Newbie audio LPS help - connections for R-Core transformer and voltage regulator - Page 1
Forum section: electronics / beginners

author: hltf
author_rank: Newbie
timestamp: December 20, 2015, 09:21:51 pm
content: Hullo:Many thanks in advance for some basic EE connection answers for a complete novice. I am an audio enthusiast engaged in my first DIY project. I am building a computer music server with a Raspberry Pi 2 Model B. To do this I am putting together a DIY linear power supply using an R-Core transformer purchased on EBay and a voltage regulator purchased on Amazon.I purchased the transformer described below on EBay from a seller in China which will be delivered to me shortly:TRANSFORMER DETAILS: 115V/230V 50W high quality Audio R-Core Transformer 15V+15V 9V+9VInput Voltage: 0-AC115V-AC230V; output voltage: AC 15V+15V(1A 4 lines ) and AC 9V+9V(1.1A 4 lines); Output Power:50WSize:96×82×47?mm; Weight: about 1kgFeatures: Japanese Nippon steel coiled silicon steel sheet; Oxygen-free copper enamel wire; With shielding wire output; Stable performancehttp://www.ebay.com/itm/140864418899?_trksid=p2057872.m2749.l2649&ssPageName=STRK%3AMEBIDX%3AIT(1) My first question is how should I connect this transformer to AC 110v mains for the input power? I am assuming I should buy a male IEC plug such as this for example:http://www.vhaudio.com/images/fi-11mcu.jpgAnd then will I need to solder the input cable connections of the R-core transformer to the posts on the rear of the IEC plug? If I do need to solder, I guess I should take it to an electrician. If I don't need to, can I make the connections to the IEC plug myself?(2) And then my second question. I will be using the 9V+9V outputs of the Chinese R-core transformer above to go into the Voltage Regulator (details and link below) which I purchased on Amazon.Electronics-Salon 5 Amps 1.5 to 32V Adjustable Voltage Regulator Modulehttp://www.amazon.com/gp/product/B00LO273K8?psc=1&redirect=true&ref_=oh_aui_detailpage_o00_s00BTW the Raspberry Pi needs 5v/2amps DC as input power delivered via Micro USB. On the Chinese R-core transformer there are four 9V outputs with each one rated at 1.1 amps. I assume I just need to connect two of these output cables to the voltage regulator's inputs, is that correct? And the other two output cables will just be extras that I don't need and therefore leave unused? Is the 9 Volt/1.1Amp output rating of the two transformer outputs adequate for the voltage regulator to deliver the 5Volts/2Amp input needs of the Raspberry?(3) Finally, what cable should I buy to connect the output of this regulator to a micro USB which is how I make the connection to the Raspberry? And where will I find it available for sale?Again many thanks in advance for your help.hltf

author: ciccio
author_rank: Frequent Contributor
timestamp: December 21, 2015, 02:54:51 pm
content: Well, I'll try to answer your questions, but I'm a little afraid to help a total unexperienced person to work with mains voltage....1) if you have money to waste, buy the "audiophool" plug you have linked to, or (better choice) a standard (and cheaper) mains cord with a molded plug (if you cannot find a suitable cord you can buy an extension cord and cut away the socket)If you live in USA the cable will have the following wire colours (hopefully):https://en.wikipedia.org/wiki/Electrical_wiringlive: black : to transformer's primary blue wireneutral :white: to transformer's primary black wireearth: green or yellow/green: to transformer's primary green/yellow wireA suitable table fuse (0,5 to 1.0 amps, slow blow) shoud be wired in series with the primary live conductor.2) On the secondary side, if you need 2 amps at 5 volts you must parallel two 9 V secondaries (hoping they are identical).The problem is that the two secondaries have the same wire colour (yellow and brown) for both start and end of the windings.You must parallel them in phase, or you will get a short circuit on the secondary.A way to determine the correct secondary phase is to wire them first in series:a) Label the four yellow and black wires as : Y1, Y2, B1, B2b) Connect Y2 to B1c) Measure the voltage between Y1 and B2Then :if the voltage is 18 or more volts, Y1 and B1 are the start of the windings,if the voltage is 0V, Y1 and B2 are the start of the windingsif the voltage is small, but measurable, then discard the transformer: the two secondaries are differentand are not suitable for being paralleled.Then connect in parallel the start and the end of the two windings to the voltage regulator's ac input screw terminals.I hope of having been clear in my explanations, but take a good advice and:Buy a suitable "wall wart" power supply and don't tamper with mains voltages.Best regards

author: Brumby
author_rank: Unknown
timestamp: December 21, 2015, 03:13:38 pm
content: For 2A from the regulator, the supply will need to be able to deliver at least 2A.You do this by connecting the two 9V windings in parallel - but there's a catch. There are two ways of connecting the 9V windings. One way will give you 18V at 1.1A and the other will give you 9V at 2.2A.Here's how you get it right using a meter than is set on AC volts:1. Wire up the mains connection as set out byciccioMake sure all mains wiring is secure and insulated.2. Connect one of the yellow wires to one of the brown wires (it doesn't matter which at this point) and measure the voltage between theotheryellow andotherbrown wire3. If the voltage is around 18V, then you need to swapeitherthe yelloworthe white wires around (not both!) - then re-test.4. If the voltage is zero (or pretty close to it) then you keep the yellow and brown that are already connected as one supply line and just join the other yellow and brown wires you have had the meter on for the other.You then end up with 2 sets of brown and yellow wires - which will give you 9V at 2.2A

author: ciccio
author_rank: Frequent Contributor
timestamp: December 21, 2015, 03:20:11 pm
content: Thanks Brumby.I was writing the post and it was inadvertently posted whe it was not finished.I edited it and re-posted before seeing yours.I was trying to explain the secondary trick: your explanation is more clear than mine.Best regards

author: Brumby
author_rank: Unknown
timestamp: December 21, 2015, 03:29:46 pm
content: No worries. We both said the same thing ... but you did raise a point about matched secondary windings. Not all secondaries will be an exact match and minor discrepancies shouldn't be a problem - but what is 'minor'?So I'll put out the question: In the above case, what would be the limit of safely being able to parallel two secondary windings with different voltages? 100mV? 500mV? 1V??

author: Brumby
author_rank: Unknown
timestamp: December 21, 2015, 03:49:22 pm
content: Here's an alternative to the parallel windings.....Run a bridge rectifier off each 9V secondary and parallel the DC sides.Depending on the voltage differences of the secondaries, at low currents, one winding will be bearing the lion's share - but as current draw increases and you get a bit of voltage sag, the other will contribute more and more. The acceptable voltage difference in the secondaries will be dependent on the maximum current required.Just a thought.

author: ciccio
author_rank: Frequent Contributor
timestamp: December 21, 2015, 08:46:58 pm
content: Quote from: Brumby on December 21, 2015, 03:29:46 pmSorry, I do not have an answer, but in my experience if the difference in unloaded voltage was less than some percent, I've never had a problem.I think that, when loaded, the winding with the higher voltage will supply a larger current, that will increase the winding temperature resulting in an increase of resistance (copper has a positive tempco), and this will tend to reduce current.It will be the same effect you get when paralleling diodes for a larger current rating.I was thinking to the OP post: in my work with pro audio I see that a switching power supply is most of the times an advantage (with the exception, maybe, of power amplifiers).With a switching supply mains induced hum tend to be a problem of the past, and most of the new pro audio gear that I see uses switch mode power supplies, and you can measure and many times hear the advantages.I'm actually testing a speaker controller with 24 bits converters and a dinamic range of more than 115 dB. It uses a switch-mode power supply, a standard 30 W block by MeanWell. No analog power supply, evenif mounted in an external enclosure, will allow this.The OP want to power an Arduino based system (a digital system) with an analog power supply, and even he wants to use an "esoteric" mains plug..., when a simple switching power pack, made by any decent manufacturer, will be more suitable for the job..Best regards

author: ciccio
author_rank: Frequent Contributor
timestamp: December 21, 2015, 08:55:28 pm
content: In my first post I forgot a very important note, that is obvious to most of us, but maybe not to the OP (when I was young I was always trying to do fireworks with mains connected experiments) :INSULATE ALL UNCONNECTED WIRES, expecially the 230V one (RED): it will be at 230V when you apply 115 V to black and blue.Insulate (from each other) all unused secondary wires (grey and white).Best regards