# Building a Power Supply..

#### PCBoy

Joined Dec 14, 2011
26
So hey, I got a project to build a Power Supply and I got me a dilemma.

So I got this 32V 3A Center-tapped Transformer and it cost a lot of money. Now, I just realized that my IC regulators (LM7815) has a maximum input voltage of 40V. Since I still have a filtering capacitor before the IC, my regulator gets a total of 32*sqrt(2) volts or approx. 45 volts.

I think this'll break my regulator and I'm thinking of a way to remedy this. So far I'm at a blank.

As much as possible I'd like to go the more economic way. I've already used up a lot of money for my parts.

Hope to hear from you guys.

Thanks!

#### R!f@@

Joined Apr 2, 2009
9,740
Why the heck you did you went and buy a 32V transformer when you have a 7815 regulator, which say only needs about 20VDC min to regulate 15VDC.

Either you make a series regulator or buy a high power resistor to get rid of the extra power that u never gonna use in ur app.

The way I see it....doing no home work is heavy on ur pocket.

#### MrChips

Joined Oct 2, 2009
22,521
Use a full wave rectifier and not a bridge rectifier. That should give you about 22V.

#### bountyhunter

Joined Sep 7, 2009
2,512
So hey, I got a project to build a Power Supply and I got me a dilemma.

So I got this 32V 3A Center-tapped Transformer and it cost a lot of money. Now, I just realized that my IC regulators (LM7815) has a maximum input voltage of 40V. Since I still have a filtering capacitor before the IC, my regulator gets a total of 32*sqrt(2) volts or approx. 45 volts.!
It will be a LOT higher than that when you measure the peak DC voltage at no load. It will blow th regs. I thought there were some LM317HV versions that handle about 60V (?)

#### PCBoy

Joined Dec 14, 2011
26
Why the heck you did you went and buy a 32V transformer when you have a 7815 regulator, which say only needs about 20VDC min to regulate 15VDC.

Either you make a series regulator or buy a high power resistor to get rid of the extra power that u never gonna use in ur app.

The way I see it....doing no home work is heavy on ur pocket.
Our professor did not make us aware of that fact. He actually recommended to us to use this 32V. In retrospect, no idea what he was thinking.

Thanks for the suggestions, guys. Gonna read into them later when I get home.

#### crutschow

Joined Mar 14, 2008
26,013
Do what MrChips recommended. That will solve your problem and that may be what your professor intended.

Last edited:

#### PCBoy

Joined Dec 14, 2011
26
Do what MrChips recommended. That will solve your problem and that may be what you professor intended.
Yeah, I think that'll work!

Thanks guys!

#### t06afre

Joined May 11, 2009
5,934
Do you have adequate cooling fins for the regulator. It could be that overheating is a part of your problem.

#### Audioguru

Joined Dec 20, 2007
11,249
Make the circuit that feeds the LM7815 like this:

#### Attachments

• 21.1 KB Views: 53

#### R!f@@

Joined Apr 2, 2009
9,740
Chips suggestion is effective here without further expense. But you will be at the extreme of any 78 series regulator if the load is at 1amp. You should consider better cooling to prevent thermal shutdown.

#### Georacer

Joined Nov 25, 2009
5,182
Is it possible to use multiple 78XX regulators in parallel, in order to increase the amperage?

#### t06afre

Joined May 11, 2009
5,934
Is it possible to use multiple 78XX regulators in parallel, in order to increase the amperage?
In some data sheets I have seen use of a external transistor to boost current. But I have not seen the use of multiple 78XX regulators to boost power. The output voltage will always differ some amomg 78xx circuits

#### bountyhunter

Joined Sep 7, 2009
2,512
Is it possible to use multiple 78XX regulators in parallel, in order to increase the amperage?
NO, that doesn't work

#### Georacer

Joined Nov 25, 2009
5,182
Hm, weird. I have used that in the Automated Slot Car Intersection, my completed project submission and I 'm quite sure two 7805 covered the current need where only one didn't. I admit that I didn't run the circuit for more than some hours total, though.

Why is it not recommended to use them in parallel?

#### crutschow

Joined Mar 14, 2008
26,013
Hm, weird. I have used that in the Automated Slot Car Intersection, my completed project submission and I 'm quite sure two 7805 covered the current need where only one didn't. I admit that I didn't run the circuit for more than some hours total, though.

Why is it not recommended to use them in parallel?
The one with the higher output voltage will provide all the current until it's current limit is reached, at which point the other unit starts providing the additional current. Thus the load is very unbalanced between the paralleled units. The will operate that way but it's not good for reliability.

This can be reduced by adding a small output resistor is series with each output, but that degrades the voltage regulation with load change.

#### PCBoy

Joined Dec 14, 2011
26
Hey thanks for all the reply. I tried the center-tapped 2 diode rectifier setup but I'm getting 28-29V instead of the supposed 22V. Any ideas?

#### R!f@@

Joined Apr 2, 2009
9,740
Now U can use a LM317HV regulator to do ur thing. Find one and we can show u how to built it.

What are the value of ur Filter caps

#### Attachments

• 471.8 KB Views: 15
Last edited:

#### t06afre

Joined May 11, 2009
5,934
Last edited:

#### PCBoy

Joined Dec 14, 2011
26
Now U can use a LM317HV regulator to do ur thing. Find one and we can show u how to built it.

What are the value of ur Filter caps
I'll browse our local market tomorrow if they have that.

Does that have a negative equivalent, btw?

Hey PC boy take a look at these Wiki papers. If you need more help just ask.
http://en.wikipedia.org/wiki/RectifierThen it comes to 50/60 Hz transformers like yours. The voltage output is always given/rated in RMS voltage
http://en.wikipedia.org/wiki/Root_mean_square
How much current do you plan to take from your power supply?