AC in for dual regulator circuit - Page 1
Forum section: electronics / beginners

author: harps
author_rank: Regular Contributor
timestamp: November 08, 2017, 11:57:22 pm
content: hello thereI am wondering regards the schematic (jpg attached) I am hoping to tune this so that it gives out DC 18v + & - with a ground)On the schematic, CON3 (AC in 3 inputs). Do i need a special transformer that gives out 2 x 24v and a ground wire ( 3 wires)?I have an AC 33v transformer, just two wires + - , is it possible to use that?thanks for help.: )

author: Jwillis
author_rank: Super Contributor
timestamp: November 09, 2017, 12:24:43 am
content: I believe you will need a 24-0-24 VAC transformer where the center tap ( 0V )is your common ground on CON pin 2 .the two 24 VAC would connect to CON pin 1 and CON pin 3.

author: harps
author_rank: Regular Contributor
timestamp: November 09, 2017, 12:37:30 am
content: thank you Jwillscheers for the info. : )

author: Audioguru
author_rank: Super Contributor
timestamp: November 09, 2017, 12:47:46 am
content: A 24V transformer produces a peak of 34V which is much too high for your 18VDC regulated output. Use an 18V transformer that has a peak of 25.5V, the rectifier drops it to 24.5V and the ripple in the filter capacitor reduces the voltage to the regulators to a reasonable 22V.So get a 18V 0-18V transformer called "36V center-tapped".

author: harps
author_rank: Regular Contributor
timestamp: November 09, 2017, 12:55:50 am
content: hi thanksWould a 2 x 18v toroidal be o.k? ( it will be for an audio preamp) like this:https://www.maplin.co.uk/p/2x18v-15va-toroidal-transformer-a76qg

author: harps
author_rank: Regular Contributor
timestamp: November 09, 2017, 08:22:01 am
content: helloThe regulator can handle 40v.I wonder, how does an 18v transformer provide a constant voltage of 22v for the regulators?i understand that the regulator requires a higher voltage of at least 3v than the desired output, as it says this in the pdf. that makes sense.That is why i thought a 24v transformer would be correct. (and that an 18v transformer would be too low)"the LM317 Operation With Low Input Voltage.The device requires up to 3-V headroom (VI – VO) to operate in regulation. The device may drop out and OUTPUT voltage will be INPUT voltage minus drop out voltage with less headroomand 3-V headroom required to support maximum current and lowest temperature."

author: rdl
author_rank: Super Contributor
timestamp: November 09, 2017, 09:13:05 am
content: How much current does this power supply need to provide?

author: harps
author_rank: Regular Contributor
timestamp: November 09, 2017, 11:09:16 am
content: HiI have the need to power:2 x mono mic preamps 17v dual ( not 18v , as i thought)1 x phantom 48v ( I will likely use a step up converter circuit which requires 10V to 38V input (24V nominal) maybe using another transformer for that?)Then i need something similar for the ADC analogue to digital converter.The ADC requires:( 15v dual) for its analogue power5v for its digital powerSo I thought 2 transformers for this project.1.1x toroidal with 2 x 24v = for the pre amps(17 dual) and ADC (15v dual analogue power)2.1x toroidal with 24v + 9v = for the phantom (48v) and the ADC digital power(5v)These transformers then give adequate power to the regulator circuits.I am not sure of the current required though.Thanks for help.

author: rdl
author_rank: Super Contributor
timestamp: November 09, 2017, 02:42:15 pm
content: I don't think you'll need anywhere near the maximum current that the regulators could provide. Considering the amount of input filter ("smoothing") capacitance shown in your schematic, an 18v x 2 transformer should be plenty. As a matter of fact, you could probably use half as much capacitance and still be good to go.https://en.wikipedia.org/wiki/Ripple_(electrical)

author: Kleinstein
author_rank: Super Contributor
timestamp: November 09, 2017, 04:26:38 pm
content: It does not sound like you need a lot of current. So way smaller caps should be enough, especially if there is plenty of voltage.The required current should be known.For the 48 V one could use a kind of votlage doubling or similar rectifier, as the current is likely low. It is a little like a 50/60 Hz driven charge pump. So no need for an extra transformer or switched mode converter

author: harps
author_rank: Regular Contributor
timestamp: November 10, 2017, 12:36:48 am
content: thanks everyonei had already made the circuit up (as shown in the schematic ) so the capacitors are larger than needed. but hopefully this wont be an issue.I purchased a 2 x 0-18 toroidal. 15va was the lowest rating in my price range. but should be o.ki shall see if i can power some of this up.I am not 100% sure on the phantom power, the mics i will use may be rather expensive so i would want it regulated and safe. I might buy a separate toroidal (2x25 in series to make 50v :?) and use a regulator tuned to 48v., shall have to research a bit more on thatthanks for your help. : )