Applying hall effect sensor or mic as frequency meter

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Hello

I want to measure frequency of impulses of the current passing during the discharge of the capacitor. Basically I thought about two options:

a) detection by microphone (spark gap makes noise)
b) Hall effect sensor

Now question is which solution would be better to my design and how to apply it? What do I need?

Hall effect sensor + low pass filter + impulse width expander ?

How should I apply hall effect sensor? What use as impulse width expander? I want to make from impulses square wave so that AVR can see signal and can adjust IGBT triggering signal width to charge capacitor appropriately to desired discharge frequency.

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Why not just measure the voltage across the test element? After all, Ohm's law will tell you the current from the voltage and the resistance.

Jim

 

Until Black Lives Matter, we do not have "All Lives Matter"!

 

 

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c) capacitive pick up as used in numerous car tachometer designs.
After simple conditioning pulse can be fed to AVR for further processing.
Mic will require amplifier - limiter. Not sure about Hall's ability to react to fast discharges.
Nevertheless readings will show the same frequency as per discharge switch control.
Secondary (and primary, too) ringing should exhibit quite short aperiodic current oscillations. The frequency, indeed, will not vary significantly.

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Quote:
Why not just measure the voltage across the test element? After all, Ohm's law will tell you the current from the voltage and the resistance.

Voltage is about 10kV and impulse current can be up to 20kA which will give a solid drop on tested element.

Capacitive pick up is interesting option but I don`t think so it will manage to register impulses up to 2000 discharges per second? I would doubt it.

Somehow I feel Hall`s sensors are way to go but impulses here are short so I don`t think so AVR would react so quickly to register them. I saw few solution basing on 555 timer to expand impulse but maybe you have better circuit designs or ideas.

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Processor reaction should not be considered as limiting factor for 2000 pulses per second.
Hall sensor shall work OK at these frequencies, too. Will take arranging good magnetic coupling. An output from digital sensor can fire at least few different interrupts or be polled. No need for an additional interfacing circuitry, though.
Capacitive pick up is very similar yet employs induction principle; all the above applies, too, in certain extents.
Since processor controls discharge, readings from the sensor shall return the same firing frequency, methinks indeed.

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At 20kA just wind few turns of wire around the main conductor and you have a current transformer. Put a resistor as load, and with a resistor divider connect it to AVR.
George.

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A photo sensor might work. It might be possible to use a blue or white LED as a sensor.

It all starts with a mental vision.

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Quote:
A photo sensor might work. It might be possible to use a blue or white LED as a sensor.

Then you will have to deal with the ambient light.
George.

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I didn`t think about it yesterday but instead of measuring voltage across the tested element I can measure it on short length of wire so in this way voltage should not be high. It should work ok but impulses would be fast few nanoseconds I think.
I don`t think AVR will manage to register impulses of few nanoseconds. So how would you extend time of impulses so I could use them i.e. to trigger external interrupt.

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Ifryd wrote:
I don`t think AVR will manage to register impulses of few nanoseconds. So how would you extend time of impulses so I could use them i.e. to trigger external interrupt.

Maybe use a One Shot. Something like a 74VHC123 will trigger on a 5 ns pulse.

Greg

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I read this post again, and like Jim's idea. It might be easier to measure if the spark is referenced to ground not Vcc. I would be concerned with transients on Vcc in the present configuration.

The spark duration for a conventional ignition coil with a typical spark plug gap of ~0.8mm, is 1 to 2 mS.

It all starts with a mental vision.

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Quote:
Maybe use a One Shot. Something like a 74VHC123 will trigger on a 5 ns pulse.

Yes I think it is a way to go...

Quote:
The spark duration for a conventional ignition coil with a typical spark plug gap of ~0.8mm, is 1 to 2 mS.

Yes as long it is single ignition coil without capacitor...