# How to split 12V DC into 8 1.5V sources?

I'm trying to find a description or schematic about how to split voltage. I want take an automotive 12V (nominal) supply and create
separate outputs of 1.5V each. These outputs should be reasonably close to 1.5V regardless of the number of actual loads attached. In other words, if I only have 4 of the outputs being used, they should still be getting around 1.5V, not 3V each.
I think I should able to wire 8 1.5V voltage regulators in series and pull the output off the regulated 3rd leg, but I'm not sure. I've done a ton of research and all I can find are schematics for 3 or 4 outputs and they are all at different voltages (like 5V, 3.3V, etc). Can anyone provide any pointers to schematics or basic instructions about what components I will need?
Thanks!
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snipped-for-privacy@hotmail.com wrote:

There is a good reason why this isn't done.
Think what happens when you put a 1.5 ohm load across one of the outputs. That should draw 1A. Which is 12W nominal from the supply. 1.5W is going to be dissipated in the load - but where is the other 10.5W going to be dissipated, assuming that you aren't using SMPSU design? OK, now think about what happens when the nominal supply voltage is actually 15v - you have an extra 3V and an extra 3W to "lose" - where are you going to lose those?
It is possible to design a bespoke analogue supply or a bespoke SMPSU, however.
For one design, you use a series of *shunt* 1.5v regulators fed from a series regulator. With constant supply voltage, the chain of shunt regulators runs at constant current, irrespective of the loads on each output (within rated output). Quite simply, as one load /increases/ the current through its shunt regulator /decreases/ - thus maintaining constant supply current and constant load voltages. The series regulator ensures that the chain of shunt regulators is fed from a constant voltage and thus each output voltage will remain constant. A possibly unfortunate side-effect is that such a design consumes full power at all times.
So, you could use a buck/boost SMPSU producing a constant 12v to a chain of shunt 1.5v analogue regulators - if power consumption wasn't an issue. If power consumption was an issue, use a single SMPSU with parallel, independent, 1.5v outputs and daisy-chain the outputs.. The latter would be my solution, in either eventuality..
-- Sue
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<%-name%>
If this is a "one of" and power is not an issue and you want all the grounds to be the same as the 12v battery, you need 8 each 1.5 volt regulators. Once you find the current you need for each load you can probably find what you need under 3 pin voltage regulators at digikey for a couple bux each. If you put a bunch in series. then seven of the negative leads are going to be at some voltage between 1.5V and 10.5 volts
peace dawg
wrote:

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<%-name%>

I'm actually using this as part of a load-controlled PWM. The schematic I have will decrease the duty cycle as the load increases. thus I think I'm already prepared to dissipate the excess if all the outputs aren't in use. However, I'm not positive this is a complete solution.
In any case, I've already got a load that is already being fed 12V (nominal) that is being overpowered. So I'm trying to break out into multiple loads so that I can get much closer to my target voltage.
Thanks for the help!
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| For one design, you use a series of *shunt* 1.5v regulators fed from a | series regulator. With constant supply voltage, the chain of shunt | regulators runs at constant current, irrespective of the loads on each | output (within rated output). Quite simply, as one load /increases/ the | current through its shunt regulator /decreases/ - thus maintaining | constant supply current and constant load voltages. The series regulator | ensures that the chain of shunt regulators is fed from a constant | voltage and thus each output voltage will remain constant. A possibly | unfortunate side-effect is that such a design consumes full power at all | times.
And it would be hard to get away from that full power consumption. What if one of the 1.5V loads uses full current, while the others do not? If they are in series, the current still needs to flow, and so it would their those respective regulators.
One question is whether it really is necessary to supply the 8 1.5V loads on an independent basis. Why not a single 1.5V source derived from the 12V with a common DC-DC voltage converter?
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snipped-for-privacy@ipal.net wrote:

Impossible, actually, if using linear/analogue regulators..

Erm, yes - but you do know how a shunt regulator works, don't you?
With no load attached, the full load current flows through the power semiconductor in the regulator. (Or, with a chain of shunt regulators, each with no load, through the power semiconductor in each regulator).
The output of each regulator is essentially in parallel with the power semiconductor - so, as the load increases, the regulator reduces the current through the power semiconductor.
Thus the supply current, with a constant voltage supply, remains constant (at the full load value). All that happens is that, as a particular load current increases, the current through the associated shunt rtegulator decreases.

I read the OP as meaning that he essentially wanted outputs at 1.5v, 3v, 4.5v, etc.
Now a number of conventional analogue regulators, each with the required output voltage and each fed from the 12v supply, would work - but waste large amounts of power in the series pass elements. eg with each output providing 1A, the input would be almost 100W, for 12W total output.
With the design I described and each output supplying 1A, the input would be around 15W (allowing for the buck-boost regulator losses), for 12W total output.
-- Sue
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| snipped-for-privacy@ipal.net wrote:
[snip]
|> One question is whether it really is necessary to supply the 8 1.5V loads on |> an independent basis. Why not a single 1.5V source derived from the 12V with |> a common DC-DC voltage converter? |> | | I read the OP as meaning that he essentially wanted outputs at 1.5v, 3v, | 4.5v, etc.
I didn't read it as that, but I can see the value in it for some things.
The rough idea that comes to mind would be a digital regulator/logic that maintains a charge on 8 capacitors in series, using a PWM or PDM method. It would obviously have to be an active system, and keep them isolated. And it most likely would not meet the "cheap and simple" requirement that is usually desired.
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snipped-for-privacy@ipal.net wrote:

These days you would just take what supply you have and shove it into a SMPSU with multiple isolated outputs giving whatever was needed. Usually very cheap and very simple.
-- Sue
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snipped-for-privacy@ipal.net wrote:

A simple switch mode power supply wit eight separate secondaries -> rectifers -> filter capacitors will allow isolation betwwen outputs, and to be connected any way you need them would be simpler and more efficient. It would also provide isolation from the power source. At low current levels the regulation would be fairly good.
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snipped-for-privacy@hotmail.com submitted this idea :

The "best" way is to use an inverter and convert the AC voltage back to DC at the required voltage.
Here is a simple inverter circuit: http://www.uoguelph.ca/~antoon/circ/555dcac.html
All you need to do is determine which transformer you want for (T1), rectify and regulate. :)
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The obvious way to do it is with a DC to DC switching circuit such as a two switch forward converter driving a high frequency transformers with eight secondaries using low drop synchronous rectifiers to provide the 1.5 volt outputs. This scheme could be designed to deliver virtually any desired current and would provide complete isolation between the outputs.
However, a quick and dirty way to do it giving approximately 1.4 to 1.5 volts per stage at low current (milliamps) would be to place 16 rectifier diodes in series, forward connected, with a ballast resistor also in series connected to the 12 volts source. The 1.5 volt outputs would be picked off every two diodes, plus on the anode, minus on the cathode of the pair.
A similar but more precision arrangement could be made with eight op-amps in a unity gain non-inverting mode connected to a series string of equal value resistors forming a multi tap voltage divider.
What you do depends on the precision you need, the current you need, and whether isolation or a common ground is required. What is your application?