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Over voltage protection circuit suggestions?

Discussion in 'Electronic Basics' started by Dan Beck, Mar 11, 2007.

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  1. Dan Beck

    Dan Beck Guest


    a couple of us on the pinball newsgroup have been wondering about a design
    for an over voltage protection circuit we would like to add to our games.

    Here is the problem: On some pinball games there is a pcb dedicated to
    providing the +5.0 DC volts for the logic boards. Apparently when this
    board fails, it fails and then sends anywhere between 10-12 VDC to the
    circuit boards. On some of these circuit boards there are ancient (read
    obsolete and expensive) TTL and CMOS chips, that this novice suspects would
    not tolerate 10-12 VDC very well.

    1. Should I be worried about sending 10-12 VDC to these dinosaur chips?
    2. If so, are there any websites or ideas you could share in regards to
    designing an over voltage protection circuit for this power supply pcb? I
    know I won't be competent with an ASCII drawing; I could certainly fax the
    schematic if you so desired.

    Thank you in advance for any and all thoughts you may have.

  2. One solution is a crowbar circuit plus a fuse. A crowbar is simple ...
    resistors, a zener diode and an SCR. Try Google.
  3. What you would want to do is put a device after the regulator that will act
    when the voltage has gone past a point(i.e., your over voltage). You could
    just use a 5v regulator if the currents are not to large.

    I would imagine that using a relay would be best and simplest. If you use a
    12V relay then it will not trip unless the voltage goes above 12V.

    Heres a site after a quick search:

    You'll need to protect the relay from kickback but thats pretty easy. Here
    the power for the relay will not be a steady 12V as in the pdf but will be
    the supply over your pcb.(so you don't need to change the pcb out) This way
    when the pcb outputs 12 volts it will then have enough to drive the relay
    but any lower and it won't. (or if you have a constant 12VDC source you can
    use that if you wanted)

    In other cases one might want to know what the pcb is actually doing. If its
    just a power supply and if the current is not to high then it might be
    easier to replace it using a modern linear regulating circuit and would be
    much more failsafe. (And then tend to have many protection mechanisms

  4. Chris

    Chris Guest

    Hi, Dan. As Mr. Schuler suggested, a "crowbar" circuit is the
    standard mode of protection for this type of problem. Here's a simple
    one which will trigger and pop a fuse if the supply exceeds about 6.2V
    or so (view in fixed font or M$ Notepad):

    | + 3AG 2A +
    | o---o_/ \o-o-----o---------o
    | | |
    | 1N4733A /-/ | SCR
    | Vz = 5.1V ^ V S2010
    | | -
    |Power o----/|
    |Supply | | Vout
    | .-. |
    | 2.2K| | |
    | | | |
    | '-' |
    | - | | -
    | o----------o-----|---------o
    (created by AACircuit v1.28.6 beta 04/19/05

    Here's a Wikipedia link which could help you understand the circuit:

    The idea is, if power supply voltage exceeds 6.3V (which even TTL
    circuits can momentarily withstand), there will be enough voltage
    across the 2.2K resistor to trigger the gate of the SCR. The SCR will
    act like a "crowbar" across the power supply, shorting it out (and
    bringing the output voltage down to about 1.2V or so). This would
    normally put a lot of stress on the SCR, but that's where your fuse
    comes in. It will open up, saving the SCR so it can save the day next

    It's a heck of a lot easier to replace a $0.50 fuse than a lot of
    obsolete TTL.

    Good luck
  5. Dan Beck

    Dan Beck Guest

    Thank you gentlemen, for the abundance of information! I think the crowbar
    circuit is the way to go.

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