I read somewhere that if you sell a product that you plan to be UL certified, this means its power supply must be UL certified as well. This means that if you supply power via a transformer (external product), this means that you have to make sure that the transformer has a UL certification. Is this statement true? If it is, how exactly can you source these UL certified DC transformers? Not once have a seen an online listing where it states that this transformer is UL certified.
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asked Sep 13, 2023 at 13:38
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1\$\begingroup\$ By "DC transformer", do you mean a DC-DC converter or AC-DC power supply? Because transformers cannot operate at DC (they will melt.) \$\endgroup\$– rdtscCommented Sep 13, 2023 at 14:19
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2\$\begingroup\$ If you go to Digikey's parametric tables for "AC DC Desktop, Wall Adapters" there is a column for "approval agency". Not every power supply they sell is approved by UL because not all their customers are re-selling in the North American market. But 1000's of them are. \$\endgroup\$– The PhotonCommented Sep 13, 2023 at 14:54
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1\$\begingroup\$ They also have products with CB approval which is apparently (though I never heard of it until just now), accepted in the US. Further, the first product I checked that is listed on Digikey as having "CB" approval, actually does have UL approval if you click through to its datasheet. I suspect this is typical, but I'm not going to check 300,000 listings to be sure. \$\endgroup\$– The PhotonCommented Sep 13, 2023 at 14:57
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\$\begingroup\$ @rdtsc AC-DC power supply. \$\endgroup\$– user1584421Commented Sep 14, 2023 at 10:29
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\$\begingroup\$ @ThePhoton Thanks a lot! Highly informative! \$\endgroup\$– user1584421Commented Sep 14, 2023 at 10:30
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If you supply the PSU to the customer, and the customer needs UL listing, then either the PSU needs its own UL listing, or it has to be certified under the overall system banner. The former approach is hugely preferred, as it allows you to swap for a different supplier if needed, and makes the full range of PSU testing the vendor's problem.
It is possible to find such PSUs (one example), though they are much more common when the PSUs are targeted at OEM integration.
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\$\begingroup\$ Thank you! Could you explain a bit what you meant with this: "though they are much more common when the PSUs are targeted at OEM integration"? Is it normal to NOT supply a PSU (a common AC-DC converter) or is this considered a bad practice? Reason is PSUs are easy to find globally, but there is trouble in collecting it - plus extra shipping cost. \$\endgroup\$ Commented Sep 14, 2023 at 10:32
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\$\begingroup\$ For normal domestic supply, UL listing is relatively uncommon, and minimizing cost is key. When a PSU supplier is targeting selling to OEMs for them to ship with end-user equipment, UL listing is more common. As noted in other comments, if you look at somewhere like digikey or mouser, UL listing is a common filter field. \$\endgroup\$– colintdCommented Sep 14, 2023 at 10:41
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\$\begingroup\$ "For normal domestic supply, UL listing is relatively uncommon, and minimizing cost is key". This is why I was hoping to omit, the PSU. However, I thought that in order to pass the ratings, you have to make sure that the PSU is UL listed. Could you pass a UL rating, as a standalone PCB, while at the sometime, the PSU included in the packaging is not a UL rated? \$\endgroup\$ Commented Sep 14, 2023 at 10:43