Building a RFID-Zapper from the scratch

Question

I would like to build an RFID-Zapper from the scratch (not based on a camera flash).

Since it is working with a high voltage, I prefer to ask first.

Does this look OK to you?

The capacitor is 330µF, handling 300V, the source is 150VDC and the coil is a self-wound coil, measuring 4.5 x 8 cm, insulated copper wire, 1mm thick, 5 windings.

Question

1. Can I plug the source directly at 150V to the capacitor or should I increase it slowly?
2. How can I know the time of discharge of the capacitor?
3. Will the coil generate a strong enough EMP to destroy the transponder?
4. Would it be possible for a second version of the RFID-Zapper to charge this HV capacitor with a low voltage battery?

Answer 1

• VOLTAGES USED IN THIS CONCEPT ARE LETHAL.

• ZAPPERS CAN KILL THE EXPERIMENTER.

• EVEN A SMALL CAMERA FLASH CAN KILL AND

• ADDING LARGER CAPACITORS AND/OR HIGHER VOLTAGES CAN KILL YOU EVEN MORE! (Being dead once is more than enough)

It's doubtful whether this query well matched to the aims of this forum.

The schema shown shows the general principle but nothing more. It's OK if that is its aim.

The cited web page is not technically competent.

The statement "Although we doubt that it has the capacity to cause any trouble aboard an airplane" is an alarming one given the intended aim of the equipment is to produce "a strong shock of energy comparable with an EMP". If taking such equipment onboard an aircraft resulted in the carrier being arrested nobody should be too surprised.

Specific questions:

1. This shows a basic lack of understanding of what you are dealing with. Direct connection of an HV cap to a supply will place a substantial load on the supply. The supply needs to be designed o deal with the inrush current or the inrush current needs to be designed o be limited. "Just doing it" is liable to cause damage.

2. It's not obvious why knowing discharge time is important in this context, but while designing, an oscilloscope works wonders. A voltmeter may be useful but far less so. If the circuit has high L & C and little R it may oscillate for a long time.

3. Can't tell. Read article you cite. Efficacy decreases with square of distance at short ranges. With cube of distance at longer ranges. Some RFID tags are liable to be destroyed at short range. As range increases it depends on how effective your coils is and coupling between coils.

4. A battery powered high voltage source is what is provided by a camera flash - so, yes, obviously. This is called in general terms, a boost converter.

It would be relatively easy to make a tag which was immune to such a zapper.

It would be easy to make a device which detected when such a unit was being used.

IANAL. Operation of such a device may make you liable for destruction of property charges. Generally speaking the cost of being found guilty of such a charge is disproportionately high compared to cost of damage done.

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Attack defense:

If I told me then you'd have to kill me :-).

But, as long as you know the sort of attack you have to face you can always defend against it. The only issue is all up system cost - at some point you detect and use a Phalanx gun* on the offenders as a cheaper option :-). (* no - that noise is not a buzz saw). In real life a Phalanx liketargeting camera could identify zappers and other "persons of interest).

For starters, tags are alwayss resonant to increase volage and thus range and power transfer capability. (See Microchip AN710). Q factor (effective voltage multiplication factor) is typically in the range of 50 to 100. It would be simple and low cost for a tag to detect gross overrvoltage and to de-Q the circuit for a selected period. Just placing a short across the inductor will do this very nicely. You now have a straight transformer with Vout down by a factor of say 50 times. As power = V^2/R you may need as much as 2500 times the power level to get this back to as it was before. But maybe only 50x depending on various factors. This de-Qing would be cheap and easy to implement in an RFID transceiver or RX IC and if zapping became common you could expect this to appear as of right. If this is not enough you can consider getting creative with your inductor such that if voltage across half of it gets above some target level then the two halves are switched into anti-phase and cancel each other out. This is nicish as it means you are not trying to sink large amounts of energy and the high voltage levels appear at the coil and not at the IC.

The switches in such a system could be something like those in a Marx generator where they break down under over-voltage and act as switch elements and stay conducting until current drops below a certain level. This could be implemented with a few transistors and resistors forming an SCR or TRIAC structure so would be cheap easy and low area to implement. It wouldn't happen overnight but if the threat was concerted it would, or something similar. Or a Phalanx gun :-).

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Marx generator working - the arcs al the way up the middle are switches! formed by an airgap designed to flashover. Replace this with an SCR and you get the same result.

Dont try this at home - use a camera instead.

Answer 2

If such a beast would work, and I'm not going to tell you how to make it work, it would also likely destroy Cell Phones, bluetooth stuff, Credit Cards w/RFID, WiFi gear, Radios, over-the-air TV, and mostly anything wireless. Also, anything with cables and/or no shielding. Oh, and don't forget the RFID's that are being used to track lost dogs.

It could also effect the magnetic stripe on a normal credit card if it were close enough.

Basically, what you're trying to do is generate a very broad-band Electro-Magnetic-Pulse. This is quite non-discriminatory and can have other unintended victims.

If you used one of these things near me, I would certainly consider legal action.

The ONLY valid reason to have such a device is for "protection of privacy". The problem is that unless you also carry around an RFID reader you will never know if your device worked properly. It could also be that the effect on the RFID tag is temporary, and the tag could start working again. So, in a pragmatic sense, it doesn't even assure privacy! Therefore, it's questionable at best to have this device at all.

If privacy is what you want, then go for some shielding (a bag or wallet). That will work much more reliably than an RFID-Zapper and won't land you in hot water.

Answer 3

The basic premise looks fine to me.

You will need to current limit the source to a sensible initial current. This is because when you first connect a source across a capacitor it looks like a short circuit.

So if you have a 150V source then using say a 15 kohm resistor would limit the current to 10mA initially. The initial power dissipation would be 1.5W, so a 2W resistor would do. If you're not so sure of what you are doing, I would advise deriving the high voltage from a current limited, low voltage source like a small battery to lessen the risk of nasty things happening if you get it wrong. A camera flash circuit as they used would be one option, the other would be to make your own boost converter run from a few AAs.
If you used a 330uF capacitor and the 15k resistor then it would take about 15 seconds to charge up. You can work out how long it takes to charge to 90% of source using 2.3 * R * C (e.g. 330e-6 * 15e3 * 2.3 = 11.385 seconds to 90%)
I would make the LC combination resonant at whatever RFID frequency the tag uses.

Obviously think carefully about possible consequences of building this, and if you do be very careful how you use it, probably best to keep it at home and use away from anything remotely sensitive. If you were using to "de-RFID" clothes or similar as is suggested in the link, you could maybe even make a shielded box to put the item in and zap. Done correctly this would greatly lessen the risk of you getting into trouble.