Change voltage without consumption

Question

Is it possible to change voltage without consuming power?

I have a 5V Arduino circuit that makes use of a wireless chip (NRF24L01) that runs on 3.3V. My current approach to get 3.3V out of a 5V source is a voltage divider using a two resistors. It is cheap and very easy to setup. The disadvantage is that the consumption over the remaining 5V - 3.3V = 1.7V is lost / wasted. I run the circuit on an AA battery so power consumption has to be as low as possible.

Now I am looking for a way to not loose any power in the process of getting a 3.3V out of 5V. One approach would be to use a DC buck step down converter. It uses very little power to do so, but it also is more expensive than the voltage divider using resistors. There is also the possiblity to use an AMS1117, but I have no experience with that.

Is there a cheap and power saving solution for getting 3.3V out of 5V?

Answer 1

a resistive divider is never an appropriate voltage regulator. It's dangerous – the voltage seen by the low voltage side depends 100% on the current it draws

The disadvantage is that the consumption over the remaining 5V - 3.3V = 1.7V is lost / wasted.

That's not even the point. You say this works, which means your resistors are using significantly more power than your 3.3V circuitry. This is a terrible design choice!

The voltage can be much higher or lower than the target current, unless you waste a lot of power through much too strong resistors; I hope you remember Ohm's law and how a loaded resistor divider is not the same as an unloaded one.

So, drop that really bad idea that everyone should tell you is a bad idea and use a linear voltage regulator. Done.

You get an actually regulated voltage, and don't waste as much power.

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You could use a more expensive but more efficient buck converter instead of a linear voltage regulator, but "5V arduino" kind of implies "not power efficient" to begin with, so my guess (!) is that this won't save a significant amount of energy.

If you want to be more energy efficient, re-base your digital logic to run on a microcontroller off the same 3.3V (or lower) rail as your NRF chip, and supply both from a switch-mode power supply.

Answer 2

First of all, you can't power anything from a voltage divider. As soon as you start pulling current out of it, you start to lose voltage on its output. Your voltage divider has to consume like an order of magnitude (x10) times the current used by your 3.3V device to keep voltage at usable levels. There is no CHEAP and POWER SAVING solution. And you will also have a series resistor before the power pin of 3.3V device.

Cheap solution is 1117 or some LM317, just open the datasheet, there is an example circuit, it's more than easy. It will still waste 1/3 of power, because that's what linear regulators do - they burn excessive voltage in a way.

Power saving solution is to use a switching voltage regulator - buck converter (step-down converter). They can have efficiency over 90%, sometimes over 95%. They cost a little more, and you can probably find a 3.3V output module for a dollar or two.

A bit of a weird solution is to put 3 series diodes to drop around 1.8V down to 3.2. I would also put a resistor between 3.3V power pin to the ground to make some minimum current of 1ma or so to make sure the diodes would always conduct something and keep their voltage drops relatively constant. 2.2k resistor looks good for that. Oh and this is also energy burning.

Answer 3

There are many ways to convert voltage, as with any engineering choice, they come with tradeoffs. It sounds like you are a hobbyist and I would recommend using a linear regulator, 78M33. If you feed it 5V it will pop out 3.3V. There are also other 7833 chips that produce more or less current, some with smaller or large packages, different pins for different applications, etc.

Below are some of the basic tradeoffs of different kinds of regulation:

Voltage divider:

• Least complex

• Least expensive

• Voltage is not regulated - will vary quite a bit depending on how much current is being drawn by the downstream load (in this case your wireless chip)

• Very inefficient, the resistors are constantly using power that doesn't go into your load

• Teeny tiny - only two small parts

Linear regulator (silicon that throttles the current up or down when voltage gets too high or low):

• More complex, the inner working of the chip are not trivial, some applications require capacitors as well for stability. Adjustable ones (317) require resistors to "set" the voltage.

• More expensive but we're only talking $1-2 for a chip (78M33, 317, etc)

• Voltage is regulated; it will stay very close to the correct voltage as the load changes

• Efficient enough for most applications - in your application it might cost a few milliamps to do the conversion.

• Very small - one small chip and a capacitor or two

Switching regulator (uses switches/transistors and inductors to charge or not charge a capacitor to adjust voltage):

• Very complex, requires a handful of components, each of which must be selected properly (guidance available in the datasheets)

• Very expensive compared to to linear regulators, requiring a chip, a handful of external parts and a way to wire them all together

• Voltage is regulated as above, but there will be more noise on the voltage due to the switching nature of the regulation

• Very efficient; Least energy is lost; 95-98% efficient

• Very large - many parts, capacitors and inductors are space hogs