On Thu, 14 Feb 2008 12:54:35 -0800 (PST), Harold Hallikainen wrote:
> It SEEMS that we end up spending thousands of dollars to get UL safety
> testing on the following:
>
> 1. A UL listed power supply
> 2. A UL listed power switch
> 3. A UL listed power inlet
> 4. A few pieces of wire and a ground lug
>
>
> It SEEMS that it should be possible to avoid all this if there were
> something like the old PC power supplies that had the inlet, power
> switch, etc. all enclosed in a box with just the DC outputs coming out of
> the box. We'd shove the power supply up against our rear panel and have
> no line connected circuitry in our product other than that "pre-approved"
> box.
>
> I think this is similar to using an external low voltage power
> supply.

No, there is actually quite a large difference in U.L.'s eyes -- using a
wall wart no high voltage is contained inside your enclosure, only isolated
low voltage via your power cable. In the case of an enclosed AC mains
operated supply inside your box you are also introducing mains voltage
inside your box. Yeah, it's in a can but it's still there and stuff
happens. That's how U.L. will look at it, even if it's a U.L. listed power
supply module.

> So.. does the group think this would reduce or eliminate UL safety
> testing costs? Does anyone know of any power supplies like this?

It depends on the specification you're trying to meet and what the target
industry expects. In the industries I design for (medical devices, fire
alarm and detection systems and industrial controls) I would expect to
actually have more U.L. safety tests because AC mains voltage is now inside
the box.

Matt Pobursky
Maximum Performance Systems

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