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Old 3rd September 2012, 08:54 AM   #1
Ronj is offline Ronj  Singapore
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Default Fixed Linear Regulators a possibly stupid question

I am not sure how dumb the below circuit is? Will it even work... Your comment please?

I have a step-down transformer with two secondary outputs each 12V-0-12V capable of around 1.3A. If I discard the center tap and do a full-wave bridge rectifier, I understand I can create a 33VDC out from each of the secondary outputs. With that and 3 Linear regulators from ST Micro, can I create something like this in the diagram I have drawn? As per it I should be able to get 500MA(at least) capable, +48VDC and +12VDC (1 Ampere capable) outputs. Or there is something inherently flawed or stupid in the below design? Your inputs are as usual, highly appreciated.

Here is the schematic
Click the image to open in full size.

Last edited by Ronj; 3rd September 2012 at 09:47 AM.
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Old 3rd September 2012, 09:49 AM   #2
AndrewT is offline AndrewT  Scotland
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how many separate windings on your transformer?

You mention centre tap. You cannot easily change that to dual secondary. A centre tapped secondary is one winding with 3 leads, one at each end and one in the centre of the winding.
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Old 3rd September 2012, 09:58 AM   #3
Ronj is offline Ronj  Singapore
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Quote:
Originally Posted by AndrewT View Post
how many separate windings on your transformer?

You mention centre tap. You cannot easily change that to dual secondary. A centre tapped secondary is one winding with 3 leads, one at each end and one in the centre of the winding.
If I don't use the center tap and keep it NC(Not connected) I still get the full-voltage swing of 24V AC. I have at least used it before that way....

The lower leg of the two 4700MFD/50V caps are intentionally set to that virtual ground point not the actual ground of the system. 7824 cannot take an input voltage above 35V(absolute max) hence the arrangement(or at least what I thought).

What I was essentially trying to do with two secondary outputs are nothing but a dual rail. but then taking only the V+ and V- as a fixed out put... where V- is considered as the system ground.

A dual rail in normal case look like this
Click the image to open in full size.

That's why those 4700MFD caps are not connected to the middle(which is actually the ground for two transformers.
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Old 3rd September 2012, 10:05 AM   #4
Mooly is offline Mooly  United Kingdom
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I can't decide if this is an elegant soltion or not

You have the -24 volts as ground. That's fine as ground is an arbitrary point. The 12 volt supply feed off the now (+) 24 volt supply. That's OK. Now you stack a separate +24 volt supply "on top" whose "ground" reference (for the +24v reg) is the already +24 volt differential from ground.

It should work but I would wonder what the transient performance is like as the "grounds" for the regs are all relying on "non ground" references... if you see what I mean. In other words is a transient load and corresponding voltage disturbance going to reflect its way into the other regs.
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Old 3rd September 2012, 10:06 AM   #5
Ronj is offline Ronj  Singapore
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Quote:
Originally Posted by AndrewT View Post
how many separate windings on your transformer?

You mention centre tap. You cannot easily change that to dual secondary. A centre tapped secondary is one winding with 3 leads, one at each end and one in the centre of the winding.
Oh by the way I have two center tapped outputs on this transformer.

That is 12 -0-12 and another 12-0-12. I was not changing one center tapped output to dual secondary. I indeed have two separate outs consisting of 6 wires in total as output in case it was not clear.
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Old 3rd September 2012, 10:10 AM   #6
Ronj is offline Ronj  Singapore
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Quote:
Originally Posted by Mooly View Post
I can't decide if this is an elegant soltion or not

You have the -24 volts as ground. That's fine as ground is an arbitrary point. The 12 volt supply feed off the now (+) 24 volt supply. That's OK. Now you stack a separate +24 volt supply "on top" whose "ground" reference (for the +24v reg) is the already +24 volt differential from ground.

It should work but I would wonder what the transient performance is like as the "grounds" for the regs are all relying on "non ground" references... if you see what I mean. In other words is a transient load and corresponding voltage disturbance going to reflect its way into the other regs.
Yeah that's where I am clueless... To me I have these parts currently in-hand, and I need to power two loads that require a 48VDC/100MA(a pre-amp section) and a small class D which will run happily at 12VDC/1.2Amps in a single chassis. Since I happen to have dozens of fixed regulators and this particular transformer in-hand(which came from a 5.1 computer system where 1 tapping was powering a sub-woofer board while other for a 5channel amp), I thought of this to maximize reuse...
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Old 3rd September 2012, 10:27 AM   #7
Ronj is offline Ronj  Singapore
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Just a question, what if I do two 24V positive regulators and series them for 48V? At least that way I can have the lower ground line consistent...

Like this?
Click the image to open in full size.

Last edited by Ronj; 3rd September 2012 at 10:35 AM.
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Old 3rd September 2012, 10:28 AM   #8
Mooly is offline Mooly  United Kingdom
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I think powering an amp is going to cause noise on the +48 volt rail.

The 12 volt regulated supply will have some noise and supply modulation on its output. That will also be seen across its input. Your ground is our point of reference. So that noise is seen across the -24 volt stabilised supply.

Because "ground" is our point of reference that noise will appear (and can be seen and measured) at the other pin of the -24 volt reg (the regs "ground pin") and that means it contaminates the +24 vol regs "ground" pin.

So I would say not good for this application as the load on the 12 volt supply will be very dynamic and changing.

Haven't time to suss it out now but how about series connecting the secondaries (as they are one transformer) and using the lower windings for the main 12 volt high current supply and using the other tappings to provide a low current high voltage rail. I think you could AC couple the winding output used for the high voltage and then rectify and arrange it so they are all truly ground referenced. Also do you need the 12 volts to be stabilised ? as there is a lot of power (heat) wasted in dropping all the excess volts in a linear reg. Would the amp run on 17 volts (single winding) or is that to high. Perhaps use a simple ripple filter to loose a few volts at much less dissipation than dropping nearer 22 volts across the reg.
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Old 3rd September 2012, 10:44 AM   #9
Ronj is offline Ronj  Singapore
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Quote:
Originally Posted by Mooly View Post
I think powering an amp is going to cause noise on the +48 volt rail.

The 12 volt regulated supply will have some noise and supply modulation on its output. That will also be seen across its input. Your ground is our point of reference. So that noise is seen across the -24 volt stabilised supply.

Haven't time to suss it out now but how about series connecting the secondaries (as they are one transformer) and using the lower windings for the main 12 volt high current supply and using the other tappings to provide a low current high voltage rail.
Appreciate the input. Yes that make sense... What about the twin positive regulator approach? and for the 12V regulated out, yes looking at it, lot of power will be dissipated away... May be I have to think something else
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Old 3rd September 2012, 11:05 AM   #10
Ronj is offline Ronj  Singapore
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Quote:
Originally Posted by Mooly View Post
I think powering an amp is going to cause noise on the +48 volt rail.


Haven't time to suss it out now but how about series connecting the secondaries (as they are one transformer) and using the lower windings for the main 12 volt high current supply and using the other tappings to provide a low current high voltage rail. I think you could AC couple the winding output used for the high voltage and then rectify and arrange it so they are all truly ground referenced. Also do you need the
I guess by series connecting the tappings I get something like this...

Trans ACV
12 ----- 48

0 ----- 36

12 ---- 24

12 ----- 24

0 ------ 12

12 ------ GND



But then how much current will the lower 2 tapping will be able to give? only half of the rated secondary current right? If only I use a twin diode full-wave rectification with both 12-0-12, I get the full current. Isnt my assumption correct?

Also I need to use discrete rectification then as the normal 3legged fixed regulators max out at 24V as well as none of them can take anything more than 35V on input.
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