Electronic – Making a neat, cost effective, efficient job of supplying 24VDC and 5VC

power supply

I'm making a device for industrial control that will internally use 5VDC (for the controller/RPi) and 24VDC (for everything else, sensors, interfacing with the industrial kit etc). The housing will be plastic, vented (passive cooling). The options I'm looking at all seem slightly sub-optimal, and it feels like this must be such a common problem with industry kit generally running higher voltages than micro controllers..

For power supply I'll need 5V/2.5A and 24V/2A. I believe I could use one of the following routes:

  1. A dual output PSU like a Meanwell RD-65B and mount it inside the enclosure.

    • Pros: tidy, single AC cable into the enclosure, cost effective.
    • Cons: brings AC safety considerations into focus for the device, generates all its heat inside the enclosure;

  2. Dual output brick power supply outside the enclosure

    • Pros: solves every problem I can think of
    • Cons: can't find one with the current capacity required. Could make one by separately enclosing one for step 1) but it doesn't seem cost effective when adding in the build time and extra components

  3. A single external brick power supply providing 24v 5A+ outside the enclosure and internally a DC-DC converter providing 5V from the 24

    • Pros: neat, and 24v 5a bricks are common
    • Cons: the DC-DC regulators I've looked at have efficiencies around 80% and I presume the rest ends up as heat inside the enclosure. Means the 24V PSU needs to be larger current capacity than necessary which bumps the price up and regulators aren't so cheap either – this is the most expensive option

  4. Two different external power supplies

    • Pros: Cheap, easy individual replacement if they break without opening the enclosure, cost effective
    • Cons: looks messy, needs 2 AC outlets

Am I missing anything?

What's the typical route chosen when different voltages are required? (I'm guessing from e.g. PC manufacture it's option 1) Is option 3 as wasteful in terms of efficiency as the specs sheets lead me to believe? 5v, 2.5A = 12.5W, divided by 0.8 is around 16W so am I right in assuming that 24VDC -> 5VDC will incur a wastage of 4 watts with an 80% efficient converter?

Best Answer

Option (3) with a buck convertor module to 5V.

But the external brick need only be 24V 3A (not 5A).

Why? 5V * 2.5A = 12.5W. An 80% efficient buck converter will consume about 16W (or 2/3A at 24V) releasing only about 3 to 4W as heat inside the box. This is likely to be much less than the dual supply would release.

Added to the 24V 2A requirement gives inconveniently slightly more than 24V 2.5A, so you need to round its current up.

Option 1, while viable, requires more space in the enclosure, and leaves you to do the mains wiring and deal with any safety approval issues that may be relevant.

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