HV protection for receiver input?

I find it hard to believe that a zapper would take out a car alarm. The energy from the ignition and sparkplugs would produce as much energy. Have you tested a zapper to see if that's really what's happening. I would think that the alarm was tested for ESD. Please Comment?

In article , Jerry G. wrote: [.... neon lamp ...]

I disagree with adding the resistor. Just find a way to live with the extra 5pF. The resistor in series with it will cause losses. The 5pF will cause a mistuning that may either be unimportant or easy to correct for.

Antenna coax is what? 75 pF per foot? I doubt the 5 pF will make any detectable difference.

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Hell, put a zener diode into the gate of a Triac and use crowbar overvoltage protection! When a high enough disturbance makes the zener conduct the triac will short the whole thing right to ground/frame until the charge is gone then release.

But slow acting.....

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Not sure about the Triac but zeners are the fastest and the gate of the Triac should only act like a diode until the whole thing turns on and backs it up.

Zeners are usually used >

Right. Triacs/SCRs are probably in the microsecond range. But the zener should accept the initial surge, and possibly all of it! :)

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This bumper business sound unlikely to me. But there's a prototype device being tested by the police here that shoots a little wire-guided cart under the wheels of any car that refuses to stop and zaps the EMU by some form of EMP, thereby bringing it to a halt.

You should be able to put a parallel pair of low capacitance Silicon diodes across the receiver input. since the receiver is looking for microvolts of signal and not 600 millivolts. You also use an RF choke to ground. This will provide a low dc impedance to ground and a high rf impedance at the frequency of operation.

Mark

In article , snipped-for-privacy@mindspring.com mentioned...

I had a cheap scanner that used semi's on the input. The first lightning bolt many miles away zapped them. Got out the soldering iron and replaced. Same thing again, later. Pi$$ poor design.

Don't waste your time. Use an inductor that has nearly zero ohms DC resistance, or your front end will suffer the same fate.

And the inductor is used how? In series it will block the RF. In parallel, it will still look like a high impedance for the fast rise time spike of a zap.

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Good point, Sam. So what's the definitive answer?

Anti-parallell diodes are a good answer, but there has to be something before it to limit the current.

Maybe an impedance-corrected pad, or a series resonant circuit, an antenna transformer..

I have seen schemes for switching antennas for radio amateurs, between different bands, between receiving and transmitting.. They give the diodes a forward voltage to turn it on or a back voltage to turn it off (low capacitance).

There has to be some useful tricks these radio amateurs use because they don't seem to have problems with broken diodes.

It just needs some more consideration than just putting two diodes directly at the input.

Well, the scanner has three front ends, UHF, and hi and low VHF. The UHF and hi VHF have inductors, and use FETs, and they aren't damaged by lightning. But they are coupled to the antenna coax with small 5 or 10 pF caps, so their inductors don't protect the lo VHF. The lo VHF uses no inductor and a BJT, MPS-H10, and it burned out with every lightning strike. That seems to me to indicate that the coils conduct most of the fault current to ground. I suppose the coils do let some of the high freqs thru, but not enough to do damage.

Hams do the same thing with their tried and true homebrew equipment. They put some DC path to ground in the incoming coax. This also conducts to ground AC current picked up by the antenna.

I think you mean the so-called "braid break."

1.5KE30CA (1500W 30V bidirectional Tranzorb). Couple of bucks each. General Semiconductor.

In article , dave- snipped-for-privacy@mailblocks.com says... | I am trying to fix a car alarm/remote locking unit. It seems that punks are | putting personal protection "zappers" to the bumpers of cars and frying the | receiver input. | | The input is very simple: length of wire serves as antenna, connected to | input of wide bandwidth transistor driving tuned circuit. | | Transistor is fried. How can I protect this from several thousand volts (I'm | guessing here; what's the typical input from one of these zappers?)? Are | air-gap devices recommended? |

It sounds a little unlikely that the input could be damaged by putting a zapper to the car bumper, more likely the zapper is placed close to the actual antenna wire.

To protect this, you could connect the end of the wire to a small florescent tube, with the other end connected to ground. A neon warning lamp similarly connected, might also be worth trying.

lasoo? ...hell, a net