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How to isolate 5V and 12V circuit

Discussion in 'Electronic Design' started by Haris, Dec 13, 2004.

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  1. Haris

    Haris Guest

    Hello everyone,

    I have extend existing circuit design by adding a 12V relay to it. AT
    the moment both 5V and 12V circuit are side by side. This doesn't work
    well. Intermitent device failure happen.

    I would appreciate opinion and instruction on how to isolate these two
    circuit. I already have some idea to put the relay and it 12V voltage
    supply on another board. But I have no idea whether this will work or

    Thanks in advance
  2. The most important points are:

    The 12 volt supply and the 5 volt supply commons connect at one point,
    only, at the relay driver

    The relay supply be bypassed to this common point with a capacitor,
    before the power heads for the relay coil.

    The relay coils have reverse voltage suppression connected across
    them. This can be a diode, a diode in series with a zener or
    resistor, or a series RC (or some other choices). The point is that
    if a driver turns off the relay coil current and there is no place for
    the inductive current to go, the coil produces a very high voltage

    These 3 solve most relay next to logic problems.
  3. CFoley1064

    CFoley1064 Guest

    Subject: Re: How to isolate 5V and 12V circuit
    If I could add one more to Mr. Popelish's list:

    * Do what you can to minimize arcing at the relay contacts, especially if
    you're switching an inductive load.. The EMI can cause upsets on logic level

    One good way to do tis is to put an R-C snubber across the inductive load.
    What this does is reduce the rate of rise in voltage across the load so the
    relay contacts can get far enough apart to avoid arcing.

    Good luck
  4. I have always totally isolated, using split planes and creepage distances,
    any area of my boards that drive relay coils. Yes, snubbers on the load and
    diodes (TVS) on the coil help. I would still supply an isolated 12V to the
    relay-driving side of my circuit and communicate through optocouplers to it.

    As an alternative, you might consider solid state relays. They might be a
    bit more expensive than the mechanical one you are using, but they won't
    kill the rest of your circuit with +/- spikes. They come in big and small
    sizes (I use them to drive 6A motors!).
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