Timed MOSFET Switch & 4060B Counter

Discussion in 'General Electronics Chat' started by masa6614, Nov 10, 2008.

  1. masa6614

    Thread Starter Active Member

    Nov 9, 2008
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    Hi,
    For a home project of mine, I am in need of a simple timed switch. My load is a LED light bank which draws ~1A. My source 7.5V from a solar panel charged battery. THis unit has the option to turn on the light from dusk til dawn, but what i want it for is to turn on for only maybe 5~6hrs, so it shuts off around the time i go to bed, i dont want it on all night. Now this unit is to be placed out of easy reach, so i can't just turn it on/off as i please, So scowering the internet, i've across this 24 hr timer circuit which uses a relay and the 4060 binary counter. ( http://uk.geocities.com/ronj_1217/c4060s.html ) , so something like circuit 2 would definately work. What i'm sort of wondering is why must i use the relay? Can't i just turn on/off the FET directly from the output of the 4060? shouldn't this reduce current usage, # of components, and boardspace of this circuit? I'm thinkin of using the IRF9520 p-channel FET, which can handle up to 6A drain current, has a Vgs(th) of 4V, and Rds(on) of 0.6Ohm. Shouldnt the only difference i should encounter be the voltage drop of (1A*0.6Ohm)=0.6V across this switch? I placed a 1kOhm resistor between the FET and the IC because i think i remember hearing that should be done due to gate capacitance of power FETs being so large, and the switching on/off of the FET can create a large transient gate current, enough to damage the IC?
    So just wondering if this would work or are there other issues that would prevent this from working that i am not seeing, or should i stick with the relay circuit?
    Thanks,
     
  2. mik3

    Senior Member

    Feb 4, 2008
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  3. Audioguru

    New Member

    Dec 20, 2007
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    The circuit does not start by itself. You must reset the 4060 for the Mosfet to turn on. When it times out then the output goes high which turns off the Mosfet and stops the oscillator.
    The next time you want to use it you must reset the counter.

    Never mind the 2V to 4V threshold voltage of the Mosfet because then its current is a minimum of only 0.25mA. It meeds 10V on its gate (a 10V supply for the circuit) for the Mosfet to turn on completely and have a on-resistance of 0.6 ohms when it is cold and a resistance of 0.8 ohms when it heats up.
     
  4. masa6614

    Thread Starter Active Member

    Nov 9, 2008
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    Ok, thanks alot for both your responses. So i'd need to use the 0.1uF pulled up and 100kOhm pulldown they have for the relay circuit for the starting, at the Reset pin in the case of the 4060B, and at the -Trigger pin in the case of the CD4047BC.
    And for the FET, looking at the performance curves, they seem to show that with a Vgs of 7V, I should be getting fine with my Drain current requirment of 1A. And so i may be seeing as high as a 0.8V drop across this FET, considering it will likely heat up pretty good being on for up to 6hrs. Thanks again.
     
  5. Audioguru

    New Member

    Dec 20, 2007
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    The resistor and capacitor at the reset pin of the CD4060 resets it one time when it is powered. You must manually reset it every time after it is powered.
    Maybe the circuit that detects daylight can reset it.

    You are gambling with the spec's and yield for the Mosfet. You can't buy a "typical" mosfet that conducts well with only 7V on its gate. You must hope that the one you bought was produced with a high gain and a product manufacturer before you didn't take all the typical and better ones. The datasheet lists a max on-resistance of 0.6V when it is cold and its gate is 10V.

    The CD4047 is a simple Cmos oscillator with a divide by 2 counter. It needs very high value resistor and capacitor values to time a couple of minutes, not hours.
     
  6. masa6614

    Thread Starter Active Member

    Nov 9, 2008
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    Thanks again for the advice, and yeah, the dusk til dawn circuit should take care of the reset. And i see what you mean regarding the Vgs turn on importance, so now i'm looking at logic-level FETs (won't give up on the FET approach, being stubborn now and wont give up til this works), such as the IRL530N.
    Also, that CD4047 datasheet gives this equation for the pulse length: t = 2.48*R*C, so with easily available values like these...
    C=470 uF
    R=15 MegOhm,
    I should get about 17,484 secs, ~5hrs
    this should work as well? or should i stick with the 4060?
    Thanks again,

     
  7. Audioguru

    New Member

    Dec 20, 2007
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    The 470uF capacitor for the CD4047 needs to have a leakage current that is far less than is available with a cheap electrolytic capacitor so 47 10uF huge film type capacitors must be used which would cost a fortune and use a lot of space.
    Maybe 470 smaller and less expensive 1uf film capacitors could be used.

    I made a circuit for my daughter. It used a timer made with a 22M resistor and a 0.47uF film capacitor. I took it in my car to her home and it didn't work. It began to work properly in 2 days when the condensation on the pcb dried out. It got cold in my car and got some condensation when it entered her home. A very small amount of condensation mess up a 22M circuit. A counter circuit using a 22k resistor would probably work fine in rain.

    The IRL530 Mosfet will work fine with a CD4060 and your 7.5V battery.
     
  8. masa6614

    Thread Starter Active Member

    Nov 9, 2008
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    Thanks a lot for your help,
     
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