Multi tap Toroid transformer for making a portable power supply - Page 1
Forum section: electronics / projects

author: 460voltclub
author_rank: Contributor
timestamp: August 26, 2020, 08:50:28 pm
content: The transformer wires are as follows:Red & Blue 120 VAC supplyOrange & Brown & Orange 34 VAC Brown is the center tap 17 VACYellow & Black & Yellow 26 VAC Black is the center tap 13 VACGreen & Green 8 VACThe transformer will be mounted in a plastic tackle type box to make it portable. There will be bridge rectifiers and voltage regulators. fuses etc.I will have panel mount female sockets for power connections. I will have 30 V, 24 V, 15 V, 12 V and 5 V supplies. My major issue is I need a negative voltage supplyfor each voltage and I can't come up with a circuit I am happy with. Any ideas would be greatly appreciated. I have a well equipped electronics work shack so fabrication is not a problem.Thank You

author: Doctorandus_P
author_rank: Super Contributor
timestamp: August 27, 2020, 12:00:11 am
content: What are the current ratings for these transformer windings, and how much current do you need?Are you able to split the windings? With some transformers the center tap is connected through on the outside, and it can be split in 2 separate windings.Part of the problem I think is that you want too much.Do you want very high quality, or do you want something reasonable easy to build?There are many choices to be made, some will be hard compromises.The 8V winding will of course be the most logical for the 5V output. This is most often for powering logic chips, uC's and such, and the simplest would be to put a switching CC-CV module on it.With the 34V winding you can make a 2x 20V power supply: ( 17Vac * sqrt(2) = 23.8V), so you have a few volts left for Elco ripple and regulation. Maybe it only goes upto 2x15V or 2x18V, but that is enough for almost all analog circuits such as opamps etc. Circuits like this usually do need much current. I think a 500mA supply with good regulation will be more useful then a 3A power supply with mediocre regulation.If you go for lower power, but higher quality, and your transformer can deliver the power, then you can put 2 of these split power supplies in parallel, and then just share GND.If you want to build a linear power supply using relays / mosfet's to switch between transformer windings is often done to reduce dissipation. But building a switching SMPS as a pre-regulator is not so difficult, and makes it easier to make-do with the transformer you have.Do you want to design it yourself? or use pre- built modules?DPS5005 seems to be quite popular, and it would make your project very easy, but no dual outputs.DPS series are buck regulators, so only go down in voltage. The DPH series can also output a higher voltage than it's input. These modules cost around EUR25. There are also older versions with just 7-segment displays and lower power ratings starting already around EUR10. A lot of these modules have an STM32 in them, which can be re-programmed. (Take a look at "opendps" for example).

author: Pawelr98
author_rank: Regular Contributor
timestamp: August 27, 2020, 09:42:30 pm
content: Center tapped windings allow symmetrical +- voltages to be formed.That was already mentioned by Doctorandus.However if negative voltage is desired from a single winding then either use the doubler circuit (basically two half wave rectifiers, can supply high current on both rails) or use a pump charge.Pump charge can create a negative voltage even with the main supply working in full wave rectifier.Look up the "electronics lab power supply" schematic (I don't recommend this PSU but the schematic shows the circuit).Over there they implemented such a pump charge circuit for negative supply.This pump charge circuit is used mostly for low loads like opamps.

author: MarkF
author_rank: Super Contributor
timestamp: August 28, 2020, 01:07:17 am
content: The center-tap windings can provide either a +/- voltage or a single voltage depending on how you connect the outputs.You can also just use half of the +/- voltage outputs.You can connect the three connection of each group in any way because that winding is floating.With this you can have (+5V), (+12V, +24V or +/-12V) and (+15V, +30V or +/-15V).