So here’s the scenario… you have a power transformer with a voltage output of 25V + 25V AC. Let’s say, you’re using this power transformer to supply power to a power amplifier project. So it’s all good.

But you also need 5 Volts DC to power some logic circuits (a microprocessor, Arduino, PIC microcontroller or similar) within the same project. Nowadays, incorporating microprocessors in DIY projects is common… thanks to the popularity of Arduino. You may be using this microprocessor to integrate an LCD screen, or perform temperature monitoring, or perform speaker overload protection.

What are your options?

1. You can use a voltage regulator and step down the high voltage DC output of your power supply to 5VDC. You can use a 7805 regulator, or an LM317HVT 3-terminal regulator. But this is wasteful, and will generate a lot of heat! 25VAC rectified to DC will be approximately 35VDC, give or take a few. If you regulate this down to 5VDC, you’ll be dissipating 30Volts in your regulator as heat! This will require a big heatsink. In this example, we’re using a 25+25VAC transformer. But what if your transformer is bigger, say 50+50VAC output, you’ll need to bring down 70VDC (after rectification) to 5VDC. Yikes, you need to dissipate 65Volts of energy. Not to mention, this may be exceeding the Safe Opearting Area (SOA) of your regulator. (See datasheet on maximum voltage differential, Vout – Vin allowed by your regulator.)

2. You can install an additional small power transformer in your project. This is feasible, and one can do this instead. The smaller transformer can be another toroid, or even a small PCB mounted transformer. The cons of this though is the cost. Buying another transformer for your project will be expensive. If this is a PCB mounted transformer, then you need to design a board to accomodate this transformer. Not to mention, the additional transformer and PCB will take up valuable real-estate space in your already cramped chassis.

Hmmmm…. adding that Arduino to your power amplifier project seems to be a lot of hassle. Now, you’re thinking of giving up the idea of having a cool-looking LCD display and push buttons and rotary encoders on your project.

But wait… there’s another solution! How about just installing additional secondary windings to your existing toroidal transformer?

It’s very easy, let me show you how:

So let’s say this is your power transformer:

What we need to do is just wrap another layer of wire around this transformer. So you’ll need magnet wire (check out eBay for sellers selling magnet wires, so you don’t have to buy several thousand feet of spool.)

I used the same magnet wire thickness just to keep things simple.

Step 1: Wrap 10 turns of magnet wire around your power transformer. I used masking tape to keep the wire in place.

Step 2: Strip the insulation from the ends. It’s coated with lacquer so you can either use a sandpaper or blade to scrape off the insulation and get to the bare copper. I personally used a thermal wire stripper since I have one.

Step 3: Measure the AC voltage you got from your 10-turn secondary winding.

In my case, I got 2.74Volts AC from my 10-turn winding.

Step 4. Compute how many volts per turn we got.

2.74Volts / 10 turns = 0.274 Volts per turn

Using this information gathered from our transformer, we can now compute how many turns we need for the voltage output we want.

I also measured the length of wire needed to make that 10-turn winding. For my power transformer size, it takes about 5.5ft of wire to make that 10-turn winding.

Step 5. I want to use 5 Volts DC to power my Arduino… and I want it regulated. So we need at least 7 Volts DC rectified so we can get 5Volts regulated output. So let’s shoot for 5Volts AC for our secondary winding add-on.

That means, we’ll need:

5VAC / 0.27 Volts per turn = 18.51 turns, round up to 19 turns

In this case, I decided to round up to 20 turns for a nice round number.

And we’ll need approximately 11 feet of magnet wire to wind 20-turns.

…. so go back to step 1, but this time winding 20-turns around your power transformer.

I had some extra wire left, so I just went for another extra turn… 21 turns total. Doing some math, we should get around 5.67 Volts AC (21 turns x 0.27 Volts / turn).

After doing this, I measure the output voltage and I got 5.8Volts. Very very close to our computed 5.67 Volts.

Perfect… so this means our 5.8VAC rectified to DC will be about 8.2VDC, which is more than plenty enough for our project.

Now, all we need to do is rectify this AC voltage to DC, and regulate this down to 5VDC. And we’re done.