give the gate 16v.
  Where are you gone get the 16v from?
  
  Modern Hbridge IC uses 4 n-channel mosfets a chargepump for the upper mosfets.
• abukharmeh Posts: 10

  2013-07-17 16:03 edited 2013-07-17 16:03

  1 million thanks to every one
  appreciate your help
• LoopyByteloose Posts: 12,537

  2013-07-18 07:08 edited 2013-07-18 07:08

  Now that you know us... come back soon and often.