5amp switching

Discussion in 'The Projects Forum' started by napst3r, Mar 29, 2011.

  1. napst3r

    Thread Starter New Member

    Mar 28, 2011
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    hi everybody

    im looking for help with regards to the switching of a 2n3055 transistor,
    the scenario we sitting with is that we have a PWM (pulse width modulator) that varies the pulse width at the output (pin 3) of the 555 timer, now we need to use that modulated signal to switch a light bulb which has a 60W rating at 12Volts, this means we need 5A into the bulb...

    can anybody help with the circuit diagram from the 555timer to the bulb

    much appreciated :confused:
     
  2. SgtWookie

    Expert

    Jul 17, 2007
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    You should post your 555 circuit schematic first.

    Consider using a power MOSFET to switch current to the lamp instead of the 2N3055. It would be much more efficient.

    What is the application for this light?
     
  3. napst3r

    Thread Starter New Member

    Mar 28, 2011
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    here is the circuit diagram of the PWM, and basically the lamp must be able to provide a temperature of 40 degrees celcius at 4cm away...
     
    • pwm.doc
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  4. SgtWookie

    Expert

    Jul 17, 2007
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    OK, here's your existing circuit removed from the Word document:

    [​IMG]

    Looks like you are using a light dependent resistor (LDR) instead of a thermistor or the like. Not sure you're going to get decent temperature regulation that way.

    Anyway, we don't know where you live. If you are in the States, you could use an N-ch power MOSFET like an IRLU8721. Digikey stocks them:
    http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=IRLU8721PBF-ND

    Basically:
    the gate terminal connects to your 555 pin 3 output,
    the source terminal connects to ground,
    the drain terminal connects to one of the lamp terminals,
    and the other lamp terminal goes to +12v.

    The MOSFET datasheet: http://www.irf.com/product-info/datasheets/data/irlr8721pbf.pdf
    shows the terminal configuration on the 1st page.
    Gate, Drain, Source.

    You need to be aware of static electricity. MOSFETs are great for switching large currents, but are very easily destroyed by static.
     
  5. napst3r

    Thread Starter New Member

    Mar 28, 2011
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    the reason for inserting the LDR onto pin 5 is to control the pulse width according to light...

    let me give you the whole scenario of the design:

    you are required to design a temperature control unit that has to perform the following:
    -the temp is to be controlled as a function of light intensity, this means the darker it gets the more heat must be generated.
    -in order to conserve energy, the part of the system that heats up must have the highest possible efficiency.
    -a user interface must inform the user of all operational statuses (eg indicator for heting element on/off)

    Design constraints:
    - to meet the requirement of high efficiency heating, the heating element must be supplied with dc chopping for this a 555 timer is needed
    - the heating element is a 12VDC lamp (type of car lamp) which provides good infra red radiation as a heat source
    -power supply of 12VDC

    design and contruction:
    -heating element to be controlled (60W)
    -max temp at 3cm away must be 40 degrees celcius
    -controlled voltage
    -light sensor LDR or a phototransitor.


    hopefully now it makes alot more sense to you..

    P.S- i am from south africa
     
  6. wayneh

    Expert

    Sep 9, 2010
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    Sarge, no concern for in-rush current at the gate? Not sure what the frequency is. I wouldn't worry at low frequency but the current could get troublesome at higher frequency?
     
  7. wayneh

    Expert

    Sep 9, 2010
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    See the MOSFET recommendation. Much more efficient than the 2N3055.
     
  8. SgtWookie

    Expert

    Jul 17, 2007
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    The MOSFET I suggested is rated for 65A continuous, has a very low Rds(on) (8.4m Ohms), and a very low gate charge (8.5nC). With 5A current, the MOSFET would dissipate about 213mW power if on continuously.
     
  9. napst3r

    Thread Starter New Member

    Mar 28, 2011
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    okay so basically i need to have a MOSFET instead of the transistor, but do you perhaps have any alternative MOSFET's because i dont think i can get that specific one in south africa...

    when i simulated that circuit with a suitable MOSFET it did not want to work...

    can i maybe suggest using 1 power mosfet, but then also using another transistor in conjuction with MOSFET to do the low amp switching from the 555 timer and then the MOSFET can do the power switching?

    but what about any base resitors and so forth to get the desired collector or drain currrents
     
  10. SgtWookie

    Expert

    Jul 17, 2007
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    I have no idea what kind of electronic components that you can get in South Africa, nor any suppliers in that region.

    What did you call a "suitable MOSFET?"

    Show us how you wired it up.

    A transistorized 555 can source and sink up to 200mA. The MOSFET I specified has a very low gate charge; so the 555 timer should be able to charge/discharge the gate directly. You could add a 10 or 22 Ohm resistor between the 555 output and the MOSFET gate.

    Once the gate of a MOSFET is charged or discharged, it does not require any current to maintain it's state; ON or OFF. A transistor like your 2N3055 requires about 1/10 of the desired collector current as the base current. Since peak currents (when the bulb is cold) can be quite high, you could require a LOT of current in the base. Even for constant ON, the 555 could not source enough current to keep the 2N3055 in a saturated state.
     
  11. napst3r

    Thread Starter New Member

    Mar 28, 2011
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    okay well for the MOSFET i used a IRFZ46N

    attatched is the circuit that is connected with mosfet
     
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  12. SgtWookie

    Expert

    Jul 17, 2007
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    An IRFZ46 is an old MOSFET with a high gate charge compared to the one I linked to.

    Try to find a MOSFET with a lower Vdss rating - 20 to 30 volts would be good.

    Even still, the MOSFET you used should have worked.
     
  13. napst3r

    Thread Starter New Member

    Mar 28, 2011
    6
    0
    okay thank you so much for all the help, i really appreciate it as i am quite new to the world of electronics...

    but today i will go and buy the MOSFET and then give it a try, im pretty sure it will work now that i have made sense of it.
    :)
     
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