My questions are:
- The answer to the original question recommended a metal junction box. Could a rigid plastic box be a safe alternative if the heat from the power supply is fairly low (or is it best to play safe and use a metal box)?
Plastic is a thermal insulator and raises hotspot temp and lowers MTBF but could be done with a fan which eventually needs vacuuming for dust and may cause noise if against a vent. Plastic also accumulates static which might cause static discharges. Conclusion: cheap choice but hotter
- Is a ventilation fan likely to be needed, or can that be determined only by experiment?
This depends on the efficiency and power utilization. A 300W fanless design might not but costs more, while 150W on a 180W unit will need cooling by excellent fanless design e.g. Meanwell, TDK Lambda (only) with thermal conduction or with a thermal-controlled fan.
I would estimate if you only use <50% of max power, a fan is not needed. Accumulated hot time just accelerates wear on capacitors and other hot parts.
- If the power block is kept external (like a laptop) with a fly lead to a socket on the lathe base, what style plug and socket would be appropriate to use that can take the likely current?
Depends on the likely current 5A 7A 10?
Perhaps the best consumer connector are the extra ones inside PC towers for HDD connections coined as "MOLEX" connectors, just a brand once famous for this style. Then use 2 pins for each +24 and 0V. It is keyed and easily inserted in line with wired pigtails. Depending on the cable length, use stranded AWG 16 for a nominal choice.
Never use a coaxial barrel Plug/Jack. They will eventually oxidize and melt.
So ultimately every choice depends on your priorities for open-frame with lid, enclosed, enclosed with fan, enclosed without fan.
- lower cost but check efficiency 85% (more heat) vs 96~98% rating = Open frame in a low dust environment, hard-wired.
- reliable? only suppliers with a good reputation and MTBF spec vs temp
- appearance? depends on how crafty you are but observe cooling, moisture, dust control and other requirements
I once disqualified PowerOne after they moved production to Mexico and if the isolated DC output was earth grounded , it failed power line transient tests in a dozen different ways due to common-mode E-stress imposed by transformer capacitance on primary parts (which took them months to fix) They only did production testing 100% of all units with DC output floating.
So I used my second source Lambda instead.
Here's one https://www.digikey.ca/en/products/detail/tdk-lambda-americas-inc/LS200-24/2336564 $86 but only 85% efficient @ 200W