Hey everyone - first time here, nice to meet you all!
I want to build myself a PSU for general use, like powering MCU dev kits and other low voltage electronic projects. I want to power it from the UK main supply (230VAC rms), and I want to have say, 3 outputs:
*0 to 20V adjustable, maybe 1A (20W max)
*5V fixed, 0.5A max, 2.5W
*3.3V fixed 0.5A max, 1.65W
So, first thing I need to do is get a DC supply from the mains AC. This supply can be rough and and I plan to sub-regulate with SMPS to get nice clean outputs. I haven't decided on a full topology just yet as I don't know yet how to make sure I won't burn out my transformer, which is the primary purpose of my visit here. Looks as though it's a good idea to use an isolated transformer for safety reasons. I have found this part 173-9579 on RS, capable of 230VAC to 2*24VAC with 60VA capability. If I connect the two transformer outputs in series I can get 48VAC with my 60VA. Using a full-bridge rectifier with smoothing capacitor will get me around 33VDC.
All good so far?
Now for the first hurdle: How much DC current can I draw from this supply while staying within my 60VA limit? Preferably I'd like to stay at around 75%, 45VA, of this for reliability (is this sensible or overkill?). So after the initial inrush to fill the smoothing cap the current pulled from the transformer will vary with my load, which in turn I want to be variable! If I use a massive amount of smoothing capacitance the inrush risks burning the transformer out. Too little and the supply has more ripple, leading to bigger gulps of current from the transformer. I just don't know where to go here... how do I manage smoothing capacitance with adjustable load and not start a fire?
Thanks in advance
G
I want to build myself a PSU for general use, like powering MCU dev kits and other low voltage electronic projects. I want to power it from the UK main supply (230VAC rms), and I want to have say, 3 outputs:
*0 to 20V adjustable, maybe 1A (20W max)
*5V fixed, 0.5A max, 2.5W
*3.3V fixed 0.5A max, 1.65W
So, first thing I need to do is get a DC supply from the mains AC. This supply can be rough and and I plan to sub-regulate with SMPS to get nice clean outputs. I haven't decided on a full topology just yet as I don't know yet how to make sure I won't burn out my transformer, which is the primary purpose of my visit here. Looks as though it's a good idea to use an isolated transformer for safety reasons. I have found this part 173-9579 on RS, capable of 230VAC to 2*24VAC with 60VA capability. If I connect the two transformer outputs in series I can get 48VAC with my 60VA. Using a full-bridge rectifier with smoothing capacitor will get me around 33VDC.
All good so far?
Now for the first hurdle: How much DC current can I draw from this supply while staying within my 60VA limit? Preferably I'd like to stay at around 75%, 45VA, of this for reliability (is this sensible or overkill?). So after the initial inrush to fill the smoothing cap the current pulled from the transformer will vary with my load, which in turn I want to be variable! If I use a massive amount of smoothing capacitance the inrush risks burning the transformer out. Too little and the supply has more ripple, leading to bigger gulps of current from the transformer. I just don't know where to go here... how do I manage smoothing capacitance with adjustable load and not start a fire?
Thanks in advance
G