transistor switching a motor

Discussion in 'General Electronics Chat' started by mattev, Feb 28, 2006.

  1. mattev

    Thread Starter New Member

    Feb 2, 2006
    4
    0
    Am posting here due to disappearance of the projects forum.

    I need to be able to turn on a motor for 10 minutes or so per day. I've extracted an LCD programmable digital timer out of a mains switch and can run this on a single AA battery 1.2 to 1.5v. The control line is 0V when OFF and 1.5V when ON.

    I'd like it when ON to power my 3-6V motor which draws about 50mA free running and about 800mA at stall @ 5V.

    I have gone to various websites to understand what NPN transistor I should use and also what base resistor to use but my calculations don't tally around hFE etc.

    Current - should I be looking to have the transistor collector current >=800mA ?

    Voltage - is it OK to have 1.5V on base and say 5V on collector?

    Base resistor - I presume I need this to reduce the current through the timer and base? Any suggestions please around what value to use?

    Capacitor - should I use a capacitor to "buffer" the motor current draw on startup?

    Grateful for any help at all.

    Thank you.

    Matt
     
  2. windoze killa

    AAC Fanatic!

    Feb 23, 2006
    605
    24
    Personally I would use a small relay to actually switch the motor on and off. You can use a transistor to switch the relay. 1.5V is fine on the base so long as you have a resistor in the emitter leg. About 470R should do it for a small relay. Place a capacitor across the relay coil as well to stop spikes.
     
  3. mattev

    Thread Starter New Member

    Feb 2, 2006
    4
    0
    Thanks. I am trying to make this as low power as possible as I want to run it of batteries (e.g. 4 AAs) for as long as possible. I presume, if I can get away with it, that just using a transistor will save power?? If not then I shall opt for a relay. Please excuse my ignorance but I presume 470R means 470 Ohms? Does it matter whether the resitor goes between my timer and the base or between the emitter and ground?

    There is a second goal I am trying to achieve with this project - to simply close a switch on another circuit for about 5 hours per day. I thought a relay would enable me to keep the 2 circuits isolated but I think holding a relay closed for that amount of hours per day is going to use up too much power. I then thought of a latching relay but as I only have a timer capable of going high/low (with minimum time period of 1 minute) I am stuck. Is there a way I could set a sequence of events like this without resorting to a microprocessor?

    timer out goes high
    -> transistor switches latching relay to ON
    set delay of say 5 hours
    -> current reverses across the relay and switches OFF



     
  4. windoze killa

    AAC Fanatic!

    Feb 23, 2006
    605
    24
    Firstly, yes 470R is 470 ohms.

    Secondly. The resistor should go in the emmitter leg. If not the transistor may burn out from the current flow throught the relay. This would depend on the resistance of the relay.

    Next. For the 5 hour problem you could build a NE555 timer. These are capable of timing anywhere from milli-seconds to days if needed. For stability you can dasiy chain a few together instead of trying to get the full time out of just one. If you use CMOS versions it shouldn't affect your power requirenment to much. Although a relay might if you have it energised constantly. You could use your other timer to trigger these timers.

    Hope this helps a little more.
     
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