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I am using a meanwell rid-85B switcher to power a legacy device that puts 3 kinds of load on the 24V supply. at startup, 1A, and at steady state, 0.1A, occasionally 0A. the specs on the supply want a minimum load of 0.3A on this supply, and 2A on the primary supply, which is 5V. my 5V load is 1A or 0A.

I can put a load resistor on each of the two supplies, 80 ohms for the 24V supply, and 2.5 ohms on the 5V, to guarantee these minimum loads to get good regulation. that's 7.2 + 10 watts of waste heat I need to get rid of.

now for the question: what would a simple circuit look like that could switch these resistors out when my real load is pulling enough current to keep the regulation in spec? I'm thinking a relay, but can't figure how to make the current sense work.

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  • \$\begingroup\$ Rather than futzing with a complicated fudge to overcome problems with the supply, why not just use a better one? \$\endgroup\$ – Brian Drummond Apr 30 '18 at 20:05
  • \$\begingroup\$ because it is a generic problem; i was pretty specific about my question. this is not a parts selection question. \$\endgroup\$ – Curt Mayer Apr 30 '18 at 20:15
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As a block diagram (for one supply):

Current sensor -> comparator with hysteresis and reference-> MOSFET controlling resistor load.

You probably want a high-side current sensor. You'll also want to make sure that the relatively fast load switching does not cause undue transients in the supply rail under all conditions (high line/low line etc.).


But generally speaking, you're probably better off specifying a supply that is designed to meet all your requirements out of the box.

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