Main question: What is the purpose of having 4 outputs?

  • If I only had pins 1 & 4, I would be able to use 115VAC when it's set to 115VAC, and 240 VAC when it's set to 240VAC.
  • For 240VAC setting: pins 2 & 3 are connected together. They seemingly add no functionality. From the diagram, it appears they would just be floating and isolated.
  • For 115VAC setting: pin 1,2 and pins 3,4 are connected together. So pins 2&3 just give you an additional pair of 115VAC outputs. Is this the only use?

Background: I'm planning to use the Qualtek 864-10/009 power entry module (http://www.qualtekusa.com/images/EMI_Filters/pdf/86410009.pdf) in a project which will basically just be a fancy box with a 24VDC SMPS and a number of output connectors.

The box may be run on 115VAC or 240VAC, so I chose this power entry module which has a double-fuse with selectable voltage output by changing the orientation of the fuse-holder.

The diagram has some features which I haven't really seen before. After staring at it enough, I think it's clear that it's just indicating which outputs have which voltages depending on the orientation of the fuse-holder (which seems to be represented by the DPDT diagram inside the dashed box).

enter image description here

This raises the question: Why give access to pins 2 & 3? Just to have an additional 115VAC output when the fuse-holder is set to 115VAC? Is it useful for setting up some kind of transformer system (given the similarity of that part of the diagram to a kind of half-transformer)?

Related Post: This other stackexchange post deals with a similar power entry module in the context of transformers, but I couldn't understand their setup or application.
What is the right way to wire a power inlet to a switch to a dual-primary transformer?


Contrary to what you seem to think such power entry modules do not include the transformer.

You have to provide your own transformer, so you need pins 2 & 3 to properly connect to it.

But, in this day and age of switching power supplies with wide-range auto-voltage selection, why use a switch-selectable AC input at all? There are areas of application for it, but it has to be a conscious design choice.

  • \$\begingroup\$ Thanks for your comment. The description got a bit lengthy, but I'm actually not using it for a transformer. I plan to use an SMPS. But it sounds like your answer to my main question is that pins 2 & 3 are just for use with transformers. That's what I wanted to know, thanks! \$\endgroup\$ – William Jan 23 at 0:42
  • \$\begingroup\$ The selectable AC input is because it will go in an application for use in both the US and Europe. \$\endgroup\$ – William Jan 23 at 0:43
  • \$\begingroup\$ @William without pins 2 & 3 that switch does absolutely nothing for you (it changes nothing), it might actually become a regulatory violation. If you are using the proper isolated SMPS you should need no switch at all. \$\endgroup\$ – Edgar Brown Jan 23 at 0:45
  • \$\begingroup\$ Thanks for the additional feedback. I just realized that you're point was that modern SMPS have auto-voltage selection so that it is redundant. I will look into that. Thanks again! \$\endgroup\$ – William Jan 23 at 0:49
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    \$\begingroup\$ Any change you could point to the relevant regulation? The point of the regulation being that if the switch doesn't change anything, it could be misleading and somehow dangerous? (Incidentally, the 'switch' is actually the orientation of the fuse holder, but I assume that wouldn't matter from the regulatory perspective) \$\endgroup\$ – William Jan 23 at 0:59

With the switch as shown, the two transformer primary windings are in series, for 230 volt operation

With the switch in the "up" position, the two primary windings are connected in parallel, for 115 volt operation.

The transformer primary windings are each designed for 115 volt. If you only used one winding for 115 volt operation, you would be limited to the same primary current as for 230 volts, so the power capacity at 115 volts would be 1/4 of the 240 volt rating (power goes by the square of the voltage).

  • \$\begingroup\$ Thanks for your comment. That's what I would expect, but the power entry module itself doesn't have a transformer built in. I'll just be using the outputs to go to a SMPS. But it seems like the consensus is that pins 2 & 3 would be useful in a transformer configuration. \$\endgroup\$ – William Jan 23 at 0:44
  • \$\begingroup\$ So perhaps the additional outputs for 2 & 3 just make transformer wiring simpler? If you needed two more connections to a transformer, couldn't you just double up the connections and have two connections each to pins 1 & 4? \$\endgroup\$ – William Jan 23 at 0:47
  • \$\begingroup\$ You need the pin 2 and 3 connections to allow switching the primary windings betwee series and parallel. If you are using a "universal" switching power supply that will handle anything from ~90 - 250 volts directly, you don't need that switching arrangement. \$\endgroup\$ – Peter Bennett Jan 23 at 1:07
  • \$\begingroup\$ Got it, that makes sense. Thanks for your input. In that case, I will look for a power entry module that doesn't have the unnecessary switching capability. \$\endgroup\$ – William Jan 23 at 1:11

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