RF to DC

[zemanekj]

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.

So the AC current running through those coils produces an electromagnetic field which in turn can be captured by high speed rectifier diodes? Thank you again, you are really helping me understand this.

 

[Reloadron]

I put this in your other thread but now see it was locked without a ghost. So I'll repost it here:

I see you have a few threads going and I want to think this came up recently here or in another forum. You may want to do some research on the term "rectenna" or "rectifying antenna". The idea being tiny diode arrays are built directly into the antenna. Doing this helps eliminate loss. I can't recall who it was but a major US company did some experiments using highly directional microwave transmissions at very high power to a rectenna receiving dish about I think a mile away. The concept was being tossed around for use in space. The merit to microwave transmission is the beam can be made highly directional.

As you see in your other thread low frequency stuff exist for cell phone charging and hell even my electric toothbrush circuits. A Google of "rectenna video" will get you some interesting stuff but the video I recall was out in I believe the southwestern US desert. I just can't remember who it was.

Ron

 

[rjenkinsgb]

So the AC current running through those coils produces an electromagnetic field which in turn can be captured by high speed rectifier diodes?

No...
The magnetic field from the "sending" coil can induce AC voltage in another _coil_

The two coils work as a transformer, but are not physically attached to the same core - just in close proximity.
http://en.wikipedia.org/wiki/Transformer


Rectifiers - diodes - are like "one way valves" are with liquids - they only allow current flow in one direction.

Four diodes suitably connected route the AC input so you get the same polarity output for either polarity of input.
http://en.wikipedia.org/wiki/Diode_bridge

From an AC input, you get unsmoothed DC out the rectifier; the output voltage drops off as the AC reduces and changes polarity, then increases again during the next half cycle.

A capacitor across the rectifier output "smooths" the variations and gives a steadier voltage.

A rectifier is just a type of diode intended for use in power circuits, rather than for small signals like audio or logic.
Bigger, in other words, rather than being fundamentally different.

eg. These are all single diodes, the one at the left is a "small signal" one, the rest are rectifier types of various current ratings.
http://ae01.alicdn.com/kf/HTB1EbLwm...7-1N5819-1N5399-1N5408-1N5822-FR107-FR207.jpg

You can also get rectifiers ready-connected in the "bridge" configuration, but normally they are only for low frequencies and not suitable for your project. Examples, purely for information:
**broken link removed**

 

[unclejed613]

So the AC current running through those coils produces an electromagnetic field which in turn can be captured by high speed rectifier diodes? Thank you again, you are really helping me understand this.

you need a second coil as a "receiver" (called the "secondary" of a transformer), and the diodes are connected to the secondary coil to change the high frequency AC into DC. the amount of voltage and current available on the secondary side depends on how much voltage and current are used to drive the primary side. the power (in watts) is calculated as E*I (voltage times current), and will be nearly equal when compared between primary and secondary. i said nearly equal, because there is always a slight amount of loss. if you drive the primary side with 10 volts at 10 amps at 40khz, and the primary and secondary have the same number of turns of wire, and the primary and secondary are really well coupled magenetically, you will be able to get 10 volts at 9 amps out of the secondary (assuming efficiency at 90%... you will never get 100% or more, it will always be less than 100%) to drive the load. you can change the ratio of the turns of wire between the primary and secondary to get more voltage (add secondary turns) or less voltage (subtract secondary turns) and the current available from the secondary will be inversely proportional to the voltage. so you could get 0.9 volts at 100 amperes with a turns ratio of 10:1 or 100 volts at 0.9 amperes with a ratio of 1:10.

 

[zemanekj]

you need a second coil as a "receiver" (called the "secondary" of a transformer), and the diodes are connected to the secondary coil to change the high frequency AC into DC. the amount of voltage and current available on the secondary side depends on how much voltage and current are used to drive the primary side. the power (in watts) is calculated as E*I (voltage times current), and will be nearly equal when compared between primary and secondary. i said nearly equal, because there is always a slight amount of loss. if you drive the primary side with 10 volts at 10 amps at 40khz, and the primary and secondary have the same number of turns of wire, and the primary and secondary are really well coupled magenetically, you will be able to get 10 volts at 9 amps out of the secondary (assuming efficiency at 90%... you will never get 100% or more, it will always be less than 100%) to drive the load. you can change the ratio of the turns of wire between the primary and secondary to get more voltage (add secondary turns) or less voltage (subtract secondary turns) and the current available from the secondary will be inversely proportional to the voltage. so you could get 0.9 volts at 100 amperes with a turns ratio of 10:1 or 100 volts at 0.9 amperes with a ratio of 1:10.

Awesome thank you!! So by connected rectifying diodes I can convert the current from AD to DC. Thank you that clarifies things. And I just need a coil of wire (the secondary) situated above the the oven coil (primary) and not connected to the primary - and then connect diodes to the secondary and boom there we go?

 

[Externet]

Yes, boom there you go.

The left coil is the 'RF driven coil' from the induction cooker; the right one is the 'RF inducted coil' ; the dots are the air gap coupling space between them; rectifiers make RF into pulsing DC; posterior filtering can smooth waveform.
The ratio of coil turns sets the secondary voltage.

 

[Pommie]

Why does the term "boom there we go" seem so appropriate here?

Mike.

 

[shortbus=]

Why does the term "boom there we go" seem so appropriate here?

Mike.

Or the ever popular, "here hold my beer".

 

[unclejed613]

Or the ever popular, "here hold my beer".

that one is sure to require an ambulance.... almost as bad as "hey.... watch this....." at the rifle range.....

 

[gophert]

"hey.... watch this....." at the rifle range.....

In the cajun to English dictionary, "hey, watch this" translates to "hey, get ready to call 911"