Switching, Regulated, Or Transformers? Choosing the Right Power Supply

!: Switching, Regulated, Or Transformers? Choosing the Right Power Supply

I received another message from Dan, this time asking about power supplies:

Subject: Frustrated with power converters

So, the past month or 2 I've been trying to figure out how different types of AC to DC power supplies work, since i have a few audio/video receivers that im trying to install PC fans inside that require that type of power.

The problem is that I don't have any power converters that are less than 18 volt DC. I've been researching capacitors, different types of diodes and ways to hook them up but I cant seem to lower the voltage. I've tried a few different ways to hook a resistor up to it and sometimes the resistor starts smoking or doesn't change much of anything. im just really annoyed with spending many hours on this simple project. I'd hate to ask you to explain multiple electronic parts since you're probably very busy so instead could you give me a short answer to this?

I've been to over 50 Google websites and cant find anything on YouTube that I understand.

So it sounds like Dan has a situation where AC to DC has been converted, but he DC is too high to use with a 12V fan. The quickest solution is to find an AC fan (they do exist, but they can be noisy, and overkill for this kind of situation). So instead let's look at the alternatives.

Let's say that Dan has 18V coming from the power supply and he needs 12V to drive a quiet PC fan.

Voltage Regulators

Where you require a large amount of current, using a resistor/potential divider is not always the best idea, as the current levels causing heating/smoking - and that's usually a bad thing. An alternative is to use a voltage regulator - this does the same thing, dissipates the excess voltage as heat, but does it in a more controlled way and lets you attach a heat sink to it. You need to work out the current required by the motor driving the fan, then multiply this by the voltage drop. So if you are trying to drop 18v to 12v and your fan needs 1/4 amp - this would be (18-12) * 1/4 = 1.5 Watts. (Watts being power and equal to volts x amps). The hookup is simple - 3 legs: input voltage, gnd and output voltage.

So your regulator is going to operate like a 1.5Watt heater, and if you don't have enough heat sinking attached it will may overheat and go into thermal shutdown.

(Like the 5V regulator did on my original laser harp design)

So if the power dissipated is not that high, the cheapest solution would be to use the regulator.

Switching Convertors

A better, more efficient method is to use a "Switching" power supply. These take the input voltage (18V) and turn it on and off very quickly (thousands of times per second). The amount of time the voltage is on, compared to the amount of time it is off is called the "duty cycle". So if I wanted to convert 18V to 12V I would need a duty cycle of 2/3. This is then fed through a network to smooth it back out to DC. I recently built some efficient DC converters for a robotics project. It was important to make them switching as they would not drain the battery as much as a regulated power supply.

AC to DC

If Dan wants to tap just the AC power then this has to be regulated and stepped down first. The old school way of doing this is to use a transformer with windings that get you close (but slightly above) the voltage you want. Eg - 110V to 16V would be a good step down. Then the 16V AC is fed into a full wave rectifier array of diodes that do full wave rectification, and then finally through 2-3 large capacitors to smooth the voltage down to something usable.

It's worth pointing out that you can get switching power supply units that run off AC directly. The AC has to rectified and smoothed first, but then the full 110V is switched on and off to generate the voltage you require. The beauty of these units is that they can run on either 110V or 220V unmodified, the circuitry just monitors the output and adjusts the duty cycle of the switching to deal with whatever input voltage you pass in.

Probably the best thing for Dan to do is check out electronic surplus websites and pick up a 12V or 5V switching "wall wart" and then install this inside the case. They are small, efficient, and sometime go on sale for a dollar or two.

Switching, Regulated, Or Transformers? Choosing the Right Power Supply

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Faqs About Voltage Converters and Transformers

!: Faqs About Voltage Converters and Transformers

If you're confused about voltage converters and transformers, we've compiled a list of facts that will make them simpler to understand. Here are the most common questions that habitancy ask, along with the answers.

What Do Voltage Converters and Transformers Do?

Voltage transformers and converters change the voltage of the current advent from a wall outlet so that it can be used for electrical appliances or buyer electronics that wish a different voltage.

When Would I Need a Voltage Converter or Transformer?

Electronics and electrical equipment are designed to use electricity at a definite voltage. If the voltage is too powerful, it can fry the electrical circuits and destroy the equipment. If it's not grand enough, the equipment won't work. different countries have different standards for their electrical outlets. North America and a few other countries provide 110-120 volts in most household currents. Most of the rest of the world provides 220-240 volts. If you travel surface the U.S., you may need a voltage transformer or converter to power any electronics that you carry with you. Likewise, if you bring electrical appliances from overseas into the U.S., you may need a voltage transformer or converter to use them in the U.S.

How Do I Tell Which Voltage Converter I Need?

You need to match the voltage and wattage needed for your appliance in order to power it without damaging it. Every electrical appliance or buyer electronics item has a label that tells you the voltage and whole of amps needed to power it. Many also list the whole of watts that it pulls from the circuit. If the wattage isn't listed, you can presume it by multiplying the whole of volts by the whole of amps.

What is a Step Up and Down Transformer or Converter?

Cheaper transformers can only change electricity in one direction. For instance a step up converter transforms power from 110V to 220V while a step down converter transforms from 220V to 110V. A step up and down converter or transformer can change electricity in whether direction. Usually, you'll have to set the converter for the right setting, but many high-end transformers automatically sense the needed voltage and adjust accordingly.

Do I Need a Converter For Each Appliance I Plug In?

No. You can plug some items or appliances into the same voltage transformer by using a surge protector. But you'll have to do a slight math to make sure that you don't damage the transformer or your electronics. Add up the wattage of all the appliances you want to plug in and make sure that it doesn't exceed the wattage of the transformer. Try to keep the total wattage output about 20% below the top whole the transformer can handle in order to list for power fluctuations.

What Kind of Appliances Can I Plug Into a Voltage Transformer?

You can use a voltage transformer or voltage converter with nearly any small appliance or personal electronics item, along with galvanic shavers, hair dryers, cell phone chargers, Mp3 players, stereos, televisions, Dvd players and recorders or coffee makers. Select a converter for appliances, such as hair dryers, curling irons or coffee makers that have a heating element and pull a lot of wattage. Transformers are suitable for electronics, such as Dvd players, televisions and battery chargers, which have electronic circuit boards.

Faqs About Voltage Converters and Transformers

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