HV protection for receiver input?

[DaveC]

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?

Example p/n's would be appreciated.

Thanks,

 

[Mark]

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?

 

[Ken Smith]

In your case you want to use it as a hard voltage clamp. Put a resistor
of about 27 to 33 ohms in series with the lamp, to allow isolation from
its capacitive characteristics. The stray capacitance of the lamp is
about 3 to 5 pF.

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.

 

[Sam Goldwasser]

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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[Mike Berger]

How do you think this would work? The tasers have two electrodes, with the
power passing from one to the other. It seems that it would be difficult to
press
one on a bumper and zap something without grounding the charge before it
hits anything electronic.

 

[Gym Bob]

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.

 

[Sam Goldwasser]

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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[Gym Bob]

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 in parallel with MOV's for the speed, beating the
MOV, of course.

 

[Sam Goldwasser]

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 in parallel with MOV's for the speed, beating the
MOV, of course.

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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[Paul Burridge]

Someone "zapping" your bumper is NOT going to affect any electronics
on your car. They would have to zap the PCB assembly directly.

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.

 

[Mark Robinson]

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

 

[Watson A.Name - 'Watt Sun']

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.

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.

Mark

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[Sam Goldwasser]

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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[Paul Burridge]

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.

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

 

[Roger Johansson]

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.

 

[Watson A.Name - 'Watt Sun']

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.

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.


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Just when you thought you had all this figured out, the gov't
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[Paul Burridge]

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."

 

[Tom L]

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

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

 

[Harry Bloomfield]

| 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.

--
Regards,
Harry (M1BYT)...

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[Gym Bob]

lasoo? ...hell, a net