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Thread: Mackie Power supply

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    Master Destroyer nosaj's Avatar
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    Mackie Power supply

    I was given a Mackie 220 watt Power supply. I would like to know roughly what value resistor I can use to test each of the rails. After they're verified what I would like to do is make this a test power supply.
    I would like to be able to vary the regulated output of each. I think this can be done with pots and triacs.
    Is this reasonable?

    Thanks,
    nosaj
    Schematic power_15s.pdf

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    Supporting Member Jazz P Bass's Avatar
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    Not too sure what you mean by :"I would like to know roughly what value resistor I can use to test each of the rails."
    It's a linear power supply.
    Resistors are not needed to 'load it down' like on a switch mode.

    I would advise that you do not exceed either supply as to current draw. (go with the fuse values)

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    Master Destroyer nosaj's Avatar
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    Quote Originally Posted by Jazz P Bass View Post
    Not too sure what you mean by :"I would like to know roughly what value resistor I can use to test each of the rails."
    It's a linear power supply.
    Resistors are not needed to 'load it down' like on a switch mode.

    I would advise that you do not exceed either supply as to current draw. (go with the fuse values)
    I wanted to put a load on the power supply to see if it could deliver the goods.

    Thanks,
    nosaj

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    Lifetime Member Enzo's Avatar
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    So use Ohm's Law. If you have an 18v supply and put 18 ohms across it, it will draw 1 amp from the supply. 9 ohms will draw 2 amps. 36 ohms will draw half an amp. Decide what you want to test for, and simple calculation will give you a resistance.

    I used to service "black box" SMPS for arcade games. I have a big 1 ohm resistor in my bench drawer so I could slap it across a 5v supply and have it draw 5 amps.

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    Master Destroyer nosaj's Avatar
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    Quote Originally Posted by Enzo View Post
    So use Ohm's Law. If you have an 18v supply and put 18 ohms across it, it will draw 1 amp from the supply. 9 ohms will draw 2 amps. 36 ohms will draw half an amp. Decide what you want to test for, and simple calculation will give you a resistance.

    I used to service "black box" SMPS for arcade games. I have a big 1 ohm resistor in my bench drawer so I could slap it across a 5v supply and have it draw 5 amps.
    Ok I have a 10ohm resistor probably a 120watt ceramic big as a kielbasa sausage.

    +18.15 rail 1.81A
    -18.22 rail 1.822A
    11.83(12v rail) 1.183A
    4.99(5volt rail) .499A
    -48Vrail (gives a voltage of 1.42VDC it collapses I guess.)

    So then I realized I could go back to the chassis and read the rated amps output

    18v rails 3.5A would use a 5.14ohm resistor 63watts
    48V rail 250mA would use a 192 ohm resistor 12watts
    12v rail 1.5A would use a 8 ohm resistor 18watts
    5v rail 3A would use a 1.6ohm resistor 15watts

    So now I have what resistors I should use to max out the power supply and what wattage they should be.

    Does this look like I'm heading in the right direction?

    Thanks,
    nosaj

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    Supporting Member loudthud's Avatar
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    I think you are in the ballpark. My guess on the +48 current limit was about 350mA. On the +/- 18V the current limit would be around 4 to 5A, but I need to know what Voltages are coming out of the transformer to guess any closer because the circuit uses a dissipation limiting scheme. This means that if you change the output Voltage, the current limit will change.

    A regulated power supply needs to have a Voltage reference. The +48 is straight forward, D37 is the reference, R36 and R37 are the feedback dividers. You can make the supply variable with a pot connected as a rheostat for R36, but it won't go lower than about 12V. There also could be stability problems. The +/- 18V is a weird circuit where the reference D27 (or D35) isn't grounded, one side hooks to the output. You might try hooking a pot in series with R14 (or R28) to lower the output Voltage. There will be a low Voltage limit of maybe 8V.

    Anytime you convert a fixed power supply to variable, you must be mindful of the power dissipation of the pass transistors. As you lower the output Voltage, you must lower the current limit or the pass transistor(s) will get too hot. In my experience with those OEM linear power supplies, the current limit needs to be about half. On the +/- 18V change R7, R8, R21 R22 to 1 Ohm.

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    Lifetime Member Enzo's Avatar
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    If the supply generally works, I'd probably assume it would produce rated outputs unless I had good reason to doubt it.

    So your 10 ohm resistor would load the 18v rails to about half, probably close enough. It loads your 12v almost all the way, good enough. 48v is not meant to provide any power, just a voltage for some light draw FETs. If it makes 48v unloaded, probably OK.

    The question is this: what do you want from this point? You want to use the 18v rails as is, sounds OK. Or add a pair of 15v Vregs for 15v rails. In a mixer, the 12v is usually not for audio, but for relays, LEDs, maybe light sticks. SO it often isn't well regulated. I had a 12v power supply from RAdio Shack on my bench for decades - I used it to check relays, a lot of 12v fans, stuff like that. Other than phantom power, I would have no use for 48v.

    Also, be mindful of the schematic, some such supplies will have separate grounds for audio and "digital" supplies. Meaning the 5v and 12v might float relative to the 18 and 48.

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