AT PSU mod - take two!

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  • nickon51
    New Member
    • Sep 2012
    • 9
    • Australia
    #21
    Re: AT PSU mod - take two!

    This is an interesting project TH3, I want to do something similar, with about the same quality supply. Please keep us up to date.
    Tom and PeteS, I have to say that I'm a bit surprised to see the 220k from the mains live go to the startup cct for the supply. That breaches the "cold" zone by a mains lead into it. I don't think I've seen that before with the driver control in the "cold" zone.
    I wouldn't be surprised if that wire from the 220K was supposed to take a right turn to the collector of Q1. It has after all, got a winding on the driver transformer, that should provide the required startup pulse.
    I can see the 330uf that you refer to, I wonder if that would charge quickly enough to power the 494 rail for start up, I somehow doubt it.

    What are you going to do for current limiting? The supply that you are using has, like mine, no current limiting. I'd be interested to see.

    Cheers
    Greg
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      • Th3_uN1Qu3
        Believe in
        • Jul 2010
        • 6031
        • Romania
        #22
        Re: AT PSU mod - take two!

        Dunno where you read that it goes to the cold side. The 220k is between base and collector of Q1, which is on the high side of the primary DC bus.

        For current limiting i will be using a shunt and an opamp to amplify the small voltage drop of the shunt to something the 494 can read. Something like this oughta do for the shunt resistor. I will be using 10mOhm. For the opamp, i believe i'll be using the MC33078.
        Last edited by Th3_uN1Qu3; 09-18-2012, 03:14 AM.
        Originally posted by PeteS in CA
        Remember that by the time consequences of a short-sighted decision are experienced, the idiot who made the bad decision may have already been promoted or moved on to a better job at another company.
        A working TV? How boring!

          Comment

          • nickon51
            New Member
            • Sep 2012
            • 9
            • Australia
            #23
            Re: AT PSU mod - take two!

            I don't think I explained that very well. I was looking at the start up resistor R2 on the Seven Team power supply. It goes from the non-isolated side to the switching transistors, which I thought were in the isolated section, but of course they are not.
            Cheers
            Greg

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              • Pentium4
                CapXon Be Gone
                • Sep 2011
                • 3741
                • USA
                #24
                Re: AT PSU mod - take two!

                Can't wait to see the improvements

                  Comment

                  • zaero
                    New Member
                    • Dec 2012
                    • 4
                    • Spain
                    #25
                    Re: AT PSU mod - take two!

                    Very interesting topic!!!

                    Regarding current limiting, it is typical to use either the spare amplifier on the TL494 or the DTC pin (4). You can always add a primary current limit with a current transformer. The not-so-crappy AT PSU have one. They usually are configured as latching, to DTC pin.

                    For sensing I have been looking at the ACS712 modules. Affordable ($3.20), isolated and very low resistance (it can even be ignored) makes it a pretty interesting candidate

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