Multiple DC Outputs using 0-24v AC output transformer

Thread Starter

artskynet

Joined Apr 6, 2014
8
Hi Everyone,

Is it possible to make a multiple DC outputs voltage like 1.5, 3, 4.5, 6, 9 and 12v in a 24v AC output transformer using LM 317 or something? Your comment is very much appreciated. Thanks in advanced.
 

MrAl

Joined Jun 17, 2014
7,673
Hi,

You could create one DC main supply and then use multiple LM317's to do each voltage.
You'll have to watch the power dissipation in each LM317 however because they will get hot if there is significant current draw.
 

ErnieM

Joined Apr 24, 2011
8,047
As Mr Al said, you need to watch the power.

Say you need 12V@1A, and 0.5A in the 5 other supplies. If you run each one in turn the 12V supply is passing 3.5 A (1A it's direct load plus 0.5A * 5 for the others). If the input voltage is 24V then it must dissipate 3.5A * 12V = 42 watts. PLUS each regulator is also burning off power for a total of 60 watts.

60 watts is enough power to light a room. Note no matter what you do (assuming you use linear regulators) this same power must get burned off as waste heat somewhere.

When you run all the regulators off the 24V input you just shift power from the 12V to the others, so the 12V now burns off just 12 watts but the 1.5V regulator goes from 5.25 watts up to 11.25 watts.
 

Thread Starter

artskynet

Joined Apr 6, 2014
8
Thank you MrAl

I guess I'll just use a piece of LM317 and put a rotary switch to select the desire output voltage.

Hi,

You could create one DC main supply and then use multiple LM317's to do each voltage.
You'll have to watch the power dissipation in each LM317 however because they will get hot if there is significant current draw.
 

Thread Starter

artskynet

Joined Apr 6, 2014
8
Thank you ErnieM for the clear elaboration.

As Mr Al said, you need to watch the power.

Say you need 12V@1A, and 0.5A in the 5 other supplies. If you run each one in turn the 12V supply is passing 3.5 A (1A it's direct load plus 0.5A * 5 for the others). If the input voltage is 24V then it must dissipate 3.5A * 12V = 42 watts. PLUS each regulator is also burning off power for a total of 60 watts.

60 watts is enough power to light a room. Note no matter what you do (assuming you use linear regulators) this same power must get burned off as waste heat somewhere.

When you run all the regulators off the 24V input you just shift power from the 12V to the others, so the 12V now burns off just 12 watts but the 1.5V regulator goes from 5.25 watts up to 11.25 watts.
 

MikeML

Joined Oct 2, 2009
5,444
The lower you set the output voltage, at any given output current, the more heat the poor LM317 has to get rid of. One of my first power supply projects back in the 1970s was a small AC Variac ahead of a ~28V transformer driving an adjustable regulator that had fold-back current limiting and was adjustable from 0 to 30V.

I used the Variac to reduce the rectified-filtered voltage upstream of the regulator to where the power dissipation in the regulator element was manageable when the supply output was only a Volt or three...

Still have that supply, and still use it...
 

ian field

Joined Oct 27, 2012
6,539
The lower you set the output voltage, at any given output current, the more heat the poor LM317 has to get rid of. One of my first power supply projects back in the 1970s was a small AC Variac ahead of a ~28V transformer driving an adjustable regulator that had fold-back current limiting and was adjustable from 0 to 30V.

I used the Variac to reduce the rectified-filtered voltage upstream of the regulator to where the power dissipation in the regulator element was manageable when the supply output was only a Volt or three...

Still have that supply, and still use it...
Can you still get the 5A regulators in the old style TO3 metal can?

The LM317 version was the LM317K, there were also 78xx types in this style at one time.

Starting with a high voltage headroom to get low voltages means a large dissipation in the regulator. A pre-regulator would at least share a large bulk of dissipation between 2 or more regulators. Its still going to need a pretty beefy heatsink, but each device is less stressed.

The external pass boost transistor described in the appnotes can just as easily be used to reduce the current through the reg chip, and hence shift some of the dissipation into the external transistor.
 

Thread Starter

artskynet

Joined Apr 6, 2014
8
Actually, I am just building a power supply with an adjustable voltage output and I want to use my salvaged transformer with an output of 0-24v AC. What will be your best solution to get those desire dc output voltage? What are the components that I am going to use then?
 

panic mode

Joined Oct 10, 2011
1,837
you need:
1. transformer to step voltage down (and for isolation),
2. rectifier (standard is full wave or bridge rectifier), you can make your own from 4 diodes or just use component (bridge rectifier) that has all diodes already in.. they must handle voltage and current higher than what transformer can provide. note, after rectification, DC voltage peak will be up to some 40% higher than AC voltage fed into rectifier. so if your transformer produces 24VAC, then you will get DC voltage of some 34V.
3. capacitor to filter out most of ripple, good rule of thumb is about 2000uF per Amp of current, also capacitor rated voltage need to exceed maximum DC peak. for example 40, 50 or 63V rated capacitors would do.
4. voltage regulator... there is tons of solutions out there and I know it can be hard to choose but... in general all solutions fall into two categories - linear and switching regulator. as mentioned by others, linear regulators are poor choice for high difference between input and output voltage. this means heavy heatsinks, lots of heat etc. switching regulators are perfect for this, in fact they work as sort of DC/DC transformer (input high voltage and low current, output low voltage and high current) maybe nice video tutorial like this will help you make your mind.

 
Last edited:

MrAl

Joined Jun 17, 2014
7,673
Hello again,

I have read now in this thread that you wish to use a selector switch to switch between output levels. That is perfectly understandable and should be possible.
There is one catch though, and that is you probably want to use a Make-Before-Break type switch so that you never leave the lower resistor (the one that goes to ground in the schematic) unconnected even for a very short time like 1ms. This will help keep the output well defined and within reason.
If you dont use a switch like this, then maybe a largish capacitor or some other means to keep the output from jumping up when the resistor is first disconnected (before the new resistor is switched in). This will prevent blowing something out that is connected to the regulator if the switch turns either on purpose or by accident.
I would also think you better use a good quality switch too.
Many people use a potentiometer, but i guess you want fixed voltages so you dont have to adjust.
Another idea is to use several switches, where each switch connects in another resistor in parallel to the first. If you have the lower resistor set for the highest output, you can then switch resistors in parallel that lower the voltage to the different levels you need like 1.5, 3, 6, 10, 12, etc.

The problem with using any switch however is that if the switch becomes disconnected either dirty or otherwise the output can jump up much higher than expected. This is why a different design is preferable, where if the switch becomes open the voltage goes *down* to the minimum value possible like 0v to maybe 1.25v approximately instead of *up* to the maximum possible which could be 20v or so.
 
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