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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?

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    \$\begingroup\$ DC-DC will only waste 4W if your raspberry pi is using 16W, mostly they consume less power. \$\endgroup\$ – Jasen Feb 9 '20 at 7:09
  • \$\begingroup\$ I'd go with first option, those are manufactured with AC safety consideration. and it's a switching regulator which is not going to produce much heat. some have a fan included inside. \$\endgroup\$ – ElectronSurf Feb 9 '20 at 7:13
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    \$\begingroup\$ "Is option 3 as wasteful in terms of efficiency?" No, it's not particularly wasteful in itself, 80% efficient is very standard for most switched-mode power supplies, both mains-connected and DC-DC included - linear supplies were the real power hog. The only extra inefficiency here is the AC->24 V->5 V double conversion, if the first 24 V PSU is 80% efficient & the 5 V PSU is 80% efficient, the overall system efficiency is reduced to 76.8%, just a loss of 3.2%, which is acceptable in my opinion. The real problem is heat dissipation, which shouldn't be too hard for 4 watts, and of course, cost. \$\endgroup\$ – 比尔盖子 Feb 9 '20 at 7:17
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    \$\begingroup\$ 1 and 3 are the ones worth looking at, depending on whether you want the heat inside or outside the block. 3 makes international use a little easier, 1 feels a bit more professional. \$\endgroup\$ – Neil_UK Feb 9 '20 at 10:12
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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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  • \$\begingroup\$ That's a good point, and I was able to find a 2.7A brick at about a third the price of the 5A one, thanks! \$\endgroup\$ – Caius Jard Feb 9 '20 at 15:12

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