Protection for RFID

[avlmaker]

I'm making a device that uses 64 small heaters on a wheel. The wheel turns and a handful of the heaters will be on at a time in a specific region. As heaters leave that region they turn off. As new ones enter that region they turn on. It occured to me that it could be useful to be able to identify individual heaters digitally. The assembly is on a slip ring. The wires running from the slip ring are used to power the heaters. It would be difficult to run additional wires from this spinning assembly so I started thinking about wireless options and it occured to me that the heater itself could be an antenna for an rfid chip. I don't necessarily need to identify heaters when they are energizes so it seemed like that might work. I modified the heater design to be more condusive to this idea, tested it out and low and behold it worked... well, sort of. I can read the number from the chip. The chip and load are in parallel so it works until you power the heater by any useful amount and then it blows the chip. I tried protecting the chip using pptc resettable fuses. That seems to help but if I suddenly apply anything above ~13Vac the chip is blown. If I ramp slowly I can go higher. I'd ideally like to safely go up to 18Vac. I'm wondering if there is a simple way to dampen the initial rush so that the pptc has time to restrict current before the RFID chip is affected. Or is there perhaps a better method altogether? Thank you for your consideration.

 

[kellys_eye]

A somewhat over complicated solution. Can't you put an index slot on the wheel and use it to start a counter and interrupt a 'beam' and simply count those that pass?

 

[avlmaker]

Thanks for writing, kellys_eye. I agree that it is a somewhat over complicated solution but that's okay. I am intrigued by the possibility of the heater also acting as an antenna. I enjoy the experimentation and creative challenge of trying to get it to work. Regarding your suggestion to put an index slot and start counting interrupts to a beam, I'm not sure how well that would work in this application. The wheel spins very slowly. ~3 rotations / hour. If I understand your suggestion properly, it might be 20 minutes before the index slot comes around.

 

[kellys_eye]

RFID is, by its nature, radiative so how would you determine position from a radiated signal that was 'everywhere'?

You would need to switch the RF on - and end up using (maybe) a slot technique to do the switching!

Without knowing how the wheel is actually drive I can't comment on alternative methods for position referencig.

 

[avlmaker]

The reader is good for a distance of perhaps an inch or less so that limits how many chips could possibly be activated at a time. If it could be limited to just one chip at a time then it could work. I have some ideas on that. It's a challenge I'm conveniently willing to ignore for the moment. Pretending that I'm just working with one heater/antenna, I'm particularly interested to know if there is a way that I might dampen the initial surge just long enough for the pptc fuses to kick in and protect the chip. Capacitors maybe?

 

[kellys_eye]

Are you actually powering the RFID chips from AC? They would usually be DC devices and require some regulation to keep within their operating parameters. What is the actual device being used?

 

[avlmaker]

When firing the heater I'm using AC controlled from a Variac.
When reading the chip I'm using an rfid reader from sparkfun attached to an arduino.



I am under the assumption that the reader induces a current via AC which powers and modulates. Perhaps the chip contains a rectifier? The chip I'm using is from a bag of rfid tags I got off of ebay. I simply clip away the existing antenna and then solder the chip to the circuit.



When I bring the rfid reader close to the heater/antenna circuit the reader reads the chip, strips out the id and sends it to the serial monitor so I can see it.

Thanks for engaging with me on this topic. I really appreciate it.

 

[kellys_eye]

You drawing shows the RFID device being powered from the same AC source as the heater...... it's supposed to get its power from the induced voltage from the RFID reader unit. The 'sender' chip will only require a very small voltage and the AC from the heater supply is likely just blowing the guts out of it!

You might be able to capacitor-couple the RFID sender device to the heater element/coil (replace the VDRs with capacitors) but this will require knowing the inductance of the heating element and the effect of placing series-capacitance in the circuit. You need to get the L (inductance) and C (capacitance) correct such that the final circuit element resonates at the RFID frequency.

 

[avlmaker]

Would it work if I replaced the VDRs with variable caps and adjust them until I find the sweet spot?

 

[kellys_eye]

Difficult to say. Without a ball park figure for inductance you would be lucky to get a variable capacitor that could cover the required span. In theory, yes. But much easier to do ballpark calculations using the approximate value for L.