Using a scrap transformer, kbp310 bridge and 7809 to make an Arduino power supply

Hi, I'm the newest member to the Arduino community. I'm from Sri Lanka and just ordered my UNO board. I'm posting this while it arrives by mail.

So I did some initial reading to get things started, and thought of building a power supply myself. (Since I'm from a poor country and can do without spending additional money for a power supply that costs as much as the board itself)

So here's my thinking. I have several old radios with a few big transformers. (like 100g) I scrapped one, used the Multi-meter to find out that it outputs an AC voltage of 16V or so. (Sri Lanka has 230V, 50Hz if that's important)

So along with my UNO board, I ordered a few diode bridges known as KBP310 (rated for 3A), and a few 78XXs of 5,9,12 and 15V. These are much cheaper than the power supply prices obviously. I ordered DC jacks as well.

What I expect to do is this. Hook up the transformer to mains AC, then get the 16V from the other side and run it through one of the KBP310s. That should give me a rectified 16V right? Then I would like to use the 78XX to get my 9V. That will go through a DC jack and make my power supply. (add a few cosmetic stuff like tapes, AC plug to make it look better and safer if needed)

So my question is, will this be an okay way of powering my UNO board? Will the voltage from 7809 be steady enough for the job? I don't care about the efficiency at all, and have heatsinks that I can scrap from other old equipment if heat is an issue. If needed, I could spare other 78XXs so it goes like 7815 at the bridge, then it goes to a 7812, and it goes to a 7809. Is that necessary and enough for a stable enough supply so that my UNO will work without issues?

I just want to avoid spending a huge amount on a power supply. Even if I use three 78XXs like above, the whole thing costs me less than 1/5th of a power supply. If I can get away with just 7809, it's like 12 times cheaper.

Sorry if my English is horrible. But I hope my question is clear enough. Any thoughts from an experienced persons on this subject are welcome. :slight_smile:

What I expect to do is this. Hook up the transformer to mains AC, then get the 16V from the other side and run it through one of the KBP310s. That should give me a rectified 16V right?

No, it will give you the peak to peak voltage or 1.414 times the 16 volts from the transformer.
You also need to be sure to use large value capacitors to filter the 100Hz pulses to give pure DC.

The DC voltage from the rectifier will be 1.414 x rated RMS voltage (as pointed out by Paul).

You also need to allow for transformer regulation (the difference between the rated voltage at full load and the actual output voltage when off load), which could be another 20% or more.
This could mean a peak DC voltage of approaching 30v, so the reservoir capacitor(s) need to be rated at least 35v and I would suggest a minimum of 1000uF, preferably more.


You can make a power supply form the transformer, diode bridge, large filter cap and 78xx regulator. That is how we made supplies in the old days. But now if I want a supply for my project I go to my local thrift store and pick up an old 5V phone charger, cut the connector and wire the 5V to the 5V pin of my Uno. The thrift store chargers are usually less than $1.50 USD. Just make sure that the output is 5V DC (look on the label on the charger).

Hi, thanks for correcting. So that makes like 23V right? Can't a 7815 handle that and give out regulated 15V? Then I can get it down again using 7809.

I was trying to use the transistors without the capacitors. Is there a reason why 78XXs won't filter out ripples, pulses?

I can't seem to understand why you need the capacitor when the 78XX is supposed to regulate the voltage. Without a regulator, of course capacitors are needed, but aren't they redundant when there's a regulator? Can you please let me know where I'm wrong?

Hmm, yeah, that's why I said I'm from Sri Lanka. Here it's other way round. Cheapest old phone chargers are around 150 Rupees and ones that are available for that price are only 5V and 1A or so. Reliable 9V 2A power supplies are like Rs. 800. (even if you don't know rupees, just think about the numeric values)

I obviously get the transformer completely free. 78XXs are just 20 Rupees here and the bridge is 20 Rupees as well. And I can get any voltage I want if this is okay. So there's my reason.

I read UNO needs at least 7V through the jack right? Everyone says 9V is the perfect voltage through DC jack. I prefer to use that instead of using 5V pin. Also once again, why do we need a capacitor when regulator is there?

A full wave rectifier (your diode bridge) will give you DC, but it will have a lot of ripple. See the gray waveform. The cap is to smooth out the humps to try to get smooted DC, the red waveform. The bigger the cap, the more smooth (less ripple) the waveform.

Take care when using Vin or the "power jack" on your Arduino. Powering through Vin or the power jack means that the Arduino and all peripherals that are on the 5V rail are powered by the onboard 5V regulator. The on board 5V regulator is not heat sinked so will supply limited current before it overheats and shuts down. I would use an old phone charger and connect that to the 5V on the Arduino, bypassing the, weak, 5V regulator. Then the rated current of the DC DC converter is available on the 5V line.

To bad about the cost of old phone chargers. They really are near ideal supplies for the Uno.

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The reality of using the 78 series of voltage regulators is they are ALL high gain amplifiers with feedback to maintain the rated voltage. Without the capacitors specified in the data sheets, they are likely to oscillate at an undetermined frequency and NOT be able to voltage regulate. So, don;t plan on using them without the bypass capacitors.

Hmm, is that so. I don't plan on using too many power draws. Can't it handle regulating like 9V at 200ma or so?

So maybe I can instead use the 9V first, and get that 9V to 5V with 7805 to the 5V pin. Then that should be okay right? It should basically be similar a good phone charger I guess.

Hmm, so let's think I really do need a capacitor. What sort of a capacitor do I need? There's a 2200uF, 25V one in one of the old radio circuits. Can I salvage and use that? There are several 1000uF 16V and 25V ones as well. (all are electrolyte)

I guess we are back to asking you to just read the data sheets on the regulators and follow their schematic and capacitor recommendations.


Is this transformer's output (secondary) center-tapped?

Ah no, that's why I use a full bridge. Just two terminals.

As other helpers already told, that was the way to build power supplies. They work really well.
Read the specs for the 78xx, especially the need for small decoupling capacitors on input and output.
Use a 7805 and give the UNO proper and reliable 5 volt directly. Skip the poor Vin to 5 volt onboard regulator. The7805 is much better. Check the 7805 temperature. Maye some heat sink is needed.

Why do you want 9volt or more, and make the Arduino work hard to get it down to 5volt.
An old cellphone charger with USB socket is ideal and safe. Just plug your USB lead in there.

Maybe you can get a car cigarette lighter USB charger cheap.
That also safely drops down any voltage (up to about 24volt) to 5volt.
It's switching supply means you don't need a heatsink.
An 7805 belongs in a museum.

My page here may help you understand PSU design from basics. Including choosing the right size of capacitor.

After rectification and smoothing a 7812 will drop the output to 12V, which you can then drive a 7805 to get 5V. The 12V output may also be useful for other projects.
The devices WILL need seperate heat sinks.

As I said, I read everywhere that 9V is the best. It also said the best range is 7-12V, so there.

I said earlier in my replies that I"M FROM SRI LANKA and old phone chargers cost way more than 78xxs and bridges. Same goes with car cigarette lighters. There are ones on ebay etc for less than a dollar but to ship them here, it takes months or have to pay like 20 times that for shipping.

Thanks, that's why I asked.