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Linear power supply w/ multiple outputs

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mkelly09

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Hello, I am attempting to build a linear voltage regulated power supply with the following requirements:

-3 outputs:
  • 1 Variable 0-30Volts output up to 1A
  • + / - matched (tracked) output for powering various op-amps (up to 22V for ua741)

I have the following design that only does the first variable output, and I think this part of the design is pretty solid:

N53PD.png

https://i.imgur.com/N53PD.png

-Note that the second switch off the secondary of the transformer is basically used to control heat dissipation in the LM317. I will hit the switch when I need high current at low voltage to reduce the voltage drop across the regulator.

I'm wondering how I could modify this to include + / - supplies for powering op-amps. I want to be able to adjust them from say 15V up to 22V, and I want them to be tracked (adjust them both with 1 pot)

Would a second transformer be the best (easiest) solution?
 

Dear mkelly
Hi
If you want create simultaneous supply for your opamp , there is a classic way available : use a RCD voltage multiplier , before your rectifier , and then limit it's amplitude with a simple zener diode , thus you'll have two voltages (+-) , as well .
Best Wishes
Goldsmith
 
goldsmith said:
Dear mkelly
Hi
If you want create simultaneous supply for your opamp , there is a classic way available : use a RCD voltage multiplier , before your rectifier , and then limit it's amplitude with a simple zener diode , thus you'll have two voltages (+-) , as well .
Best Wishes
Goldsmith
Click to expand...

Thanks for the response goldsmith.

Do you know where I can read up on this RCD voltage multiplier? I can't seem to find much via google. What does RCD stand for?
Also, i'm assuming this will allow me to add two additional voltages to the above design? (that is what i'm looking to do)

Thanks.
 

Sorry if my words are confusing . just , use 2 diode and two capacitor , to obtain a simple voltage doubler . this way , will give you two voltages with a common ground . i have used this way for many times .
Good luck
Goldsmith
 
mkelly09 said:
Hello, I am attempting to build a linear voltage regulated power supply with the following requirements:

-3 outputs:
  • 1 Variable 0-30Volts output up to 1A
  • + / - matched (tracked) output for powering various op-amps (up to 22V for ua741)

I have the following design that only does the first variable output, and I think this part of the design is pretty solid:

N53PD.png

http://i.imgur.com/N53PD.png

-Note that the second switch off the secondary of the transformer is basically used to control heat dissipation in the LM317. I will hit the switch when I need high current at low voltage to reduce the voltage drop across the regulator.

I'm wondering how I could modify this to include + / - supplies for powering op-amps. I want to be able to adjust them from say 15V up to 22V, and I want them to be tracked (adjust them both with 1 pot)

Would a second transformer be the best (easiest) solution?
Click to expand...

Since the transformer has a center tap, use the center tap as the ground (0V). Connect the other 2 ends to the bridge rectifier. The + output of the rectifier will be +VDC. The - output of the bridge rectifier will be -VDC. Connect the LM317 input to +VDC. The circuit gnd shown in the circuit will now be the transformer center tap. With -VDC input, use an LM337. The circuit will be similar to the LM317 circuit. Download the datasheet. You can also find a lot of circuits online. To have a matched output, use a dual potentiometer so that adjusting the position of the knob adjusts the resistance of two variable resistors together (one in LM317 circuit, one in LM337 circuit). Something like this:
**broken link removed**

Hope this helps.
Tahmid.

---------- Post added at 01:39 ---------- Previous post was at 01:19 ----------

Here's what you want:
**broken link removed**
 
Tahmid said:
Since the transformer has a center tap, use the center tap as the ground (0V). Connect the other 2 ends to the bridge rectifier. The + output of the rectifier will be +VDC. The - output of the bridge rectifier will be -VDC. Connect the LM317 input to +VDC. The circuit gnd shown in the circuit will now be the transformer center tap. With -VDC input, use an LM337. The circuit will be similar to the LM317 circuit. Download the datasheet. You can also find a lot of circuits online. To have a matched output, use a dual potentiometer so that adjusting the position of the knob adjusts the resistance of two variable resistors together (one in LM317 circuit, one in LM337 circuit). Something like this:
**broken link removed**

Hope this helps.
Tahmid.

---------- Post added at 01:39 ---------- Previous post was at 01:19 ----------

Here's what you want:
**broken link removed**
Click to expand...

Thanks for the reply, but what I am looking for is to add in two more outputs to the above circuit so I have a total of 3. I understand everything you've said, but that would only introduce a second negative supply.

Would it be possible to introduce something like the following circuit through a second transformer, or perhaps it would be cheaper to use the voltage doubler that goldsmith was suggesting?
3czjs.jpg
 

Ok, so you need a variable output and another +/- output.

So, for the variable, you have the LM317 section above.

If the +/- outputs need to be fixed, just use LM7824, LM7924 or LM7815 and LM7915. The 78XX input comes from the +VDC as mentioned in the previous post. The 79XX input comes from the -VDC mentioned previously. So, use the same +VDC input for LM78XX and LM317. You don't need a voltage doubler or a second transformer. But, make sure that the transformer can handle the total current.

Hope this helps.
Tahmid.
 
Although the following tracking voltage regulator is for a fixed positive one, it works if it is variable too.


Added:
If you have a transformer with a center tap (say ground) and you expect the DC positive and negative currents are almost equal or relatively small (if not equal) for the transformer, two diodes are enough; one for each side to get a positive and negative voltage with respect to ground.
 

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Last edited:
Thanks for the responses guys. I decided that for the additional outputs all I will need is + / - 15V for op-amp work. I've also added a current limiting circuit for my adjustable voltage output. I've heard this can be useful.

I've updated the schematic below:




Can someone confirm that this is the correct way to do the -15V?
The second filtering capacitor at the top, is this correct? (C5)

My primary concern here now is power dissipation on the LM317. In my original post I had overcome this issue by using a switch to manually change the tap of the transformer when I needed low voltage @ higher current.. is there a solution for this now?

for example, if I needed 5V @ 1A the voltage drop across the regulator would be quite large..
 

Hi mkelly,
I dodn't understand why you have 2 diodes D4 and D5 on the output. They will just degrade your load regulation and make your output voltage very inaccurate. Also from the LM317 datasheet I see https://www.ti.com/lit/ds/symlink/lm317.pdf the Theta jA is 53C/W so with an ambient temperature of even 50C you can only dissipate (150-50)/53 ~ 2W. Take at least a 10% margin on that so say 1.8W. So with 5V 1A even your first scheme will cause too much dissipation on the LM317. One option is to use a multiple tap transformer if you have one, another I think more elegant way is to use a switching regulator.
 
goldsmith said:
See my previous post , i told you the correct way and simplest way .
Click to expand...

So you suggest putting a circuit like the following one before my bridge rectifier?

vd.GIF


If i took the middle of the caps as ground then I would have one + and one - for the supply. I would still have the concern about power/heat dissipation on the LM317 TO-220

aryajur said:
Hi mkelly,
I dodn't understand why you have 2 diodes D4 and D5 on the output. They will just degrade your load regulation and make your output voltage very inaccurate. Also from the LM317 datasheet I see https://www.ti.com/lit/ds/symlink/lm317.pdf the Theta jA is 53C/W so with an ambient temperature of even 50C you can only dissipate (150-50)/53 ~ 2W. Take at least a 10% margin on that so say 1.8W. So with 5V 1A even your first scheme will cause too much dissipation on the LM317. One option is to use a multiple tap transformer if you have one, another I think more elegant way is to use a switching regulator.
Click to expand...

The diodes were put there to be a quick fix for the LM317 to regulate down to zero volts.

I'm using the TO-220 package for the LM317, so I can dissipate a bit more power than what you've mentioned but it still won't be enough. Is it very difficult to use a switching regulator?
 

mkelly09 said:
The diodes were put there to be a quick fix for the LM317 to regulate down to zero volts.

I'm using the TO-220 package for the LM317, so I can dissipate a bit more power than what you've mentioned but it still won't be enough. Is it very difficult to use a switching regulator?
Click to expand...

In that case you should take the feedback to the Adjust pin from the output i.e. after the diodes. Switching Regulators are not too difficult to use. If you pick a already designed application circuit then all you have to pay attention to is the layout of the circuit. You can use tps54240 which I found from a cursory search of your requirements Vin > 36V, Vout up to 30V 1A output. This regulator uses minimum components and you don't have to worry about any complex compensation components. JUst follow their recommendations in the datasheet and you should be good

https://www.ti.com/lit/ds/symlink/tps54240.pdf

Well, thinking about it more, since you need an adjustable output, the switching regulator is trickier and you would have to go through the inductor and other component calculations, but they do have a good Application Design Guide section to guide you through it.
 
Last edited:

Tahmid said:
Since the transformer has a center tap, use the center tap as the ground (0V).
Click to expand...

If you do that then u are using only half wave of the complete transformer for one polarity. In this case you have to use larger capacitor banks because ripple voltage will be high. and one other thing if any one can explain? in this config transformer gets noisy and gets more hot than normal, i have experienced it with toridal power transformer since i didn't have E shaped power transformer with center tap.
 

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