3 ms·

Can someone explain why the transformer is on the DC side of the circuit in the diagram he draws at the beginning of the video?

by 0xffff2 11y ago

sitharus 11y ago
This is a switch mode power supply. In order to use a smaller transformer it uses very high frequency AC rather than 50/60Hz mains frequency. In order to do this it rectifies the mains input to high voltage DC (330/220Vish, depending on the mains voltage) then using a power transistor rapidly switches this on and off at around 1-10kHz. Then the transformer outputs low voltage AC which is re-rectified and filtered to produce low voltage DC. The switch mode chip monitors the output voltage and modulates the switching frequency to maintain a constant output voltage. If you look closely at the board (and he talks about this in the video) there's a separation between the high and low voltage sides. Only the transformer and certain specialized components should cross this (Y-class capacitor and optoisolators). Cheap power supplies often miss this out which could allow a small bit of loose solder being able to send mains voltage to the output. Look at other videos by Big Clive to see the horrors. The high voltage side is a bit of a misnomer. It's really just the non-isolated side - the chip runs on 5v or similar, but it's not completely separated from the high voltage DC so isn't safe for human contact.
- rlpb 11y ago
  > the chip runs on 5v or similar How does the chip get 5V, out of interest? Given that it's not allowed to be connected to the non-isolated side?
  - sitharus 11y ago
    Normally though an additional winding on the transformer. Switch mode transformers have three or four sets of windings. The primary (high voltage), secondary (low voltage) a bootstrap winding (for the chip) and sometimes a flyback winding. The bootstrap winding takes off a variable voltage depending on the local supply and uses a linear regulator to produce the required voltage. For a better explanation than I can manage in text the EEVBlog channel goes in to all sorts of regulators and how they work: https://www.youtube.com/watch?v=cM7t1Mpu7s4&list=PLn-ZIFhWxhWpbwaHZNTBWoOmeyeVPCq_i https://www.youtube.com/watch?v=cM7t1Mpu7s4&list=PLn-ZIFhWxh...

Can someone explain why the transformer is on the DC side of the circuit in the diagram he draws at the beginning of the video?

by 0xffff2 11y ago

Can someone explain why the transformer is on the DC side of the circuit in the diagram he draws at the beginning of the video?

sitharus 11y ago

This is a switch mode power supply. In order to use a smaller transformer it uses very high frequency AC rather than 50/60Hz mains frequency. In order to do this it rectifies the mains input to high voltage DC (330/220Vish, depending on the mains voltage) then using a power transistor rapidly switches this on and off at around 1-10kHz. Then the transformer outputs low voltage AC which is re-rectified and filtered to produce low voltage DC. The switch mode chip monitors the output voltage and modulates the switching frequency to maintain a constant output voltage. If you look closely at the board (and he talks about this in the video) there's a separation between the high and low voltage sides. Only the transformer and certain specialized components should cross this (Y-class capacitor and optoisolators). Cheap power supplies often miss this out which could allow a small bit of loose solder being able to send mains voltage to the output. Look at other videos by Big Clive to see the horrors. The high voltage side is a bit of a misnomer. It's really just the non-isolated side - the chip runs on 5v or similar, but it's not completely separated from the high voltage DC so isn't safe for human contact.

rlpb 11y ago
> the chip runs on 5v or similar How does the chip get 5V, out of interest? Given that it's not allowed to be connected to the non-isolated side?
- sitharus 11y ago
  Normally though an additional winding on the transformer. Switch mode transformers have three or four sets of windings. The primary (high voltage), secondary (low voltage) a bootstrap winding (for the chip) and sometimes a flyback winding. The bootstrap winding takes off a variable voltage depending on the local supply and uses a linear regulator to produce the required voltage. For a better explanation than I can manage in text the EEVBlog channel goes in to all sorts of regulators and how they work: https://www.youtube.com/watch?v=cM7t1Mpu7s4&list=PLn-ZIFhWxhWpbwaHZNTBWoOmeyeVPCq_i https://www.youtube.com/watch?v=cM7t1Mpu7s4&list=PLn-ZIFhWxh...