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Voltage conversion from 24VAC to 5VDC is quite useful, because a lot of home automation devices use 24VAC, including sprinkler solenoids, home surveillance systems etc. Having a conversion module makes it easy to use a single power supply, without a separate 5V adapter for your control circuit. There are plenty of resources you can find online about it. But these resources are rather scattered. So in this blog post I will summarize and discuss the common choices.

AC to DC Rectification

Before we begin, the first step is to have a rectifier that converts voltage from AC to DC. The common choices are half-wave rectifier (which requires just one diode) or full-wave rectifier (which requires four diodes). For simplicity, I will use half-wave rectifier as an example. The typical schematic of a half-wave rectifier is as follows:

It’s simply a diode followed by a capacitor to smooth out the rectified AC waves. As we know, diode only allows current to flow in one direction, so after the AC voltage passes through the diode, only positive voltage remains. The diode must be selected based on the maximum reverse voltage and the maximum current. One thing easy to forget is that when we talk about 24VAC, we mean the RMS (root-mean squared) magnitude of the voltage is 24V. Since AC voltage is a sine wave, the peak voltage is actually 24 * sqrt(2) = 34V, which is quite a bit higher. The maximum reverse voltage applied on the diode is therefore 34 – (-34) = 68V, which is when the AC voltage runs to the negative peak. So a diode with 70V peak reverse voltage is sufficient.

In practice, transformers that are rated 24VAC usually have a higher no-load voltage, which can vary between 26VAC up to 28VAC. This is typical, and the voltage is supposed to drop close to 24VAC under maximum load (i.e. the current rating of the transformer). As a result, when the circuit is powered on, the transformer can output a peak instantaneous voltage of up to 28 * sqrt(2) = 39.6V.

In the schematic above, I’ve picked a 1N4002 diode (70V reverse voltage, 1A current) and a 100uF/50V capacitor. These should work well for common scenarios. Note that the voltage output on the capacitor is approximately 34V – 1V (diode’s forward drop voltage) = 33VDC. Again, when the transformer is well below maximum load, the output voltage can go as high as 39.6V – 1V = 38.6V.

So next time if you see a power transformer rated 24VAC, after rectification, gives 39VDC, don’t be surprised!!

Now that we have a DC voltage, the next part is to step it down to 5VDC. We want it to be regulated, so that the voltage won’t fluctuate much. There are a variety of solutions:

1. Zener Diode

Probably the simplest solution is to use a Zener diode. As we know, a Zener diode can force the voltage across it to remain constant (break-down voltage) when it’s in the break-down condition. This condition is met when the current flowing through it (in reverse direction) is at least a few milli-amps (5mA typical) but less than the maximum current allowed (e.g. the diode’s power rating divided by its break-down voltage). For example, a 5V/1W Zener will remain in break-down condition when the reverse current is between 5mA and 1W/5V = 200mA. The typical schematic is shown as follows:

Here resistor R1 is used for current limiting. Assume D2 is a 5V Zener diode, and the circuit on the right-hand side draws about 180mA current. R1 must be selected such that the current flowing through it is 180mA plus at least 5mA to keep D2 in break-down condition. So we have R1 = (33 – 5) / 0.185 = 150 ohm. Note that D2 should be rated at least 1W, because in case of open-circuit, it needs to absorb the entire 185mA without burning out.

Now let’s take a look at the power rating of the resistor R1. Since the current flowing through it is 185mA, the power is 0.185 A * 0.185 A * 150 ohm = 5.1 Watt. Wholly crap — this is gotta be a big resistor, isn’t it :). Well, this is the unfortunate drawback of a Zener diode based regulator, that is, it can waste a lot of power and require a bulky resistor. Fundamentally, it regulates the voltage by converting the voltage differential to heat. In this case, the voltage differential is quite big (33V vs. 5V), and the current draw is fairly large (180mA) too, so it ends up wasting a lot of power in heat.

Another drawback is that to increase the current draw, we must decrease R1. Otherwise, if the output circuit starts to draw, say 250mA, that will take D2 out of its break-down condition, and the output voltage is not regulated any more. So overall it is only suitable if the current draw is constant and small (e.g. tens of milliamps).

2. Linear Regulator

Another simple solution is to use a linear regulator, such as the popular 7805. The typical schematic is as follows:

The circuit is quite simple, and the output current can vary across a wider range. However, linear regulator shares the same drawback with Zener diodes, that is, it fundamentally works by converting voltage differential to heat. As a result, it wastes the same amount of energy (5.1 Watt in this case) in heat. This is not only a matter of waste, but also it requires a large heat sink to dissipate the heat, otherwise the regulator will burn and smoke. So clearly not an efficient solution. In fact, the efficiency of a linear regulator is the ratio between the output and input voltages. In this case, it is 5 / 33 = 15.15%, which is very poor.

3. Switching Regulator

Now we have come to my favorite topic: switching regulator, also known as switching converter, or switch-mode power supply (SMPS). It uses transistors and a reactive component, namely inductor, to convert voltages much more efficiently. Ideally the switching converter works by simulating the following:

That is, imagine there is an automated switch between the input and output. When the switch is turned on, it connects the input to output, and when it turns off, the input and output are disconnected. This essentially generates a square wave with 33V peak voltage, and the duty cycle is determined by the switch. Suppose the duty cycle is 15.15%, as long as the switching frequency is sufficiently high, at the output it would seem as if you have a constant voltage of 33 * 15.15% = 5V. That’s it, simple!

The main advantage of switching regulator is that since there is no resistive element, theoretically there is no energy loss at all, so the conversion efficiency is 100%! Of course in practice there will be some energy loss due to the imperfections of electronic components. Still, even at 75% efficiency, we are talking about a power waste of only (5V * 0.18A / 0.75) – (5V * 0.18A) = 0.3 Watt, much better than the 5.1 Watt waste you saw previously with a linear regulator.

The schematic above may look very simple. But it doesn’t tell the whole story. Implementing the switch is more complicated than you might think. That brings out the drawback of a switching regulator, namely cost and circuit complexity. It typically involves a transistor or MOSFET that functions as a digitally controlled switch, an oscillator circuit that generates a control square wave, a voltage reference and feedback module that monitors the output voltage, and finally a current sensing or thermal shutdown module that protects the regulator. That’s why switching regulators are typically provided as integrated circuits.

MC34063
Probably the cheapest and most widely used switching regulator is MC34063. The volume pricing (quantity 100+) is only 20 to 30 cents. Dave Jones at the EEVblog has a nice video tutorial about how to use MC34063. Also, there are a lot of MC34063 calculators you can find online, which will help you figure out the component values and parameters.

The schematic on the left below shows what I have been using for OpenSprinkler. MC34063 has a maximum input voltage of 40V (and some manufacturers make it 45V), so it’s perfect for our purpose. The main peripheral elements include inductor L1 (150uH), Schottky diode D2 (1N5819), timing capacitor CT (which controls the switching frequency), current limiting resistors Rsc (0.5 ohm), and feedback resistors RT and RB. This circuit can provide 5V 300mA output. The image on the right below shows a picture of the switching regulator section on OpenSprinkler 1.42u DIY kit.

On OpenSprinkler Pi, Rsc is reduced to 0.33 ohm (three 1 ohm resistors in parallel) in order to provide higher current required by RPi. The picture on the left shows the switching regulator section on OpenSprinkler Pi, which uses all surface mount components.

MC34063 is quite flexible. It’s not only useful for step-down voltage conversion, but it can also do step-up conversion (i.e. the output voltage is higher than input voltage), and voltage inversion. On the other hand, it requires a number of peripheral components, and picking the right component values can be tricky, especially if the output current can vary across a wide range. It’s also prone to noise (remember those annoying humming noise from cheap power adapters), and its maximum current is limited to 1.5A.

Overall if you want a cheap switching regulator, and your circuit draws roughly a fixed amount of current well below 1.5A, then MC34063 is a great choice to consider.

LM2596
More recently I’ve started using LM2596 as a replacement for MC43063. I came across it when I was shopping for a modular step-down converter and noticed this one from Amazon.com. LM2596 provides up to 3A output current, requires only a small number of peripheral components, and is more reliable and less noisy. In fact, when I started working on OpenSprinkler, I have used a similar product LM2574 for a while, but that has a current limit ot 500mA, and the switching frequency is much lower.

Here is the new design of the voltage conversion section in the upcoming OpenSprinkler 2.0 and OpenSprinkler Pi 1.1:

It uses LM2596-5.0, which has a fixed output voltage of 5.0V. The number of peripheral elements is minimal, and the circuit design is very clean. The main downside is that it is considerably more expensive than MC34063. So the extra capabilities don’t come for free 🙂 Still, for reliability and clean design, I have decided to adopt it for all future circuits.

4. Other Solutions

The above has summarized the common choices I’ve learned through my experience. There are certainly other solutions as well. For example, you can use a transformer to step 24VAC down to 5VAC, then from that point on you can use a rectifier followed by a linear regulator to convert it further to 5VDC. This is fairly efficient because transformers can have high efficiency, and the linear regulator in this case is also efficient because the voltage differential is small. However, transformers are bulky and expensive. and this solution is not suitable if the input voltage varies across a wide range.

Another choice is to use a capacitor for current limiting, in conjunction with a rectifier and a 5.6V Zener diode for voltage regulation. The idea is similar to solution 1 above, except it uses the capacitor’s reactance (instead of resistor’s resistance) to limit current. Since there is little energy loss, this is very efficient and is similar to the transformerless power supply design, which is frequently found in small wall adapters. Unfortunately, to provide sufficiently high output current (more than tens of milliamps), you will a capacitor that has high capacitance (e.g. 100uF) and is non-polarized. This is not easy to find in real life.

Finally, you may be wondering why not use a resistor-based voltage divider to split 5VDC out of the 33V rectified input? Well, this is a terrible idea in almost any circumstance I can think of. The reason is that the output voltage will fluctuate considerably depending on the current draw. In other words, it is not regulated. So I can’t think of any real use of it other than providing voltage reference.

That’s all. I hope this blog post provides useful information for your own power supply design.