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Thread: Proper way to load a power supply

  1. #1
    Senior Member
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    Proper way to load a power supply

    My display is getting bigger so I decided to upgrade power supplies. I bought an Astec ALS Series 350W supply. On the first output, it produces 5V @ 40A. This is the only output I was planning to use since my strips are 5V but I am told that I need to load the three additional 12V outputs as well. What is the best way to do that? Do I need to put a load on all three?

  2. #2
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    Hard to tell the exact design of the PSU but would suggest a quick mulitmeter check, if the multiple 12V +ves and -ves are connected to each other you can load them as one, if they are electrically seprate +ve1 not seperate to +ve2). Would have expected at least one -ve 12V rail. The actually loading is a bit suck it and see. Basically you want enough current flow to keep the regulator happy without setting anything on fire or running up a power bill needlessly. Starting point would be a standard 0.25W resistor which would be 0.25W/12v = ~0.0208A = 20mA current and 12V/0.0208A = 576ohms. Actual resistor values would be a choice of 560 and 620, with 620 being a bit safer.

    If that works and all voltages stay stable you could try running out to 1K for cooler operation, if it doesn't then you need to get something hulkier and look at heat sinks. Also note that some power supplies run a sense circuit that will need to be tied back to ensure stable drive levels.

  3. #3
    My experience with switching power supplies with multiple outputs is that generally the regulation of all outputs are tied to a single output (referred to as cross-regulation)... so you really only have a minimum load requirement on that one output (usually +5V for PC and arcade game power supplies). If you're using one of the other outputs, putting a dummy load on it can help keep its voltage down nearer its optimal value... but there's no feedback back to the controller, so it's not necessary if you're not using it.

    This page briefly discusses minimum loads and cross regulation: http://power-topics.blogspot.com/201...lation-on.html .

    DogP

  4. #4
    Senior Member Jp3141's Avatar
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    As far as I can see from the spec for these supplies, they are specified down to zero load on the 12 V outputs. Therefore you don't need to load them. On the other hand the 5 V are specified only from 3A load -- unloaded, the output might drift high; you'll have to measure, but it may be only 100 mV or so. In general with these types of supplies, cross-regulation is not a significant issue.

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