Power Supply Transformer question - Page 1
Forum section: electronics / beginners

author: made2hack
author_rank: Frequent Contributor
timestamp: May 26, 2015, 02:04:06 pm
content: Hello all,So I have a massive 1000VA transformer that I am rewiring / recycling for a desktop power supply. I have a few questions.I was reading about SMPS topologies, namely how buck / boost are generally not isolated (ie you rectify straight from mains) and flyback is isolated (via high freq. trafo).However, my question is this. Can I use my trafo to first isolate and drop down voltage from 230VAC to something more sensible such as 30VAC? (or thereabouts?) . Does that mean that I can not use buck boost anymore? Or can I still use it ?Ie, in my schematic I've drawn what I am conceptualizing however I'm not sure whether this idea is valid.I would like the desktop supply to be able to deliver 36V / 2A (or thereabouts).

author: opty
author_rank: Regular Contributor
timestamp: May 26, 2015, 02:30:55 pm
content: Quote from: trs80 on May 26, 2015, 02:04:06 pmSure you can,Let me google that for you, ah here you go:http://jayakody2000lk.blogspot.ie/2015/05/3a-power-supply-for-usb-devices.htmlhttp://elect.wikispaces.com/3A+Power+supply+for+USB+deviceshttp://www.elektroda.pl/rtvforum/topic3030107.html[in Polish - implementation of the above projects]

author: PSR B1257
author_rank: Frequent Contributor
timestamp: May 26, 2015, 03:04:13 pm
content: You have connected the diode bridge back to front

author: Cliff Matthews
author_rank: Unknown
timestamp: May 26, 2015, 06:45:36 pm
content: Quote from: trs80 on May 26, 2015, 02:04:06 pmThe objective you're looking for is so attainable (without using 5 kilos of iron). Safety's a priority, so why not wire that nice core into a bench isolation transformer? Then, maybe wrap-on another isolated secondary for the project.

author: Richard Head
author_rank: Frequent Contributor
timestamp: May 27, 2015, 05:53:57 am
content: You can do that but you are throwing away half the advantage of going switchmode. (ie Low weight)

author: nanofrog
author_rank: Super Contributor
timestamp: May 27, 2015, 06:39:18 am
content: You can do it, but it's a waste of that large a transformer.Personally, I'd recommend using it for a linear PSU, whether it be regulated (i.e. lab supply) or unregulated (i.e. audio amplifier PSU). Bit overkill for a regulated lab PSU, but you could increase the number of outputs in order to fully utilize that 1kVA (i.e. 6 * 30V/3A + 2 * 6V/5A, all variable).

author: made2hack
author_rank: Frequent Contributor
timestamp: May 27, 2015, 09:22:27 am
content: yeah,the idea is to have it as the base of a linear / switch mode desktop psu. the reason I want to try a smps topology is more of as an added project. I have room for tons of windings.However, I have one question: According to calculations viaLudens Coils & Trafos, ie: t = V(RMS) / 4.44 / m2 / Hz / T , I should have approximately 410 Turns / 220V (or around 2 turns / volt). (my core size is 40mm x 50xx = 0.002 m2). This is assuming 1.2Tesla. If I assume 1Tesla, then it would be around 500 Turns / 220 V, or 2.5 Turns / Volt.However, when looking at the existing primary winding, it looks closer to 600 ~ 660 turns. The wire used for the primary is ~ 1mm cross section. So I estimate about 70 turns per layer (it has around 72 mm space). Plus around 8 layers (giving approx 560 turns) and the final few to get to around 630~660 turns.When I wound 9 turns as a secondary, I was getting around 2.6V (leading me to believe in the 3 turns / volt figure). Now, does it matter if I only put in the E-cores without fastening them? I mean I stuffed them and turned the thing on (big vibrations). Is it possible I was loosing 2 volts by not having the core solid? Or there is not that much loss in voltage?Because, via the math, I should have 2 turns / volt for this size core, and not 3 turns / volt.