Hello,
Just wanted to double check my thinking is sound for a power supply I am working on.
I am building a PCM56 DAC, my first digital design, a simple NOS implementation. I came across the idea of using a balanced isolation transformer prior to the mains transformers to reduce mains-coupled common-mode noise.
I had looked into a custom toroidal for this purpose, but had not found a transformer maker for the job.
Instead, I am looking at an off-the-shelf option. Maximum secondary current draw for my design at 120VAC is ~70mA. I am looking to use this transformer from Triad Magnetics, VPM240-100. It is a dual primary (120VAC, 240VAC) dual secondary (120VAC, 240VAC) medical grade toroidal with electrostatic shield and hum band.
Secondary rating for 120VAC (parallel) is 208mA, for 240VAC (series) is 104mA.
https://www.mouser.com/datasheet/2/410/VPM240_100-844597.pdf
What I would like to do is use both primaries and secondaries in series, as if running at 240VAC, but with 120VAC on the primary and ground the meeting point of the two secondaries (virtual center tap) for balanced +/-60VAC balanced. Again, max secondary current draw for my design is 70mA.
I think this should work, but thought I would run it by you folks, thanks.
Just wanted to double check my thinking is sound for a power supply I am working on.
I am building a PCM56 DAC, my first digital design, a simple NOS implementation. I came across the idea of using a balanced isolation transformer prior to the mains transformers to reduce mains-coupled common-mode noise.
I had looked into a custom toroidal for this purpose, but had not found a transformer maker for the job.
Instead, I am looking at an off-the-shelf option. Maximum secondary current draw for my design at 120VAC is ~70mA. I am looking to use this transformer from Triad Magnetics, VPM240-100. It is a dual primary (120VAC, 240VAC) dual secondary (120VAC, 240VAC) medical grade toroidal with electrostatic shield and hum band.
Secondary rating for 120VAC (parallel) is 208mA, for 240VAC (series) is 104mA.
https://www.mouser.com/datasheet/2/410/VPM240_100-844597.pdf
What I would like to do is use both primaries and secondaries in series, as if running at 240VAC, but with 120VAC on the primary and ground the meeting point of the two secondaries (virtual center tap) for balanced +/-60VAC balanced. Again, max secondary current draw for my design is 70mA.
I think this should work, but thought I would run it by you folks, thanks.
Running the primaries in series but only feeding in 120VAC will have the effect of making the transformer's regulation much poorer. I think it would be a good idea to check that the DAC's own mains transformers are going to be OK with a rather saggy AC feed. By which I really mean check your regulators have enough headroom for operation on low mains.
Thanks, abraxalito, I will keep that in mind and keep an eye on the secondary voltage. If it becomes an issue I'll change strategies. My mains transformer primaries are rated for 115VAC, so I do have that working in my favor if there are significant losses across the isolation transformer.
Just one other thing - the inter-winding screen is the main thing attenuating the common-mode noise. It needs as low an inductance connection to earth as possible to get the best performance. The longer the wire to it, the more noise gets through.
I should have pointed out that by 'the wire' I didn't only mean the wire inside your box. All the wiring back to mains earth should be included😱
Oh okay, in that case I will call an electrician and have a dedicated mains earth connection added to my home just exterior to where the DAC is plugged in, will keep that wire short too.
Just kidding 🙂 I'll do what I can control and hope for a good result. Given this is my first swing at any DAC design, the bar is pretty low! 😛
Just kidding 🙂 I'll do what I can control and hope for a good result. Given this is my first swing at any DAC design, the bar is pretty low! 😛
You're taking a lot more pains over it than I did on my first swing at DAC design so you deserve a good result!