I need a stepless variable output voltage regulator using LM317 regulator. I have a 2x15V trandsformer. To cool the regulator I need to run a 12 volt fan as well. So, I have to get 30vDC for the regulator and on the other hand, 15V DC for the fan as well from same transformer.
Anybody who has a simple schematic, please post.

 

If you put the two 15V windings in parallel you will get about 19V across the smoothing capacitors which is plenty for the 15V output that you want. It will also dramatically reduce the dissipation in the LM317 and ease the heat problem. The safest way to get 15V for the fan is to use another LM317 set for 15V.

 

Anybody who has a simple schematic, please post.

Even if this isn't homework, you should put more effort into your project than someone on AAC.

LM317 datasheets will have examples of how to do what you want for 1.2-15V. Why do you think you need 30V input to get 15V out? Transformer secondaries are rated in VRMS. 15VAC will give you 1.414*15 peak DC. What is the current rating for the transformers?

Why do you want 15V to run a 12V fan? How much current does it need? Why do you think you need a fan?

 

Even if this isn't homework, you should put more effort into your project than someone on AAC.

LM317 datasheets will have examples of how to do what you want for 1.2-15V. Why do you think you need 30V input to get 15V out? Transformer secondaries are rated in VRMS. 15VAC will give you 1.414*15 peak DC. What is the current rating for the transformers?

First of all, I am not a sturdent. I am just a hobbiest and amateur non-educated electrinics guy.
Why do you want 15V to run a 12V fan? How much current does it need? Why do you think you need a fan?

On related datasheet, sample circuit daigram shows that Vin>= 28V. That is why I intend to use a 30V (secondary) transformer. My transformer gives 2x15V as output. Max. 15VDC is enough for me. There is no label on the trafo to show current rating. But I can imagine it may give a little more than 2,0A.

Even if this isn't homework, you should put more effort into your project than someone on AAC.

LM317 datasheets will have examples of how to do what you want for 1.2-15V. Why do you think you need 30V input to get 15V out? Transformer secondaries are rated in VRMS. 15VAC will give you 1.414*15 peak DC. What is the current rating for the transformers?

Why do you want 15V to run a 12V fan? How much current does it need? Why do you think you need a fan?

My transformer gives 30VAC between 2 outer legs where as it gives 15VAC betweeb middle and any orher leg. So I think I can run safely an 12V DC fan with an 15VDC output. LM317 is installed on large aluminium fin plate and I think I have to cool this al. finplate somewhat like
in PC power supply boxes.

Also, I need some power for digital voltmeter display. I intend to use 5VDC for voltmeter and to get this from a used car USB phone charger cisrcuit which will be connected to fan DC converter circuit as well. I suppose that it works safely with 15V inlet..

 

Add a 7812 for the fan. Running 15VAC will not give you 15VDC as dl324 mentioned above.
All the info you want is in the data sheet.
http://www.ti.com/lit/ds/symlink/lm317.pdf
If you have 2 windings of 15V in series you can use a 2 diode rectifier (push pull rectifier) with the centertap = 0V.
That save you a couple of diodes and the diode voltage drop.
Use the data sheet to calculate the pot you require for the variable volts.
We could show you the lot but that is not how you need to learn.
so..
Look up "push pull rectifier"
7812 regulator
LM317 regulator with calculating the resistors required.

NOTE! The LM317 and 7812 pinouts are not the same. A trap for beginners.
And the other related trap has to do with the heatsinking. See if you can figure that one out.
At least draw up a circuit of your design with component values and submit it for perusal.
The best way to learn is to try to figure it out first, than ask for help when you get stuck.
There is tons of info for this on the net.

 

See

 

I had forgotten the LT1083 parts so had a look to see what the prices are now only to find they are "no longer manufactured"
So Maybe sticking with LM317 for this project could be a good idea.

There are a few LT1083 chips in my parts supply from years back just longing to be built into something.

 

Hello,

For 5 Amps you could have a look at the LT1084:
https://www.digikey.com/products/en...t=0&page=1&quantity=0&ptm=0&fid=0&pageSize=25
If 3 Amps will do, have a look at the LT1085:
https://www.digikey.com/products/en...s/pmic-voltage-regulators-linear/699?k=lt1085

Bertus

 

Hello,

For 5 Amps you could have a look at the LT1084:
https://www.digikey.com/products/en...t=0&page=1&quantity=0&ptm=0&fid=0&pageSize=25
If 3 Amps will do, have a look at the LT1085:
https://www.digikey.com/products/en...s/pmic-voltage-regulators-linear/699?k=lt1085

The Spice model (LT1084) has a defect. Strong pulsations were obtained with an output voltage of 1.25 V. When I lowered the input voltage (AC), everything became normal. Perhaps the model has a limitation on the power dissipated. By the way, on the top picture I showed the dissipated power chip. So I took a more powerful chip.

 

Add a 7812 for the fan. Running 15VAC will not give you 15VDC as dl324 mentioned above.
All the info you want is in the data sheet.
http://www.ti.com/lit/ds/symlink/lm317.pdf
If you have 2 windings of 15V in series you can use a 2 diode rectifier (push pull rectifier) with the centertap = 0V.
That save you a couple of diodes and the diode voltage drop.
Use the data sheet to calculate the pot you require for the variable volts.
We could show you the lot but that is not how you need to learn.
so..
Look up "push pull rectifier"
7812 is good idea. Thanks.
7812 regulator
LM317 regulator with calculating the resistors required.

NOTE! The LM317 and 7812 pinouts are not the same. A trap for beginners.
And the other related trap has to do with the heatsinking. See if you can figure that one out.
At least draw up a circuit of your design with component values and submit it for perusal.
The best way to learn is to try to figure it out first, than ask for help when you get stuck.
There is tons of info for this on the net.

 

7812 is good idea. Thanks.
What you mean by dl324? Is it a component you propose?

 

Hello,

If you want a fixed 12 Volts an 7812 regulator will do.
The 7812 is good for 1 Amp.
If you want 2 Amp, you at least need a 3 Amp regulator like the 78T12.

Bertus

 

On related datasheet, sample circuit daigram shows that Vin>= 28V. That is why I intend to use a 30V (secondary) transformer.

Drop out voltage for LM317 is around 3V. To get 15VDC, 15VAC should be sufficient. Depending on the rectification method used and the size of the filter cap.

My transformer gives 2x15V as output. Max. 15VDC is enough for me. There is no label on the trafo to show current rating. But I can imagine it may give a little more than 2,0A.

If you guess at current capacity, you could be wrong.

So I think I can run safely an 12V DC fan with an 15VDC output.

What makes you think you know more than the manufacturer of the fan?

 

Hello,

One of the fans I use is the NMB4710KL.
The 12 Volts version is allowed to work on 6 to 13.8 Volts.
The 15 Volts you mention would not be wise.

Bertus

 

Hello,

One of the fans I use is the NMB4710KL.
The 12 Volts version is allowed to work on 6 to 13.8 Volts.
The 15 Volts you mention would not be wise.

Bertus

Thanks, for reply.
I am using an old PC power supply box to enclose my voltage regulator, which has a fan on it already.

 

If you want to get a real 2A for your device then you can use the bridge diode rectifier as Bordodynov shown in #6, and use the LM317 and 2N3055 like Adjustable 2-30 volt power supply with the LM317.

You can also using SVR 5K + VR 2K to replace R2, the SVR 5K is used to setup the highest voltage as 15V, you can in series with a resistor to the output of LM317 to reducing the current get into the 2n3055.

 

My transformer gives 2x15V as output. Max. 15VDC is enough for me. There is no label on the trafo to show current rating. But I can imagine it may give a little more than 2,0A.

If the transformer is rated for 2A rms output current, then the DC output should be no more than about 1A due to the high RMS peak currents drawn by the rectifier-filter.
Drawing 2A DC will overheat the transformer.

 

Even if this isn't homework, you should put more effort into your project than someone on AAC.

LM317 datasheets will have examples of how to do what you want for 1.2-15V. Why do you think you need 30V input to get 15V out? Transformer secondaries are rated in VRMS. 15VAC will give you 1.414*15 peak DC. What is the current rating for the transformers?

Why do you want 15V to run a 12V fan? How much current does it need? Why do you think you need a fan?

Sorry, I posted my idea circuit without schematics drawing.
In the meantime, I prepared the following schematics and my intension is to build this as prototype on a drilled board.
Please, now you make your comments on this schemetics.

 

If you want to get a real 2A for your device then you can use the bridge diode rectifier as Bordodynov shown in #6, and use the LM317 and 2N3055 like Adjustable 2-30 volt power supply with the LM317.

You can also using SVR 5K + VR 2K to replace R2, the SVR 5K is used to setup the highest voltage as 15V, you can in series with a resistor to the output of LM317 to reducing the current get into the 2n3055.

I almost give up my project idea, because it seems that it is far beyond my range to build an adjustable voltage regulator.
I plan to go electronics shops to buy a ready device next days, because I nead one.

 

Sorry, I posted my idea circuit without schematics drawing.
In the meantime, I prepared the following schematics and my intension is to build this as prototype on a drilled board.
Please, now you make your comments on this schemetics.

You don't need C2 and C3. Apart from that it looks OK.