Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Need help with confirming if this 555 timer circuit with transistor will work?

  1. Oct 21, 2009 #1
    I am an electrical engineer student at SIU and I'm working on an personal project where im trying to use the following items to supply current between 450-600 mA:

    555 timer
    2N2222 transistor
    4 AA batteries in series for 6V
    Resistors and capacitors on 555 timer while just resistors on transistor

    I set up the circuit to have the 4 AA batteries supply 6 volts to the 555 timer. With a couple of resistors, and capacitors i have a square wave output with about 400 ms on and 300 ms off. I wanted to use the timer to supply the 400 mA but it could only send 200mA max so i decided to put an transistor (2N2222) on the output of the 555 timer with an resistor to reduce the current to the base. I also ran the emitter to the ground.My question is that if I also ran the output of the timer (around 5V) so that when the timer is on this output will supply voltage to two small value resistors in parallel and use the current from those to supply the needed current to my ouput that requires the value.

    I tried this as well in multisim and I got the current that i needed confirmed by connecting a multimeter in series with the collector of the transistor.

    So, i wanted to know if this circuit would actually work if i built it with the transistor? If not what other way could i use my timer to source another component to give me the desired current. I liked the timer because i would like to have the current needed to be turned off and on as well.

    If anybody could help with this or has any advise on a better way to implement a circuit i would really appreciate it. Im new to the forum so i hope to learn some valuable information from here, while continuing to learn about ee :)...... Have a nice day everyone
  2. jcsd
  3. Oct 21, 2009 #2
    To use the 2N2222 transistor as an open collector output, (as you stated) tie the emitter to ground, and supply it with ~20 mA base current. Because the NE555 output doesn't completely go to ground when it is low, use a separate equal resistor tied between the NE555 output and ground as a pull down. The NE555 should be able to source 40 mA. The 2N2222 avg power dissipation will be close to the max dissipation limit, and the VCE sat at 500 mA is ~1 volt, so you might substitute an npn with a TO-220 case.
    Bob S
  4. Oct 21, 2009 #3


    User Avatar
    Science Advisor

    Just to be sure, would you like to draw up your circuit and post it? Just a screen grab from your simulator would be OK. (Push PrtSc on your keyboard and then paste it into MS Paint)

    It sounds like there is a problem doing what you ask.

    If you have a 555 driving a transistor in common emitter mode, there will be a phase problem with using both the 555 and the transistor to supply your current.

    See attached diagram. If you have something like this, the transistor will conduct when the 555 output is high.
    But if you connect it as in the second diagram, current will only flow in the red wire from the load if the 555 output is low.
    So, the output from the 555 will not assist with giving you extra current in the load when you need it, but it will make the transistor hotter when the 555 output is high.

    As BobS suggests, you would need to get a power transistor that could easily produce the sort of currents you need. These can be mounted on a piece of metal used as a heatsink so the transistor doesn't get too hot.

    Attached Files:

Share this great discussion with others via Reddit, Google+, Twitter, or Facebook