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# RF to DC

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#### zemanekj

##### New Member
I have looked all over the place but have not been able to find a definitive shopping list of what I would need to make an RF to DC circuit. Any help?

are you trying to provide wireless power to something? or is this something like a radio receiver?

What RF?
What DC?

ak

are you trying to provide wireless power to something? or is this something like a radio receiver?
provide wireless power

To what?

when the question of sending power by radio was being investigated in the late 1930s (a feasibility study for a "death ray" capable of delivering sufficient wattage to kill a sheep at 100 meters, very likely "100 watts at 100 meters") the result was the calculation that to succeed the transmitter would need to have an ERP of 5 gigawatts. this is one reason we don't have electricity delivered to our homes by radio. the real "killer" here is first an inverse cube law within a wavelength or two, then an inverse square law after that. sending power wirelessly has been a dream for a long time, and there are devices that can charge up a cell phone battery when the transmitter and receiver are about 1/2 inch or less apart. the two devices have to use a large surface area pancake coil, so it's more of a transformer than a radio system. any further apart than that, and the amount of energy per square centimeter drops off rapidly, and even the transformer coupling factor is unusable.

By, "wireless power", are you talking about charging a cell phone sitting on top of a charging mat (transferring power 5mm) or are you talking about running a motor when the RF source is several meters or kilometers away?

I have looked all over the place but have not been able to find a definitive shopping list of what I would need to make an RF to DC circuit. Any help?
The first thing you need is an RF to power converter, otherwise known as an antenna. This converts the electromagnetic energy that hits it into AC power at the RF frequency. It is common to carry the energy away from the antenna using a transmission line, typically of 50 ohms characteristic impedance. There are many types, and coaxial transmission cable is very common. It is pretty easy to calculate the voltage or current you will have on this line, knowing the power. The more difficult thing to calculate is the amount of RF power that you will get from the antenna. You might apply the Friis equation if receiving from a distant transmitter. If the transmitter is not distant, you might have to estimate the amount of power based on near field coupling, and these equations you'll have to look up.

So, the next thing you need to do is to convert the AC to DC. This is most easily done using a rectifier circuit, either a half wave configuration, a full wave rectifier, or perhaps a bridge type. The configuration would be the same as you would see in a DC power supply. The type of diode you choose will depend on the RF frequency. After the rectifier you will need a smoothing filter, that is, a capacitor. That's the essence of what you need and you might find these functions in an IC that is designed for the task.

For the rectifier diode, perhaps consider an SMS7621-005LF. This part is well suited to RF use. The smoothing capacitor should be a ceramic type, of, oh, say about 1000pF.

By, "wireless power", are you talking about charging a cell phone sitting on top of a charging mat (transferring power 5mm) or are you talking about running a motor when the RF source is several meters or kilometers away?
Motor when the RF source is a few mm away

Motor when the RF source is a few mm away

What RC frequency and source? Be patient with us for the questions. We've seen people trying to collect FM or WiFi to power pumps to who knows what. Could you briefly describe your target setup. And specify if anything is fixed (e.g. RF frequency) vs open for optimization.

A few mm is really close. Have you had a look at how the Sonicare brand of wireless toothbrushes charge their batteries? They are wireless.

I think you might find that frequencies lower than RF might be more efficient.

Motor when the RF source is a few mm away

For very short distances like that, the principle is two tuned circuits on the same frequency plus a simple diode bridge rectifier.

The concept is no different to a mains transformer and rectifier, except the transformer coils are not physically attached to each other.
With tuned circuits, you likely need to use coil taps for the power in & out (rather than the full lengths of the coil) so you do not mess up the resonance.

For really short distances with fairly large cores they do not even need to be tuned, they can just work as an air-cored transformer.
The drive frequency to the "transmitter" part needs to be high enough to give good coupling.

If you are trying to pick up power from an existing transmitter (eg. a CB radio antenna), then you must use a tuned circuit made for the correct frequency, with a tapped output to the rectifier. Experiment with the tap position to get the best efficiency.

This is the principle of a tapped coil:

This is a basic transformer feeding a bridge rectifier, which converts AC to DC. For RF rather than mains, the diodes need to be high speed types.
https://www.electronics-notes.com/images/diode-full-wave-bridge-rectifier-with-capacitor-01.svg

There is more to it, but that gives some of the basics.

Another "wireless" system is just a bright light source and solar cells / panel to run the motor..

[Edit - spelling].

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Motor when the RF source is a few mm away
that doesn't have to be RF then. you will find few rectifier diodes that operate above 1 or 2 Mhz. if you look at how induction stovetops work, you will find they feed "pancake" coils (a flat spiral of wire with one connection in the center, and the other at the outer edge) with 40khz sine waves (it's RF, but well within the capabilities of most high speed rectifier diodes ([these] for example).

that doesn't have to be RF then. you will find few rectifier diodes that operate above 1 or 2 Mhz. if you look at how induction stovetops work, you will find they feed "pancake" coils (a flat spiral of wire with one connection in the center, and the other at the outer edge) with 40khz sine waves (it's RF, but well within the capabilities of most high speed rectifier diodes ([these] for example).

Thank you for the response...I'm not sure I understand what you're saying though...could you break it down for me a bit more?

How much power do you need to transfer? A few watts (under 5), several hundred watts? Or something in between?

Wireless chargers for mobile phones are typically 80k to 110kHz. They are normally 2 to 5 watts. Some now higher - up to 18 watts. The mid-power (5 to 18 watts ) chargers use up to 300kHz but most brands still around 100k for the more popular units even for the higher power - just heavier coils.

If you replace the pot with a coil, a decent near Kilowatt motor can be powered with an induction stove... But only at ~24KHz

If you replace the pot with a coil, a decent near Kilowatt motor can be powered with an induction stove... But only at ~24KHz
So is this an AC or DC system? Does it make a difference?

Thank you for the response...I'm not sure I understand what you're saying though...could you break it down for me a bit more?
right above this post, Externet posted a picture of what i was talking about. those are coils out of an induction stove. they are generally run at high power (up to about a kilowatt), and work by inducing high frequency AC currents in the bottom of a metal cooking pot. since the energy is transferred directly, the glass cook surface barely even gets warm (except where it's in contact with the cooking pot). a second coil placed above the first one would be able to pick up most of the energy from the coil without much loss (again the closer, the better). the coil receiving energy would use high speed rectifier diodes to convert the high frequency AC to DC.

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