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I've noticed this "feature" on a number of travel-oriented voltage converters - they either are meant to be used for small 50W loads, OR for larger loads up to 1600W. But you can't run a small load on the higher setting.

Here's an example of one which requires you to switch between the two:

enter image description here

Here's another which reads:

"1600 Watt Output for Personal Appliances or 50 Watt Output for Small Electronics"

I can imagine that the switch is physically engaging one of two coils, perhaps. But ... why? If it were left on the 1600W setting and someone powered a 50W item in that configuration, what problem would that cause? How do these devices actually work such that this design is important?

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2 Answers 2

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There are two options- an autotransformer and a triac phase control with a fixed setting to yield the correct RMS voltage for appliances such as hair dryers which are more-or-less resistive loads.

The autotransformer is a 60Hz type similar to this one from B&H Photo in NYC:

enter image description here

It may seem small for 50W but that's because it's an autotransformer (meaning it really only has to deliver 25W) and also it's pretty crappy- it will run warm.

The 1600W portion looks like the below (my own photo)

enter image description here

This is the internal circuit of an ordinary phase control dimmer used for incandescent bulbs with the knob and pot replaced by a fixed resistor (maybe) and trimpot. This is a particularly horrible construction and is not only unreliable but likely rather dangerous- The springiness in the leads is what keeps the triac in dubious contact with the silicone insulator and heatsink.

Here is the curve of RMS voltage vs. conduction angle from here: As you can see, the conduction angle has to be ~65° and therefore peak voltage will be around 300V for a 230VAC mains vs. about 168V for a 120VAC mains.

enter image description here

If you run a 120V-only electronic device on the 1600W setting it could well be destroyed because it will see peak voltages well in excess of the rating.

There is very little need for either of these in 2021. You can buy compact travel dual voltage hair dryers and most electronics that would use the 50W setting is already universal voltage and only requires a plug adapter. I also would not want to try my luck with the hair dryer setting on a $500 CAD Dyson hair dryer.

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    \$\begingroup\$ Is the aposematic orange on that heatsink to remind the user that it is not earthed, so you got to unplug the device without touching it? \$\endgroup\$
    – bobflux
    Commented Nov 3, 2021 at 20:18
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    \$\begingroup\$ @bobflux It was purchased for me about 15 years ago by an associate from a surplus place (Active in Toronto for those who remember the Queen Street Gorilla) as a fine example of utter disregard for safety and reliability. Most of the adapter things are unapproved and many are a bit dangerous (missing grounds being common). This one takes the cake though. \$\endgroup\$ Commented Nov 4, 2021 at 1:25
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    \$\begingroup\$ That has to be the sketchiest mounting I have seen on a mains connected product. Is it UL listed? \$\endgroup\$
    – winny
    Commented Nov 4, 2021 at 8:07
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    \$\begingroup\$ @winny No. The beauty of every plug and voltage adapter I've seen is that they lack any safety or EMC approval markings whatsoever (save maybe a useless CE mark). After all, they are typically specifically stated as not intended for use in the country of sale ("for export only). One I bought at HK airport had the (locally useful) UK style plug taped off and the clerk extracted a pledge from me that it would not be used within HK. Since many of them allow a 3-pin US/Canada style plug to be inserted with the ground pin left open, they can't be approved even if they are well made. \$\endgroup\$ Commented Nov 4, 2021 at 12:24
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    \$\begingroup\$ I never imagined one can market a triac regulator for a "voltage regulator". Now that I know it is possible, I wonder how these people avoid jail time. \$\endgroup\$
    – fraxinus
    Commented Nov 4, 2021 at 17:42
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There is probably an auto-transformer for the 50W setting, which will convert 230V into 115V and work with most low power appliances...

And either a dumb diode or a phase cut triac for the 1600W setting, which will only work with resistive loads like a hairdryer. That will reduce power in a resistive load, but:

  • Peak voltage at the output is the same as the input, so if you connect a SMPS or transformer based power supply that only works on 115V and not 230V, it will blow. Even if it's a phase cut triac, you'll get a chopped up sine wave but the peak is still 230V...

  • If it's a diode, there is a huge amount of DC, so if you connect a transformer-based power supply, it will probably overheat.

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  • \$\begingroup\$ A hairdryer also contains a fan motor \$\endgroup\$ Commented Nov 3, 2021 at 18:57
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    \$\begingroup\$ It can't be a dumb diode, if you do the math. More of a phase control. \$\endgroup\$ Commented Nov 3, 2021 at 18:58
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    \$\begingroup\$ @StayOnTarget, If the fan motor is a universal motor, then it will run on DC. \$\endgroup\$ Commented Nov 3, 2021 at 18:58
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    \$\begingroup\$ Most hair dryers have low voltage DC motors, which are run from a bridge rectifier and a voltage divider consisting of the heating coils. \$\endgroup\$ Commented Nov 3, 2021 at 18:59
  • \$\begingroup\$ @SolomonSlow I wasn't thinking about the DC but instead was referring to that fact that it wouldn't be a purely resistive load \$\endgroup\$ Commented Nov 3, 2021 at 19:01

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