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I never realized the main transformer element worked on rectified AC. Could someone explain why we rectify the AC power before the transformer? I always thought

I never realized the main transformer element worked on rectified AC. Could someone explain why we rectify the AC power before the transformer? I always thought transformers worked on AC power anyways.

cristoperb 5y ago
In switching mode power supplies like this, the mains AC is rectified and then chopped up into higher frequency pulses and fed to the transformer. The higher frequency allows for smaller transformer. It also allows for voltage regulation because the input pulse width (in this case via the UC3842B controller IC) can be altered based on the sensed output voltage.
RL_Quine 5y ago
50/60hz requires a large transformer core. Switching power supplies rectify to DC, then drive the transformer at potentially megahertz which increases efficiency and substantially reduces the size and weight of the core. For similar reasons aircraft use 400hz 115v supply as it makes all of the power conversion electronics dramatically smaller.
- londons_explore 5y ago
  To add to this... A transformer converts electrical energy into a magnetic field and back into electrical energy. All the energy from a single 'sine wave' must be stored in the magnetic field inside the transformer briefly before coming out of the output. The transformer core is made of steel usually. For a given weight of steel, there is a maximum amount of magnetic energy it can store. That means that at low frequencies like 50/60Hz, transformers end up very bulky.
  - whoisburbansky 5y ago
    Wasn’t immediately clear from GPs comment and my admittedly limited physics instruction, but the way you explained it made it seem incredibly obvious in retrospect, which is a rare skill, thanks!
- jeffbee 5y ago
  The standing field in a massive 60Hz transformer also necessitates robust inrush protection. With a transformer across the mains, losing the input power momentarily can cause a huge surge current when the input is reapplied. With the bridge and filter first, the inrush from being momentarily switched off won't be severe.
- adrianmonk 5y ago
  I think they are asking this: why not use the transistor to chop the 50/60 Hz AC into potentially-megahertz AC? Then pass that chopped AC to the transformer, which could still be small. The motivation would be to try to eliminate the full bridge rectifier and the high-voltage capacitors. I'm not an EE, but I would guess the simple answer is it's because you can't pass a AC through a transistor, since a transistor not only switches but also acts like a diode. I think you could work around that (build a circuit that switches AC with transistors), but there must be a reason why you wouldn't want to. Perhaps it's because it's simpler to just go ahead and rectify, or perhaps there's an advantage to having capacitors on the high-voltage side.
  - kens 5y ago
    One problem is that when your 60 Hz AC goes to 0, your chopped AC will go to 0 too. You'll need to store energy somewhere to get across the milliseconds of no power. And your transistor, transformer, etc will need to be larger to handle the peaks.
  - dfox 5y ago
    Circuit that tries to do this directly from AC is going to be more or less equivalent to active rectifier followed by traditional SMPS power stage (including some kind of DC bus with some nontrivial smoothing capacitor). In case when the whole thing is only supposed to work in one direction and input is low fixed frequency AC such construction is simply not worth it.
  - tedd4u 5y ago
    Nowadays it’s not an issue as much since the advent of digital seat-back entertainment but often, listening to analog passenger aircraft audio loops you can hear the 400Hz come through. 400Hz is noticeably sharper than G but between G (392 Hz) and G# (415 Hz) on the standard musical scale (A = 440Hz). So the 400 Hz clashes with everything.
    - bonzini 5y ago
      Is this a standard or does it only apply to one aircraft maker?
megous 5y ago
I built some 50Hz transformer power supplies when I was younger. :) They were 2x the size of the ATX power supply, produced like 60-100W of power, and weighed something like 2kg. Friends building class A audio amps were quite fond of toroidal transformer power supplies, too. They are easy to build, much less complex than switching power supplies. But as others say, less efficient and I'd guess more expensive, these days, unless you have some spare toroid transformer and high capacity low voltage capacitors laying around, already. Just the toroidal transformer for the 800W power supply would weigh like 8kg or so. :)
- bonzini 5y ago
  I had one of those for the VIC20, it got really hot and indeed it was large and heavy.

cristoperb 5y ago

In switching mode power supplies like this, the mains AC is rectified and then chopped up into higher frequency pulses and fed to the transformer. The higher frequency allows for smaller transformer. It also allows for voltage regulation because the input pulse width (in this case via the UC3842B controller IC) can be altered based on the sensed output voltage.

RL_Quine 5y ago

50/60hz requires a large transformer core. Switching power supplies rectify to DC, then drive the transformer at potentially megahertz which increases efficiency and substantially reduces the size and weight of the core. For similar reasons aircraft use 400hz 115v supply as it makes all of the power conversion electronics dramatically smaller.

londons_explore 5y ago
To add to this... A transformer converts electrical energy into a magnetic field and back into electrical energy. All the energy from a single 'sine wave' must be stored in the magnetic field inside the transformer briefly before coming out of the output. The transformer core is made of steel usually. For a given weight of steel, there is a maximum amount of magnetic energy it can store. That means that at low frequencies like 50/60Hz, transformers end up very bulky.
- whoisburbansky 5y ago
  Wasn’t immediately clear from GPs comment and my admittedly limited physics instruction, but the way you explained it made it seem incredibly obvious in retrospect, which is a rare skill, thanks!
jeffbee 5y ago
The standing field in a massive 60Hz transformer also necessitates robust inrush protection. With a transformer across the mains, losing the input power momentarily can cause a huge surge current when the input is reapplied. With the bridge and filter first, the inrush from being momentarily switched off won't be severe.
adrianmonk 5y ago
I think they are asking this: why not use the transistor to chop the 50/60 Hz AC into potentially-megahertz AC? Then pass that chopped AC to the transformer, which could still be small. The motivation would be to try to eliminate the full bridge rectifier and the high-voltage capacitors. I'm not an EE, but I would guess the simple answer is it's because you can't pass a AC through a transistor, since a transistor not only switches but also acts like a diode. I think you could work around that (build a circuit that switches AC with transistors), but there must be a reason why you wouldn't want to. Perhaps it's because it's simpler to just go ahead and rectify, or perhaps there's an advantage to having capacitors on the high-voltage side.
- kens 5y ago
  One problem is that when your 60 Hz AC goes to 0, your chopped AC will go to 0 too. You'll need to store energy somewhere to get across the milliseconds of no power. And your transistor, transformer, etc will need to be larger to handle the peaks.
- dfox 5y ago
  Circuit that tries to do this directly from AC is going to be more or less equivalent to active rectifier followed by traditional SMPS power stage (including some kind of DC bus with some nontrivial smoothing capacitor). In case when the whole thing is only supposed to work in one direction and input is low fixed frequency AC such construction is simply not worth it.
- tedd4u 5y ago
  Nowadays it’s not an issue as much since the advent of digital seat-back entertainment but often, listening to analog passenger aircraft audio loops you can hear the 400Hz come through. 400Hz is noticeably sharper than G but between G (392 Hz) and G# (415 Hz) on the standard musical scale (A = 440Hz). So the 400 Hz clashes with everything.
  - bonzini 5y ago
    Is this a standard or does it only apply to one aircraft maker?

megous 5y ago

I built some 50Hz transformer power supplies when I was younger. :) They were 2x the size of the ATX power supply, produced like 60-100W of power, and weighed something like 2kg. Friends building class A audio amps were quite fond of toroidal transformer power supplies, too. They are easy to build, much less complex than switching power supplies. But as others say, less efficient and I'd guess more expensive, these days, unless you have some spare toroid transformer and high capacity low voltage capacitors laying around, already. Just the toroidal transformer for the 800W power supply would weigh like 8kg or so. :)

bonzini 5y ago
I had one of those for the VIC20, it got really hot and indeed it was large and heavy.