Applying DC bias on a high voltage rectified ripple 570VAC down to offset  zero - Page 1
Forum section: electronics / beginners

author: andre123
author_rank: Newbie
timestamp: January 28, 2020, 01:29:22 pm
content: I am working on a linear power supply with a 3phase input. I wondering if there was a possibility to shift the rectified(ripple) power signal shown below which spans between 560VAC to 505VAC ( 55VAC difference @130 ohm load) down near to the zero so imainge it could span between 0 to 55VAC .If the voltage above is adjusted, then i imagine the voltage supply will need to go into a power transformer with a ratio of 3:1 then a go through a diode in series(half-wave rectification) then if needed go through a linear regulator for ripple reductiona and voltage regulation within reasonable expectations.My target is to setup a 12V ~100mA linear power supply from 3 phase(4-wire) 330V(max)/phase 50Hz.I've tried to use this high pass filter to adjust the bias butit shifts mostly under zero, several iterations got me no-where.under very small loads @100K ohm load, i'm getting results above zero around 20VAC, can't get it near that when load increasesNote: I am bound to have galvanic isolation ( Transformer ) in my application.I would really appreciate if someone would kindly share their thoughts.

author: SilverSolder
author_rank: Super Contributor
timestamp: January 28, 2020, 01:40:18 pm
content: Quote from: andre123 on January 28, 2020, 01:29:22 pmBut you said the 3 phase could vary from 550V to zero, or did I misunderstand that?

author: andre123
author_rank: Newbie
timestamp: January 28, 2020, 01:43:10 pm
content: Quote from: SilverSolder on January 28, 2020, 01:40:18 pmI meant in that context that a single line is 330VAC , but when all lines are connected+ rectified the output voltage line spans around 550 as shown in the first picture.RegardsAndre

author: Deactivated-1
author_rank: Frequent Contributor
timestamp: January 28, 2020, 01:48:37 pm
content: I am not completely sure of this, but that's not how diodes work. they don't add up the voltage. if 1 phase of the 3 rectified outputs ~400volts 3 of them (if the same voltage) will also output 400volts. but with more current.

author: SilverSolder
author_rank: Super Contributor
timestamp: January 28, 2020, 01:50:18 pm
content: If all you want is 12V DC at 100mA... why not just connect a regular 1 phase transformer between any two of the phases, and have the usual bridge rectifier + filter cap + regulator on the other side of that transformer?

author: Deactivated-1
author_rank: Frequent Contributor
timestamp: January 28, 2020, 01:53:08 pm
content: Quote from: SilverSolder on January 28, 2020, 01:50:18 pmyes, and if you really need a lot of current just use a 3 phase transformer. I think arc welders output somewhere around 12volts, maybe you can dissect one and put a high current bridge on it, then just a LOT of caps.edit: oh you need just 100ma?! then I guess you won't need like 200amp of welding transformer-nes ^also, I think putting a transformer between just 2 of the phases won't work very well cause the voltages don't sum up to 0?!or just hammer a steel rod in the ground to get ground

author: SilverSolder
author_rank: Super Contributor
timestamp: January 28, 2020, 02:01:03 pm
content: Quote from: ELS122 on January 28, 2020, 01:53:08 pmDon't forget, the secondary side of the little transformer will be floating - so you could connect minus of the 12V supply to ground, if you wanted to, for example.

author: KaneTW
author_rank: Frequent Contributor
timestamp: January 28, 2020, 02:01:46 pm
content: A 3-phase full-wave rectifier will give you 3 sqrt(2)/pi * V_RMS DC out, ignoring diode drop. So a 400V fed full-wave rectifier (with 230V line-to-neutral) will output 540V.Why 3-phase, though? At 12V/100mA there's basically no reason to use 3-phase.

author: andre123
author_rank: Newbie
timestamp: January 28, 2020, 02:27:30 pm
content: Quote from: SilverSolder on January 28, 2020, 01:50:18 pmI'm working on a 3 phase energy meter that requires redundant lines; say 1 or 2 lines fail or even the neutral , the meter should continue working. If i miss in clarifying something please let me know.BRAndreQuote from: KaneTW on January 28, 2020, 02:01:46 pmI'm working on a 3-phase energy meter that needs to operate on all 3 phases and neutral as a redundancy action, so in the core i'm trying to build a normal 2 wire (line and neutral) input power supply.BRAndre

author: Wolfram
author_rank: Frequent Contributor
timestamp: January 29, 2020, 05:55:04 pm
content: Three single-phase mains transformers connected in star on the input and delta on the output, followed by a three-phase rectifier would do the job. Neutral is not needed on the primary side as the secondary delta connection will force the primary star point to the sum of the phase voltages (at least for the fundamental 50 Hz component). With loss of a phase, the rectified voltage would probably be lower (by a factor of sqrt(3)/2), and there would be more ripple, but for 100 mA draw it should be easy to dimension the filter capacitors and regulator to account for this.

author: Deactivated-1
author_rank: Frequent Contributor
timestamp: January 30, 2020, 01:28:01 am
content: Quote from: Wolfram on January 29, 2020, 05:55:04 pmmaybe it's also possible to use cheap china dc-dc buck converters and feed them from rectified 3 phase mains.but it's probably gonna be hard finding a buck converter that can hold 600+V on the input