My first power supply

[queennikki1972]

They say you learn a lot from building power supplies. And I am. My first power supply is straight forward linear power supply. Not a ton of power but enough to power small circuits and toys.

120vac - 12vdc 500ma center tap to bridge rectifier to smoothing - to regulator -to small caps .

Powers on and functions great, still waiting on my fan to come in.

[OM222O]

I can't be sure but it seems like you have 2 LDOs in parallel 
One will back feed the other due to voltages not being exactly the same which is bad
also did you consider a basic voltage adjustment pot? most LDOs are adjustable via a voltage divider circuit which you can make from 1 fixed resistor and 1 pot to allow crude adjustability of the voltage.

[queennikki1972]

Each are LM7812 and LM7805. Not adjustable. Not intended to have pot on my first one. Fixed voltages

[OM222O]

oh! that makes more sense now. just a quick note, for your smoothing stage you have only used a large electrolytic capacitor. LDOs usually specify a 100n ceramic capacitor on their inputs for noise suppression which you can put in parallel with the electrolytic one. Dave also has a great playlist detailing how to make a decent adjustable power supply which you can watch here:

[queennikki1972]

I added the smaller disc caps to the outputs but if needed i can add to the input as well.
And I will review your link

Thank you

[MarkF]

You should also have some protection diodes and a minimum load for each regulator.
10mA to 15mA for the LED power ON indication works great as a minimum load.  "Kills two birds with one stone."

   

[Tek Tech]

My first power supply is straight forward linear power supply. Not a ton of power but enough to power small circuits and toys.

It's super-cute and serves your project needs. Most importantly: you made it as your first project!
Congratulations!

BTW, everyone here will give you an idea how to make it still better. You can use the advice for your next project, but I'd suggest that you leave your first baby intact as is.

[kripton2035]

remembering my first project 40 years ago, that was a mains-powered clock... that blew instantly once plugged in !
I did not keep it


anyway congrats for your first project, hope there are more to come.

[ocset]

Ive attached a simulation in the free ltspice of a linear power supply.....its always helpful to do a quick sim of a cct before building it, as it helps highlight the little details that can sometimes be forgotten.

As you know, SMPS can help you with the efficiency

By the way, if you wish,  here is a link to my free SMPS course (there is also some stuff on linears in there)
This is the link from my google drive

https://drive.google.com/open?id=1cmk81KxXA7-EliHHWr8ZimpEp7HCMIcY

Please tell me if it doesnt work.

[queennikki1972]

@MarkF I found the 1n5395 obsolete and expensive. I also found this page that shows alternatives.

https://www.el-component.com/diodes/1n5395

Can you confirm that for example the RL207 would be a fair replacement for a circuit like the one above?

I have many of the 1n4000 series diodes but they do not show to be a replacement

[ocset]

almost any diode that is rated to at least VIN would be ok to replace that 1N5395.
...yes rl207 is fine
https://www.rectron.com/data_sheets/rl201-rl207.pdf

[MarkF]

You can use anything in the 1N539x series.  1.5A diodes with 50V to 1000V rating. 
   For example https://www.mouser.com/ProductDetail/Vishay-Semiconductors/1N5393-E3-54?qs=sGAEpiMZZMtbRapU8LlZD0HbIjlpuZ44OLW52lnUrFQ%3D

You can use diodes from the 1n400x series.  But, they are only rated at 1A.  The MC78xx and MC79xx regulators are also only rated for 1A.  Your choice.  I just would not push the maximum for the diodes or the regulators.

You can also use a bridge instead of individual diodes.  I needed to buy diodes so I used the same value for all of them.


1N539x datasheet:   https://www.vishay.com/docs/88514/1n5391.pdf

[james_s]

I can't be sure but it seems like you have 2 LDOs in parallel 
One will back feed the other due to voltages not being exactly the same which is bad
also did you consider a basic voltage adjustment pot? most LDOs are adjustable via a voltage divider circuit which you can make from 1 fixed resistor and 1 pot to allow crude adjustability of the voltage.

When I was about 12 I built a power supply with 3 7812 regulators in parallel, it may not be advisable to do but it did work and I used it regularly for about 20 years before I finally rebuilt it with a switchmode regulator. Ironically that blew up within a year.

So the point is, I wouldn't do something like that in a commercial product but sometimes you can get away with things.

[MarkF]

If you are just adding the protection diodes to your old project, use the 1N400x diodes you already have.

[queennikki1972]

Yes MarkF, its a small transformer and fairly low current I was thinking that too about using the 1n4000 series for the small power supply, but I did order some 1.5amp diodes today so I'll have them. I am taking notes on everything you guys have suggested. I have no need for the variable pots on this project as I have other power supplies on hand. And I read up on why you have a third binding post for earth ground and that chassis ground (ac ground )should not be same as - negative binding post by default. i hope I said that right.

[n3vti]

Your power supply looks awesome!


Maybe I should get back to rebuilding mine at some point... need to get a bench first.

[queennikki1972]

Treez the linked work great. Thank you

[MarkF]

Yes MarkF, its a small transformer and fairly low current I was thinking that too about using the 1n4000 series for the small power supply, but I did order some 1.5amp diodes today so I'll have them. I am taking notes on everything you guys have suggested. I have no need for the variable pots on this project as I have other power supplies on hand. And I read up on why you have a third binding post for earth ground and that chassis ground (ac ground )should not be same as - negative binding post by default. i hope I said that right.

Not exactly.  You want "earth ground" and "chassis ground" to be connected together for safety reasons.  Chassis ground being the metal enclosure you put your project in.  You don't want any voltage on the metal enclosure.  When you bring the earth ground to the front panel, you have the option to tie it to any of the other outputs as a reference.

The "common ground" (labelled as such in my earlier drawing) is your circuit reference in most cases.  Typically, people call it ground but it is floating and not tied to "earth" or "chassis" ground for most power supplies.  If the common is left floating (i.e. the secondary transformer windings are NOT tied to earth ground), you can put multiple power supplies in series to create other voltages.  In my triple supply above, you can tie the +5V COM output to +12V output.  You would then have +17V, +12V and -12V outputs referenced to the +/- COM connection.

[m3vuv]

why not go down the lm327 route with pass transistors?

[ArthurDent]

MarkF –

You want "earth ground" and "chassis ground" to be connected together for safety reasons.  Chassis ground being the metal enclosure you put your project in.  You don't want any voltage on the metal enclosure.  When you bring the earth ground to the front panel, you have the option to tie it to any of the other outputs as a reference.

Here is one of the first little supplies I put together a few decades ago that illustrates that above principle. It was just a small linear supply board with transformer that I removed from some piece of scrap equipment that put out a couple of hundred milliamps on each output. It was convenient to test small circuits so I added a switch for +/-12 and +/-15 VDC and built it into this small metal case. As you can see, the common for the 3 outputs is not connected to the earth/line/case ground but the ground terminal is available on the front panel.

This project is old enough so it actually uses a line powered neon panel light and not an LED

[Tek Tech]

When I was about 12 I built a power supply with 3 7812 regulators in parallel, it may not be advisable to do but it did work and I used it regularly for about 20 years before I finally rebuilt it with a switchmode regulator. Ironically that blew up within a year.

So the point is, I wouldn't do something like that in a commercial product but sometimes you can get away with things.

I've done the same thing: stacking several 78xx in parallel for more Amps and less heat and it worked surprisingly well. I'd suggest, for best results they all should be from the same batch.

[MarkF]

When I was about 12 I built a power supply with 3 7812 regulators in parallel, it may not be advisable to do but it did work and I used it regularly for about 20 years before I finally rebuilt it with a switchmode regulator. Ironically that blew up within a year.

So the point is, I wouldn't do something like that in a commercial product but sometimes you can get away with things.

I've done the same thing: stacking several 78xx in parallel for more Amps and less heat and it worked surprisingly well. I'd suggest, for best results they all should be from the same batch.

If you put several regulators in parallel, you should put a series resistor on each output to limit the current due to small differences in the output voltage of each regulator.  Refer to example from the datasheet:

   

[rsjsouza]

Good project and good looking supply - congratulations! Power supplies are always pretty cool as starting projects - I still remember the one I assembled with my dad in the 80's.

Back in the day we did not have access to quality fans, thus we had to rely on large heatsinks for the bypass transistors (large TO-3 2N3055). Hey, at least the power supplies were quiet. 

As you progress, you may want to pay a visit to Tanner Electronics in Carrolton if you haven't yet been there. It has quite a great selection of parts for projects. BG Micro is another local store (in Garland) that also has a good selection - they are an online shop, but you can always save on shipping by arranging local pickup.

[Tek Tech]

If you put several regulators in parallel, you should put a series resistor on each output...

Yes, I should, and I never did. But it worked quite well, trouble free. However, using chips by different manufacturers would definitely require resistors on the outputs.

BTW, I also did the trick building home-made light dimmers long ago. I would connect two triacs in parallel (straight, no resistors) to reduce the heat. It was very effective.

[queennikki1972]

So, I did go back and add the protection diodes, green banana jacks tied to PE, larger heat-sinks and a larger transformer of 12v 1.8 amps. I did not use the adjustable pots as the LM7812 and LM7805 are not variable. I measured the ripple and it looks good. I am purchasing a electronic dc load after December so Ill be able to test the load. I've tested about .25 amp load and seems to run nice and cool.

I've since built a variable supply with LM317T and working on a dual adjustable supply of 1.5-60vdc. With each type I learn more. After I fully understand the linear supplies Ill try some switch-mode supplies or some built using IC's and Transistors.