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Challenge of Voltage/Digital standards differences between USA and Singapore

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bgd
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Re: Challenge of Voltage/Digital standards differences between USA and Singapore

Postby bgd » Tue, 04 Sep 2018 12:42 pm

So these are used to step down from say 240 to 120.

What about phone chargers? Are they step down transformers with an inverter? Given the number of stories about phone and charger fires, are we looking at auto transformers as being the likely cause?

I see the problem with the design. Can good quality materials get around the inherent problems, or would the cost increase negate the advantage (cost) of an auto transformer?

And thanks for the very clear explanation.

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Re: Challenge of Voltage/Digital standards differences between USA and Singapore

Postby Strong Eagle » Tue, 04 Sep 2018 8:09 pm

bgd wrote:So these are used to step down from say 240 to 120.

What about phone chargers? Are they step down transformers with an inverter? Given the number of stories about phone and charger fires, are we looking at auto transformers as being the likely cause?

I see the problem with the design. Can good quality materials get around the inherent problems, or would the cost increase negate the advantage (cost) of an auto transformer?

And thanks for the very clear explanation.


Modern phone chargers (and most low voltage power packs these days) no longer use a transformer. Instead, they use circuits containing zener diodes, a curious electronic component which allows the designer to take just a slice of the input AC sine wave and output it as a low voltage for USB, etc.

Like all electronic components, zener diodes (and the associated circuit components) have maximum working values, and they produce heat. Cheaply made devices, with insufficient provision for cooling components, or use of components that run close to their maximum capabilities, increase the risk of fire, shorts, and meltdowns.

You are correct, use properly sized materials, of good construction, with appropriate heat sinks. Of course, this costs money and takes up more space.

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Re: Challenge of Voltage/Digital standards differences between USA and Singapore

Postby PNGMK » Tue, 04 Sep 2018 10:58 pm

Actually they are switched PSUs I think. Extremely high frequency down verters. AC rectified to DC and then switched into a extremely high frequency wave form and transformed down to a voltage and then rectified again. The extremely high switching frequency allows for a tiny transformer and full isolation.
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Re: Challenge of Voltage/Digital standards differences between USA and Singapore

Postby Strong Eagle » Wed, 05 Sep 2018 2:42 am

PNGMK wrote:Actually they are switched PSUs I think. Extremely high frequency down verters. AC rectified to DC and then switched into a extremely high frequency wave form and transformed down to a voltage and then rectified again. The extremely high switching frequency allows for a tiny transformer and full isolation.


I learn something new everyday. Thank you. DC to DC converters (variations of the buck circuit) are used for high efficiency voltage changes (high to low).

I see that non-isolated converters are used for small voltage changes while large voltage changes use an isolation transformer. I also see that DC to DC converters are tricky in that overall circuit capacitance and inductance can affect the performance of the device.

This means I now need to go break open a couple of old chargers to see what is inside them. For example, I bought a cheap replacement charger for an old HP laptop. The old charger weighed in a a couple of pounds, clearly a traditional transformer. The new one (same wattage) weighs in at a few ounces.

I thought it was simply a transformerless full wave rectified, zener diode voltage controlled device, as below. It would be cheaper but could certainly lead to some shocking conclusions if the zener diode were to fail. Now I need to see if it at least has a transformer of some sort in it.

Image

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Re: Challenge of Voltage/Digital standards differences between USA and Singapore

Postby PNGMK » Wed, 05 Sep 2018 6:46 am

That's why people die when their cheap Chinese knock off apple charge for their phone dies. No isolation and the ground of the case and ear buds goes high potential to earth. There are cases where their ears are burned from the shock as the common audio ground goes high voltage.
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Re: Challenge of Voltage/Digital standards differences between USA and Singapore

Postby Strong Eagle » Wed, 05 Sep 2018 10:00 am

Totally off topic, and... since you mentioned high frequency switching... the class D amplifier is quite a cool beast... a non-linear amplifier that creates a pulse waveform, whose length is proportional to the voltage of the input signal. Non-intuitive to me but pretty slick.

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Re: Challenge of Voltage/Digital standards differences between USA and Singapore

Postby PNGMK » Wed, 05 Sep 2018 10:52 am

I've never been much of an amp / audiophile guy. Bad hearing etc...
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Re: Challenge of Voltage/Digital standards differences between USA and Singapore

Postby Strong Eagle » Wed, 05 Sep 2018 11:36 am

PNGMK wrote:I've never been much of an amp / audiophile guy. Bad hearing etc...


I have CHS syndrome myself... but I do like a sound system that puts out the bass. 44 Hz is the lowest note on a bass guitar. I've got a woofer that will make 32 Hz at 3 dB rolloff... amazing how much more expensive things get to get down even a few more Hz.


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