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I'm still boggling at the way that power is not conducted electrically across the power supply but is instead air-gapped and pass magnetically, via transformers

I'm still boggling at the way that power is not conducted electrically across the power supply but is instead air-gapped and pass magnetically, via transformers. Isn't that a lot of power?! A modern PC can easily draw 500W or more. Does that mean there's some enormous magnetic field in my power supply? Why doesn't that pose a problem for the things around it? (I cheerfully admit to being very very good at manipulating bits, CPU opcodes, and TCP/IP packets and knowing next to nothing about electrical engineering.)

coryrc 5y ago
The magnitude of the magnetic field is small and mostly conducts through the ferrite for maximum efficiency; it's the frequency which allows so much power to be conducted. Also almost nothing in your computer is affected much by moderate magnetic fields. Go ahead, stick magnets all over the motherboard!
kens 5y ago
The magnetic field is mostly confined to the core of the transformers, so your power supply isn't going to start attracting things like a giant magnet. This internal field is pretty strong, though, up to 0.3 Tesla in a ferrite core. In comparison, a refrigerator magnet is 5 millitesla and the Earth's magnetic field is 30 microtesla.
jaidan 5y ago
That’s how power transformer work in a utility network! We have 120000000w (120MVA) at work. So 500w isn’t much for an isolation transformer.
nine_k 5y ago
Air-gapping is a nice side effect, but the real point of using transformers is that they trivially transform voltage. Altering voltage by resistance is enormously wasteful, and can only lower it. Altering voltage by quickly turning it on and off (PWM) only works with certain inert types of load, like electric motors; it works for LEDs only because the inertia of the eye. But this is close enough to AC anyway. Also, the energy of an electromagnetic wave grows as the 4th power of its frequently, IIRC. This is why old power supplies which directly used the 60Hz frequency from the socket were so bulky, while modern power supplies, which convert the mains electricity to high-frequency AC, can use physically small and lightweight transformers.
- formerly_proven 5y ago
  > Also, the energy of an electromagnetic wave grows as the 4th power of its frequently, IIRC. This is why old power supplies which directly used the 60Hz frequency from the socket were so bulky, while modern power supplies, which convert the mains electricity to high-frequency AC, can use physically small and lightweight transformers. It's much more roundabout than that. A transformer has a magnetizing inductance (you can measure this with an LCR meter by leaving all other windings open). This is generally a pretty big inductance (several henries for mains transformers). When you supply a waveform, that inductance will cause a small current to flow. That's the magnetizing current. The problem is saturation - if the voltage over time gets too large [=the magnetic field in the core], the transformer's core will saturate (that's also why transformers cannot handle DC; any DC offset will accumulate over time and cause saturation). This causes the magnetizing inductance to drop to approx. zero. When that happens, the magnetizing current becomes huge, basically a short circuit. This means that you need to avoid saturation. Note that the strength of the field depends on the number of turns - each turn reduces the impressed field. So you want as many turns to avoid saturation as necessary, but as few turns as you can get away with (each turn adds resistance, and needs space inside the transformer). And this is why mains transformers are so big. Because the frequency is low, voltagetime is large. So they need many, many turns (a few hundred to thousands) in order to prevent their steel cores from saturating. So the primary winding is a very long wire, which has a lot of resistance. So you need to increase the size of the core to fit a thicker wire, in order to avoid resistive losses. Now that bunch of wire doesn't fit any more through your transformer, so you need to use a bigger core, but wrapping N turns around that needs even more wire and so on. So you end up with a big, heavy transformer. For a high-frequency switch mode power supply, the frequency is high, so voltagetime is orders of magnitude lower (e.g. 100 kHz vs. 50 Hz = 2000x lower), so they need far fewer turns. So their primary winding's wire is short, so it can be a) a little bit thinner and b) takes up much less space! So you get a small, light transformer. But the core, it doesn't really care about power, like at all. It's all about not driving it into saturation, and having windings that can carry your current. Those two factors make the cores of mains transformers so big.
  - R0b0t1 5y ago

coryrc 5y ago

The magnitude of the magnetic field is small and mostly conducts through the ferrite for maximum efficiency; it's the frequency which allows so much power to be conducted. Also almost nothing in your computer is affected much by moderate magnetic fields. Go ahead, stick magnets all over the motherboard!

kens 5y ago

The magnetic field is mostly confined to the core of the transformers, so your power supply isn't going to start attracting things like a giant magnet. This internal field is pretty strong, though, up to 0.3 Tesla in a ferrite core. In comparison, a refrigerator magnet is 5 millitesla and the Earth's magnetic field is 30 microtesla.

jaidan 5y ago

That’s how power transformer work in a utility network! We have 120000000w (120MVA) at work. So 500w isn’t much for an isolation transformer.

nine_k 5y ago

Air-gapping is a nice side effect, but the real point of using transformers is that they trivially transform voltage. Altering voltage by resistance is enormously wasteful, and can only lower it. Altering voltage by quickly turning it on and off (PWM) only works with certain inert types of load, like electric motors; it works for LEDs only because the inertia of the eye. But this is close enough to AC anyway. Also, the energy of an electromagnetic wave grows as the 4th power of its frequently, IIRC. This is why old power supplies which directly used the 60Hz frequency from the socket were so bulky, while modern power supplies, which convert the mains electricity to high-frequency AC, can use physically small and lightweight transformers.

formerly_proven 5y ago
> Also, the energy of an electromagnetic wave grows as the 4th power of its frequently, IIRC. This is why old power supplies which directly used the 60Hz frequency from the socket were so bulky, while modern power supplies, which convert the mains electricity to high-frequency AC, can use physically small and lightweight transformers. It's much more roundabout than that. A transformer has a magnetizing inductance (you can measure this with an LCR meter by leaving all other windings open). This is generally a pretty big inductance (several henries for mains transformers). When you supply a waveform, that inductance will cause a small current to flow. That's the magnetizing current. The problem is saturation - if the voltage over time gets too large [=the magnetic field in the core], the transformer's core will saturate (that's also why transformers cannot handle DC; any DC offset will accumulate over time and cause saturation). This causes the magnetizing inductance to drop to approx. zero. When that happens, the magnetizing current becomes huge, basically a short circuit. This means that you need to avoid saturation. Note that the strength of the field depends on the number of turns - each turn reduces the impressed field. So you want as many turns to avoid saturation as necessary, but as few turns as you can get away with (each turn adds resistance, and needs space inside the transformer). And this is why mains transformers are so big. Because the frequency is low, voltagetime is large. So they need many, many turns (a few hundred to thousands) in order to prevent their steel cores from saturating. So the primary winding is a very long wire, which has a lot of resistance. So you need to increase the size of the core to fit a thicker wire, in order to avoid resistive losses. Now that bunch of wire doesn't fit any more through your transformer, so you need to use a bigger core, but wrapping N turns around that needs even more wire and so on. So you end up with a big, heavy transformer. For a high-frequency switch mode power supply, the frequency is high, so voltagetime is orders of magnitude lower (e.g. 100 kHz vs. 50 Hz = 2000x lower), so they need far fewer turns. So their primary winding's wire is short, so it can be a) a little bit thinner and b) takes up much less space! So you get a small, light transformer. But the core, it doesn't really care about power, like at all. It's all about not driving it into saturation, and having windings that can carry your current. Those two factors make the cores of mains transformers so big.
- R0b0t1 5y ago