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protection for chip

Discussion in 'Electronic Basics' started by kell, Aug 21, 2005.

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  1. kell

    kell Guest

    I need to protect a 555 timer circuit that I am going to install on my
    motorcycle. I'm not using a voltage regulator with this circuit. It
    will have to run at the vehicle system voltage. This is because the
    circuit uses a couple of constant current sources that require a
    certain amount of headroom.
    I tried a scheme using a couple of common diodes on the power line (one
    diode placed across the line followed by a diode in series) and a zener
    across the chip's power pins, all to no avail. I fried two chips.
    What kind of buffering or protection can I use to block the electrical
    transients?
    I am thinking of an RC filter next. I don't think this circuit draws
    more than about 15 mA, so if I use a resistor of a few dozen ohms it
    won't drop much voltage. What kind of capacitor should I use? Also
    looking for other ideas.
     
  2. Edward Rawde

    Edward Rawde Guest

    I'd use a series diode followed by a capacitor or two across the rail. A
    couple of hundred uF electrolytic in parallel with a couple of hundred nF
    ceramic should do.
    If that's not sufficient then add a suitably rated one of these
    http://www.google.com/search?&q=transil

    If you can tolerate the volt drop then a resistor in series with the diode
    may help.
    If you remove the diode then your circuit may go haywire when you start the
    vehicle.
     
  3. Gareth

    Gareth Guest

    I think you were on the right lines with the diode and the zener. The
    problem is that the resistance of the vehicle power supply is very low,
    so when the zener tries to clamp the voltage it just cannot sink enough
    current. This is easily fixed by adding a resistor before the diodes.

    You may find this application note useful:

    http://www.st.com/stonline/products/literature/an/3584.pdf

    --
     
  4. kell

    kell Guest

    Here's how I solved the problem:
    Today I remembered that in my box of stuff I have some tiny 1mH chokes
    (they look about like 2 watt resistors). So I used the choke, changed
    out the old 1 watt zener for a much fatter one, and a ceramic bypass
    cap. Works now.
     
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