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ColecoVision Internal Power Supply

Discussion in 'Power Electronics' started by ikrananka, Jul 29, 2010.

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  1. ikrananka


    Jul 10, 2009
    I am in the process of building an internal power supply mod for the 1980s ColecoVision games console. This is based on an open frame power supply with triple output ( I have built and tested a prototype unit and it works great, is much lighter than the standard external CV power brick and has the advantage of only requiring a standard household power cord to connect the CV to the mains supply. However, I am not an electrical engineer and have concerns over safety and grounding but do not know the answers. So, looking for some advice on the following (hope someone can help):

    1.In one corner of the power supply PCB there is an earth/ground symbol marked with a trace running from a capacitor to a circular trace running around the adjacent mounting hole. I believe that I should have this connected to ground - is this correct?

    2.One of the other mounting holes also has a circular trace running around it (connected to another capacitor) but does not have an earth/ground symbol next to it. There is NO electrical connection between the two circular traces - should this second trace also be grounded?

    3.The other two mounting holes do NOT have traces around them. Nevertheless, I plan on mounting the power supply using metal screws and metal standoffs. The bottom screws will be outside of the CV case, can be touched, and so in theory there could be an internal short to these screws - do you agree therefore that all four screws/standoffs be grounded?

    4.Do I need to provide some kind of physical shield in case a user pokes a metal objects through the CV air vents thus preventing them from touching the power supply? Or is this overkill? I can see quite a number of consumer electronic items that I have that I could easily poke something inside and touch the internals (not that I ever would mind you).
  2. shrtrnd


    Jan 15, 2010
    Trace the circuit. You've got power 'ground' which is the return for your electronic connections, and may have a different 'ground', which is for the electrical safety of users. Don't connect circuit ground to the electrical safety ground.
    1) If it comes from the capacitor, it's a circuit return ground.
    2) Don't add grounds, or you'll get yourself in trouble with the ciruit. Some board manufacturers leave a circle trace around the chassis screw holes, for assembler to line-up the screws easily. I'd check where this trace GOES, is it part of the circuit?
    3) If you're sure there's no electrical connection to the chassis mounting screws, there is no electrical connection.
    4) Up to you if you want a mesh screen to prevent people from poking items into your chassis (like small kids?)
    Trace the circuits, don't add grounds. If it's part of the electronic circuit, you need it.
    Not being familiar with the circuit, can only give you things to watch-out for.
    If you get the assembly together and it works, check your questionable additions with a voltmeter, so nobody gets shocked. And make sure it's fused, so there's no question.
    I'd be looking for an old ColecoVision schematic if I were you.
  3. ikrananka


    Jul 10, 2009
    Both come from capacitors so they must be circuit return grounds. I have had the unit running without any grounds connected and it works fine.
    I have decided to use nylon standoffs to mount the board and so there will now be no electrical connect between the circuit return ground traces and the mouning screws.
    I'm using a standard 2 pin power entry module, unfortunately there is just not enough room to mount a power entry module that includes a fuse.
    I already have one. This is what allowed me to identify the DC inputs needed for the CV.

    Many thanks for your help - much appreciated.
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