H bridge and motor

Discussion in 'The Projects Forum' started by shico90, Sep 10, 2010.

  1. shico90

    Thread Starter New Member

    Jul 22, 2010
    I'm building my first robot and I am having a problem with the H bridge
    the output of the H bridge is about 6 volt with no load when I connect 9 v battery, but when i connect the H bridge to the motor the voltage drops to 3 v which is very low for the motor to operate
    I'm using this circuit
    please help how to increase the voltage supplied to the motor
    I'm using 2222 and 2907 transistor
    Thank you
  2. Dyslexicbloke

    Well-Known Member

    Sep 4, 2010
    I am not able to help much with transistor switching, only have basic, and possibly inaccurate knolage, but I can say they are probably either not fully on or not fully off.

    You will need to give details of what you are driving the bases with.

    I use small brigdes like this with modifed RC solanoids and drive them with PWM.
    Several of my first attempts burnt out because the transistors were not fully on.
    and I distroyed another one because my ground wasnt right and the back emf from the motor caused both sides to be on at the same time ....
    I will be watching this to see what the folk here who know what they are doing tell us how it should be done.
    Sorry I cant be more help
    Last edited: Sep 10, 2010
  3. sgardner025

    Well-Known Member

    Nov 5, 2009
    What are the specs on that motor, and how are you driving the transistors?
  4. SgtWookie


    Jul 17, 2007
    Well, they didn't really give you the whole schematic for the H-bridge.

    You can control it with a pair of single-pole double-throw switches.

    If you're trying to drive it from a microcontroller, you will have problems, as a uC usually can't operate with more than 5.5v for Vcc.

    Also, the "rule of thumb" for calculating base resistors is:
    Ib=Ic/10, or the base current needs to be 1/10 the desired collector current, so:
    Rb = (Vin - Vbe) / (Ic/10), where:
    Rb = the base resistor value; select the nearest standard value of resistance
    Vin = the voltage applied to the side of Rb opposite from the base
    Vbe = typically 0.7v; but consult the datasheet for Vbe vs Ic
    Ic = desired collector current.

    Both the PN/2N2222 and the PN/2N2907 are rated for 800mA max collector current, but the practical limit for constant current flow is around 500mA. If you go much higher than that, you'll burn them up. (That's another "rule of thumb"; de-rate a transistor's rated current by about 1/2.)

    So, if you want 500mA collector current and are using a 9v supply, then:
    Rb = (9v-0.7v) / (500mA/10)
    Rb = 8.3v / 50mA
    Rb = 166 Ohms.

    A table of standard resistance values is here:
    Use the E24 (green) columns. You can order E48 and higher values if you absolutely need to, but more precision costs more.

    So, 160 and 180 are the closest standard E24 values.
    160/166 = 0.964
    180/166 = 1.084
    So, the 160 Ohm value is closer to what we need - but you could most likely get by with the 180 Ohm resistors.

    I previously determined that Vin-Vbe was 8.3v, so 8.3v/180 Ohms = 46.111mA
    That's within 10%, so we're good.

    Now it's important to determine the wattage rating required for the resistors.
    P = EI, so 8.3v * 43.111mA = 357.8mW; and double it for reliability; 715.6mW
    So, your base resistors will need to be rated for 1 Watt, as 715.t6mW is not a standard wattage rating.

    However, 9v "transistor" batteries have very limited current source abilities; they are usually used for powering circuits like a radio over a long period of time; the draw might be 10mA.

    That brings us back to "how are you sourcing/sinking current to/from the base resistors?"
    Last edited: Sep 11, 2010
  5. Dyslexicbloke

    Well-Known Member

    Sep 4, 2010
    Its no wonder I blow transistors up all the time ..... well some of the time anyway.
    That another post for the scrap book .... and i didnt ask anything :)
  6. John P

    AAC Fanatic!

    Oct 14, 2008
    I have a bad feeling here--is this 9V battery the little rectangular kind that powers smoke alarms and radios? It won't run anything but the tiniest motor, and not for long. It's only able to supply a small current, and if you overload it, you'll see the voltage drop.

    You can use whatever drive transistor you like, but if the battery can't supply the current, you won't get anywhere.

    Or, maybe it's a totally different battery. But you didn't say.