Help with making a DC to DC split rail supply - Page 1
Forum section: electronics / projects

author: Fireagle
author_rank: Contributor
timestamp: June 07, 2015, 08:49:32 am
content: What i like help with is designing a circuit that takes a 12 volts DC input and splits it into a positive, zero and negative output rails so that i can test devices such as op amps and the like that i have salvaged, also so i can power a signal generator amplifier that can be loaded at about 3 ampsDesign restrictions:All parts suggested should be able to be hand soldered with ease because i do not have access to a reflow machine so ether through hole or large SMD parts please.I only have access to standard 1.6mm FR4 copper board.Single sided layout would be good so i can get the board made at my university for about $10.If there is a way of not using a transformer in the power circuit then that would be awesome due to the cost and time of sourcing such a part.I have attached what i have designed so far, "B2" is the signal generator amplifier circuit if there is any problems or improvements that you can suggest please let me know also the top diagram is the overall layout of the product.This is my first posted on this forum so if i have posted in the wrong section please feel free to move it admins.

author: mikerj
author_rank: Super Contributor
timestamp: June 07, 2015, 09:34:35 am
content: Take a closer look at the configuration of your bipolar push pull output stage. You have two forward biased BE junctions in series which will switch both transistors on at the same time. You should be looking at an emitter follower here, not a common emitter. You need to consider the biasing as well.A virtual 0v rail can be made by simply using a potential divider to get half of your incoming supply (i.e. 6v) and then buffering this voltage with a high current, unity gain stage. This requires no transformers or inductors, but has the following disadvantages:a) If you are sourcing/sinking 3 amps into your virtual 0v rail, one of the transistors will be dissipating 6*3=18 Watts. Not a massive figure, but you will need to use adequate heat sinking.b) Your virtual 0v rail is not at the same potential as mains ground with this scheme, but the 12v supply 0v rail could be, in which case clipping a scope probe ground to the virtual 0v rail would cause a short.Any other scheme is going to require switching regulators. The simplest would be just to use an inverting regulator to create a -12v rail, and then use your +12v rail directly. However, if you want to draw 3 amps from the -12v rail this will require a reasonably hefty regulator.

author: void_error
author_rank: Frequent Contributor
timestamp: June 07, 2015, 10:43:22 am
content: That's a fair amount of current to deal with, think switchmode, not linear. There are a bunch of relatively easy to make TL494 converters out there but you'll most likely to have to wind your own transformer.

author: Zero999
author_rank: Super Contributor
timestamp: June 07, 2015, 10:49:18 am
content: That won't work because the transistors are AC coupled to the op-amp.One thing you can do is add transistors that bypass the op-amp's internal transistors, when any current over what is required to power the op-amp is drawn. The disadvantage is it's less stable than emitter followers.

author: mikerj
author_rank: Super Contributor
timestamp: June 07, 2015, 12:13:36 pm
content: Quote from: Hero999 on June 07, 2015, 10:49:18 amThat was my immediate thought, but the circuit the OP shows is for his amplifier, not his split supply generator. It still won't work though

author: Fireagle
author_rank: Contributor
timestamp: June 08, 2015, 01:49:19 pm
content: Quote from: mikerj on June 07, 2015, 09:34:35 amFirst i have no clue as to what your first two paragraphs are talking about so diagrams would be handyAnswering a) I was planing to put the project inside a aluminum case like to ones shown on a past eevblog videos so heatsinking should not be a problem.Note on b) So if i use a transformer to isolate the 12 volt in a switch mode power supply configuration then this would solve this issue ?finally which regulators would you recommend ?Quote from: void_error on June 07, 2015, 10:43:22 amI was already in that mindset. Taking a quick look at the TL494 datasheet i only see ways of making a step down positive supply, So how you make a negative rail ?Quote from: Hero999 on June 07, 2015, 10:49:18 amSo get rid of the capacitor ? and somehow add a transistor ?Quote from: mikerj on June 07, 2015, 12:13:36 pmthank for noticing and again how would one fix this ?

author: mikerj
author_rank: Super Contributor
timestamp: June 08, 2015, 11:08:06 pm
content: Quote from: Fireagle on June 08, 2015, 01:49:19 pmI gave some tips in the first sentence of the first post I made.Your output stage just won't work, both transistors will be switched on pretty hard by the resistors connected across the base pins. This is because you've connected them as common emitters, when you actually want an emitter follower circuit, i.e. swap top and bottom transistors. You then need to deal with the biasing of your push-pull output stage, and this is a whole topic in itself, but some possibilities are discussed inthis recent threadand you'll find a lot of information if you search on the web for "push-pull" or "class b" biasing methods.You other mistake is that you have taken the feedback directly from the op-amps output, so the op-amp can't correct for any non-linearities in your push-pull output stage.You very probably don't want to AC couple the op-amp to the output stage either. If you directly couple it then you can use the op-amp to control the quiescent voltage at the output by biasing the non-inverting input pin to 0v. You may want to AC couple the input if this amplifier is for audio etc.

author: Fireagle
author_rank: Contributor
timestamp: June 09, 2015, 08:43:53 am
content: Quote from: mikerj on June 08, 2015, 11:08:06 pmsorry about not realizing that you posted twice. The main reason i am using an op-amp is the signal gen i have can only source 50mA and that is not enough to even do basic design testing so i would like to buffer that signal before using it in a amplifierso attached is a redraw of the "B2" and what i have done so far with the "B1" box in ni Multisim 13.0 where "L1" and "R3" are fill in for a transformerone last question to those in the know, how does one fix a 100v overshot ? because that will kill everything in the circuit