the spec:
1) portable like other wallwart
2) dual rail +ve and -ve psu (tracking -ve), adjustable voltage using trimpot (Vref in schematics)
3) max voltage is up to transformer max voltage (whatever transformer i can find in my junk stock)
4) max ampere is 8A since i have a bunch of 13007 NPN (and of course to the transformer max ampere rating)
5) current limiting and the plethora of protection is currently omitted for simplicity. i will proceed to this when everything is working.
abstract:
there's example on the net (link below) but either not up to the spec, too complicated for my head to understand, or/and i dont have the parts. so i will reinvent the wheel by trying as simple as i can get with parts that i already have. my very weakness is in the transistor circuit. basically i just put them together and tinker until they work. the best i can get so far is by using optocoupler to adjust -ve rail voltage according to Vref control adjustment (using dual comparator with +ve voltage feedback control). and i'm not sure if my transistor arrangement for -ve rail is correct, maybe this is a novel invention from me or i'm just redoing people mistake in the past, thats why i need advice, i got no luck for -ve power transistor design in the net, or i just simply dont get it.
interested at links:
0-15 Volt Tracking Power SupplyConstructing the +12 Volt Supply0-50V 2A Bench power supplyconclusion:
from testing powering dc motor (i dont have other easy to access "higher power device" right now), so far so good (i mean dc motor did rotate fast CW and CCW resp +ve and -ve rail), but T2, R9, R10, and esp T7 can get hot very easily albeit only around 0.1A is running the motor, maybe i need to tune the value some more or change the arrangement.
obsolete. refer to latest update
problem:
the biggest problem is -ve rail is not following exactly the +ve rail magnitude. ie when i set Vref to somewhere in the middle between V+ and GND, the -ve rail seem to be pulled to GND potential, but when Vref is near GND or V+, then only then the -ve rail is closely tracking the +ve rail (not exactly same, but close). i thought the optocoupler is just like a isolated ON/OFF switch which will follow the comparator output (switching) but i'm not sure whats going on, so need big help on this. refer to schematics below, i dont have spice model for PC817 in Tina software, so i just represent it with a LED and a transistor, but the external connection is like shown. its from my try and error to get the best -ve tracking i can get. please help on there, i'm sure it will just take a blink for you to understand my circuit. thanks.
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update 28th Dec 2011: (portpsu1b)the first design on -ve rail is in the Art Of Electronics 2nd Ed. entitled as "The Design of The Blunder!". so... its a retard. changed the plan, i used the N-MOSFET now, free from JunkYard Dept. re-arrange and re-testing a bit, now its better, but still,
far from perfect. tracking is better than 1st design.
tracking will not work if either side of the rail is heavily loaded, and the other is not. the 2nd LM393 in 1st design is also a retard, so changed to become voltage follower of the 1st LM393 output. so the latest is portpsu1b.png please review it. the 1st design (portpsu.png) is obsolete, dont talk about it anymore unless absolute necessary
ps:
1) 1x C945 and 1x LM393 dead. RIP.
2) please translate the label in the picture as:
(a) BC394 = 13007
(b) !NPN = C945
(c) TLC393 = LM393 or LM193
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update 28th Dec 2011: (portpsu1c, few hours later)in the cold of the night, idea popped out, changed the tracking system on the 2nd/-ve rail comparator, it turned out similar to circuit posted by Hero below, now i know what it does
, now... tracking is... perfect! (portpsu1c) unless the transformer is saturated (overloaded where it cannot maintain its original sine output). and another problem is excessive heat when +ve -> -ve rail is used to power a load (only 1.5A to load). have no idea why, will figure out later, some bug sneaking i believe. for low powered device (<1A) i think this unit is now usable, thanks to me for my novel invention!
and err, to Hero999 my friend
the 2nd design (portpsu1b.png) is obsolete, dont talk about it anymore unless absolute necessary, but... its just hours ago? i believe no one catched that!
review, C&C are highly welcomed!
ps:
1) death toll increased to 2x C945, 3x LM393 and 1x schottky. RIP.
2) translation above still apply with addition:
(d) the Vref system is not a fixed voltage divider as shown. its actually a trimpot, go figure.
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update 24th Jan 2012:just to add the note (without bumping
), i've found similar technology has been used almost 30 years ago, and more involved than mine. the link...
http://michaelgellis.tripod.com/power4.html happy chinese new year and have a good day!
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update 7th Feb 2012:the latest version psu1e (skip the 1d, there's not much in it). it will become my temporary power supply to build more... -ve rail still oscillating, +ve rail now using LM317 as regulator (controlling base of pass elements), if i struggle to clean the -ve rail any longer, i'm risking a divorce!
so i have to call it off for a while. the performance is as in
reply #12 & #13 below. i hate to bump it, but i have to for the sake of completeness and tidyness
any improvement advice is welcomed.
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update 19th Feb 2012:i think this is the finale, urghh i'm tired of this. psu1f... has been tested/used for few weeks/days or so. does the job!
major changes... now -ve rail wont eyeing on +ve rail anymore (heavy loading on +ve rail wont be reflected anymore on lesser loaded -ve rail). instead both +ve and -ve rail will use Vref from LM317 as a reference (the -ve rail will invert it first and then regulate the -ve rail from there). the LM317 is overkill i know, a simple zener + volt divider(trimpot) + magnifying opamp can do the same job. but, i have much more LM317 stock than zener.
Regards,
Shafri.