[Re: Ldi/dt of an inductor]

Because that's what inductors do: they try to keep the current constant.

Think of a flywheel: difficult to get spinning and hard to stop once spinning.

An inductor stores energy in the form of a magnetic field which takes energy to build and releases energy when it collapses.

When there's no current flowing through the coil and its energised, energy is needed to build the field. If the current is suddenly interrupted, the field collapses which induces a high voltage spike which forces the current to carry on flowing by arcing over the switch contact.

[Re: Relay]

What's an SPS relay?

Single Pole Single What? Throw, I'd guess?

No conversion is necessary, just only use one of the poles or better, connect them in parallel to increase relay life.

[Re: power surge kill power supply/motherboard]

The noise is nothing to do with the high voltage protection, it is generated by the high frequency switching transistor in the supply and the mains.

Extra filtering will reduce the noise but there's no way to eliminate it completely.

[AC Clamp on current meter to measure dc current??]

It's not possible.

Cheap AC clamp meters use a current transformer principle which only works on AC currents, for DC you need a hall effect sensor. A DC clamp meter will cost more but  that's life. ;)

[Re: need help asa]

Thay may have been breaking the law too. It could have been a pirate station.

[Are these 2 caps the same?]

The more expensive one is rated to 100V.

The cheaper one is probably rated to 50V but it doesn't say.

[Re: coaxial cable and GHz frequency]

What's the point of the 10k resistor?

It's in parallel with 1R so it's not going to make any difference.

He needs something to trigger the alarm, he could configure the other half of the op-amp as a comparator with a small piezo buzzer connected to the output so it makes a noise when the LNB is stolen.

Watch the current consumption of the buzzer though, the LM358 can only source 10mA and loading the power supply too much is probably a bad idea. Luckily you can get <10mA piezos but they're not that loud. If you want a very loud siren to sound when it's stolen, you need an external power supply whether this be a battery or a mains power supply (wall wart).

[Remote control vibrating alert bracelet]

A wireless doorbell?

Is he deaf or is it just that he can't understand you?

If he can't hear then you could add a motor with a weight on the shaft to the wireless doorbell to make it vibrate as well as chime.

[Re: coaxial cable and GHz frequency]

That won't work, you need to measure the DC current flowing though the co-ax, not just the voltage.

You need to break the shield and wire the resistor in series. The break and the resistor need to be as small as possible to avoid ruining the properties of the transmission line.

I'd recommend a low inductance thin metal film surface mount resistor. I'd also recommend adding a filter capacitor and possibly ferrite beads to the input of the amplifier, op-amps aren't designed to work at these sorts of frequencies and might do funny things when exposed to an 18GHz signal.