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PSU supporting both 120v and 240v.
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dcbrown73:
I've dabbled in low voltage DC electronics for a while, but I'm looking at learning and building a lot more.  I intend to build Bob Heil's Pine Board AM Transmitter and part of it is building a power supply for it. 

That got me thinking about PC and server PSU's in how they can accept both 240v and 120v inputs.  How exactly do they handle the different voltages?   Does it just detect if it's 240v and use a center tap or the entire winding if it detects 120v?  Sorry if this sounds like a stupid question.

Thanks,
Dave
Dave:
Unlike linear power supplies, which use hefty laminated iron core transformers, computer power supplies use smaller, high frequency switching transformers. The latter support a much wider input voltage range. There is no need for any tap switching, the regulator simply adjusts the PWM duty cycle to get the correct output.

I would avoid a switchimg converter for your specific application, because your RF circuit might not interact well with the noisy power supply.

If you want to learn more about switching converters, I recommend you first try to wrap your head around simpler topologies, like the buck converter.
IanB:

--- Quote from: dcbrown73 on June 10, 2019, 05:46:53 pm ---I've dabbled in low voltage DC electronics for a while, but I'm looking at learning and building a lot more.  I intend to build Bob Heil's Pine Board AM Transmitter and part of it is building a power supply for it. 

That got me thinking about PC and server PSU's in how they can accept both 240v and 120v inputs.  How exactly do they handle the different voltages?   Does it just detect if it's 240v and use a center tap or the entire winding if it detects 120v?  Sorry if this sounds like a stupid question.
--- End quote ---

For a simple linear power supply using a transformer, such as you might build yourself, this can be done using a transformer with two 120 V primary windings. You put them in parallel for 120 V mains and in series for 240 V mains. This is most simply done using a DPDT mains switch.

I have a power supply where you have to open the case and manually reconfigure the wires, on the assumption that you make such configuration changes very rarely.
tooki:
Think of a switch-mode power supply as a device that simply takes little sips of power from the primary side and sends them to the secondary, just often enough to average out to the necessary output voltage. With a higher input voltage, it simply takes shorter sips of power from the primary.

For example, imagine if the primary side is 120V, and you need 12V at 1A. Then (depending on the transformer winding) you might only draw power from the primary 5% of the time. If the load increases to 2A, you might need to draw power 10% of the time. At 240V, though, with the 2A load it'd only need 5% of the time.

Look into PWM (pulse width modulation), as it's a really good way to get a very rough understanding of the principles.
Doctorandus_P:
Have a look at voltage doublers:
https://duckduckgo.com/?q=psu+"voltage+doubler"&iax=images&ia=images

The principle is quite simple.
For SMPS power supllies on 230Vac the input is a simple full bridge which rectifies to about 310Vdc.
Then there are 2 small modifications to have it run on 120Vac.
First the elco is split in 2 elco's in series.
Then the center tap between the elco's is shorted to one of the input terminals.
This means the top elco is rectified to around +150Vdc, and the bottom elco is rectified to around -150Vdc, and together they are 300V, about the same as from the full bridge.

Additonal advantage is that 200V elco's can be used instead of 400V elco's, Disadvantage is that somewhat bigger elco's must be used because of the (2x) half wave rectification for 120Vac.

--------------------------
With smaller power supplies it's more common that they simply accept a wide change of input volatage.
With a bit of forethought it's not too difficult to design an SMPS for which the input voltage can have a 1:4 ratio.

If you have a look at the very common small smps modules from Ali / Ebay / China with for example a LM2596 or MP1485, they have a wide input voltage, and they simply adjust the PWM duty cycle internally to regulate the output voltage.



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