Sorry! I neglected to mention that the transformer will be used in a lab bench power supply.
The range is 0-30VDC in 0.01 Volt inc/decrement & 0-5A in 0.01A inc/decrement. If I can do it I would like to be able to provide miilivolt, milliamp values at least below 1V, 1A if not the full range.
I’m not asking for instructions on a whole build just the transformer specs or how to derive them.
Thanks for all the useful input thus far.
Mike V
You're talking about a pretty serious PS - especially now that the current has been kicked up to 5A.
You want to make 30VDC - that means you need to drop something across the regulator, say 6V so the DC voltage input needs to be 36V. As a first approximation, the DC output of a full wave rectifier is 1.4 time the RMS AC voltage so you need a transformer in the range of 26V.
You're talking about 5A at 26V or 130 Watts. The VA rating needs to account for the power factor of the rectifier and that number probably isn't known. So, maybe 200 VA would do it.
That should get you in the ballpark.
HOWEVER... When you try to output 1V at 5A, the regulator has to drop (36V - 1V) * 5A or around 175 Watts - quite a space heater.
And this is why commercial power supplies have multi-tapped transformers with the appropriate tap chosen by relays based on output voltage setting.
I can't seem to build a direct link but you can Google 'mouser 530-MPI-200-28' and get to a 28V 200 VA transformer but the max primary is 230V so the secondary will be a little higher if you drive it with 240V. You can just go to mouser.com and enter the part number if the search doesn't work.
This power supply topic comes up every week. The problem is, the specs are not realistic. 5A is pretty high for a general purpose supply. Why 30V? Just because Rigol can do it? Even my Rigol DP832 can't output 5A at 30V. It's a $500 supply with dual 30V/3A and a single 5V/3A capability. But it's $500! And I will never use the higher currents at 30V and, since I don't play with 74xx logic, I probably won't get to 3A on 5V either. The other day I brought up a complete Z80 system including the CP/M operating system and I think the system took less than 200 mA. It uses 74LSxx logic plus a CMOS processor chip.
Transformer specs don't favor the DIYer. It's easy to get a multi-tap toroidal transformer if you want to design it and buy 1000 of them.
I bought the DP832 because I could, not because I needed it. I can get 5V/2A wall warts all day long. I can get 12V wall warts or 6V. If I wanted to, I could build a +-15V supply for op amp projects with 7815 and 7915 1A regulators.
Here is our host's video series re: a lab power supply:
Here is another of our host's power supply projects:
https://www.eevblog.com/projects/usupply/These two series may give you some ideas.