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
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
An externally hosted image should be here but it was not working when we last tested it.
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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.
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
An externally hosted image should be here but it was not working when we last tested it.
That's why those 4700MFD caps are not connected to the middle(which is actually the ground for two transformers.
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.
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.
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.
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... 🙂
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?
Like this?
An externally hosted image should be here but it was not working when we last tested it.
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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.
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.
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 🙂
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.
There's no reason why this or your first circuit wouldn't work well.
The two secondaries are completely isolated so you can put them in series all you like.
jan didden
The secondaries can be used at the full nominal rating. If it is 1.3 amp at 12-0-12 vac then that is a 31.2 va transformer but, you can only draw 1.3 amps per winding. So if one "half" or 12 volt winding is left unused then you still must not exceed the 1.3 amp max rating.
I'm a bit confused just what your transformer is and how its configured. I think that is what Andrew was trying to get at too.
In the first post you say it has two separate 12-0-12 windings ? So in post #1 the transformer has either 6 or 8 wires from the secondaries depending how they are connected in the transformer.
For a low curent high voltage rail a doubler can be a good solution.
I'm a bit confused just what your transformer is and how its configured. I think that is what Andrew was trying to get at too.
In the first post you say it has two separate 12-0-12 windings ? So in post #1 the transformer has either 6 or 8 wires from the secondaries depending how they are connected in the transformer.
For a low curent high voltage rail a doubler can be a good solution.
As I said my tranformer has 6 outputs wires on secondary and two wires for primary.
Here is a picture.
Blue-Black-Blue is one set of 12-0-12
And Yellow-Black-Yellow is the other set of 12-0-12
An externally hosted image should be here but it was not working when we last tested it.
Yes, you can ignore the centre tap ans use the transformer as a dual 24Vac.
Yes, you can series connect the secondaries for 48Vac or series connect the bridge rectifiers for ~30Vdc & 60Vdc.
Yes, you can series connect the secondaries for 48Vac or series connect the bridge rectifiers for ~30Vdc & 60Vdc.
If we discard the center tap and go for 24V the current capability goes down by half right? That's something I didn't think of..
So if I regulate down the resultant 32VDC (from a 24V AC rectification) to 12VDC instead of using center-tap 16VDC(from 12AC-0-12AC) the former one will be able to supply only half the current the latter one could supply as the VA of the transformer is fixed.
So at approx32VDC it will have less current capability than 16VDC? I am confused 🙁
So if I regulate down the resultant 32VDC (from a 24V AC rectification) to 12VDC instead of using center-tap 16VDC(from 12AC-0-12AC) the former one will be able to supply only half the current the latter one could supply as the VA of the transformer is fixed.
So at approx32VDC it will have less current capability than 16VDC? I am confused 🙁
I have tried to redesign it with a simple zener-series regulator(emitter follower) and combining the unregulated DC.
What do you all think about this? This uses the same transformer but one of the secondaries are using the center tap to rectify to 16V DC instead
What do you all think about this? This uses the same transformer but one of the secondaries are using the center tap to rectify to 16V DC instead
An externally hosted image should be here but it was not working when we last tested it.
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From the 48V rail, I am looking to power a pre-amp section. Which was originally running with out regulation. I don't think that section is going to draw anything more than 100mA. So should I limit the Ic through transistor someway? Like adding a Rc may be? Is it necessary?
My Vout equation will change if I do something like that... Or is this just fine?
Also calculation of the base resistor Rb. voltage across Rb is Vin-Vz right?
Vin= 33.936(24 * 1.414) + 16.968(12* 1.414) = 50.904
So that is Vrb 51V-49.35= 1.554
Approx 15mA current Ib for hard saturation(correct?) at VBEon and Izt=5.5mA
So total current through Rb is Ib+Izt = approx 22mA
Rb= 1.54/22mA = 70Ohm... Nearest Value is 68Ohm... That's how I got it... Am I correct?
My Vout equation will change if I do something like that... Or is this just fine?
Also calculation of the base resistor Rb. voltage across Rb is Vin-Vz right?
Vin= 33.936(24 * 1.414) + 16.968(12* 1.414) = 50.904
So that is Vrb 51V-49.35= 1.554
Approx 15mA current Ib for hard saturation(correct?) at VBEon and Izt=5.5mA
So total current through Rb is Ib+Izt = approx 22mA
Rb= 1.54/22mA = 70Ohm... Nearest Value is 68Ohm... That's how I got it... Am I correct?
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Your calculations seems OK but they make me nervous 🙂 Seeing high voltage zeners and low value resistors... there's no "compliance" or overhead.
I'm not trying to find fault, its just that I wouldn't do it that way 🙂
You could arrange a voltage doubler from the 24 volt winding. That would give nearer 60 to 65 volts volts DC in its own right. That could then be referenced to the main 12 volt ground.
I think it could also be done by series connecting the 24 volt winding to the 12 volts and using an AC coupling/diode/AC coupling etc arrangement to produce the high voltage rail. I'd need the transformer to play with though to come up with that scheme.
I'm not trying to find fault, its just that I wouldn't do it that way 🙂
You could arrange a voltage doubler from the 24 volt winding. That would give nearer 60 to 65 volts volts DC in its own right. That could then be referenced to the main 12 volt ground.
I think it could also be done by series connecting the 24 volt winding to the 12 volts and using an AC coupling/diode/AC coupling etc arrangement to produce the high voltage rail. I'd need the transformer to play with though to come up with that scheme.
can you post dc resistances of your traffo, primary and secondaries?
and can you define exactly your dc load requirements?
you mentioned 48volts at 100mA, so that is 4.8 watts, what about your 12volt rail?
once you know your loads, we can then look at your traffo and judge whether it is up to task....
and can you define exactly your dc load requirements?
you mentioned 48volts at 100mA, so that is 4.8 watts, what about your 12volt rail?
once you know your loads, we can then look at your traffo and judge whether it is up to task....
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