# Capacitive Coupling circuit questions

#### Peaches41

Joined Dec 15, 2016
70
Thank you all for viewing my posting here in the forum,
I have included a circuit diagram here to show what my present thoughts are on this topic. I am looking to capacitive couple the signal from a spark plug wire from a motor car and use this to monitor only. I know there are other ways to do this but I want to use this method to prove my thoughts on building the circuit.
I am winding a solid core wire(.5mm thick) 'x' times( I have varied from 1 to 7 times) around the plug wire. This then goes into my circuit as shown.
I am using a led as the load to show that the circuit is functioning properly. I am wanting to have .01A current thru the led.
I do not know what the resistor R1 should be here. Do I calculate this value based on the voltage from the coil? I thought this would be the correct way to design this but my end result at test (distributor spin fixture) did not work as I used 3MEG for R1 assuming 30kv from coil. I should see the led on for approx. 470mS when using the values for R2 and the cap. R3 is calculated with a 3.1 drop and a Vf to the led of 2v.
I welcome your guidance in helping me discover how I can make this circuit work.

Thank you,
Peaches

#### wayneh

Joined Sep 9, 2010
17,498
Have you actually built and tested this? It feels overly complex. Do you want the LED to just flash or stay lit?

And this is not capacitively coupled, which is what I’d recommend if you just want flashes. It could be as simple as one capacitor and maybe a couple resistors. This design is more of a power harvesting design.

#### noweare

Joined Jun 30, 2017
115
Thats got to be some pretty dangerous voltages. How about using wound slug inductor that is oriented perpendicular to the wire it will pick up the field of the wire and produce much smaller voltage to work with. Right now you are actually increasing that voltage with multiple turns (depending on the coupling). From there you can use a voltage divider if necessary to reduce the voltage even more.

#### DickCappels

Joined Aug 21, 2008
10,230
That is capacitive coupling.

You will be unlikely to get more and a few tens or at most few hundreds of microamps on average unless you have a more substantial capacitance than the one indicated here.

I don't think the 1k resistor performs any function other than to keep the voltage from being high enough for the LED to glow

What you desire to accomplish is not clear. What is your eventual objective?

Once friends and I would run #22 solid wire from an array of neon lamp on the dashboard through the firewall and wrap one wire from each neon lamp around one of the spark plug leads. The common connection for the array of neon lamps was grounded to the dashboard. It looked very cool and high-tech.

#### wayneh

Joined Sep 9, 2010
17,498
That is capacitive coupling.
What is? The capacitor in the diagram is for storage, not coupling. The coupling to the LED is direct. It might work better without the capacitor (or R2). Actually it might work best with nothing more than the LED and the coil. But you could put a capacitor in series to limit the current surge from each pulse. Then the winding of the coil would be less critical as long as it's 'enough'.

#### crutschow

Joined Mar 14, 2008
34,689
• Change R1 to 1kΩ (or less
• Eliminate R2. It's just draining off power that could be used to light the LED.
• Change the zener to a 1N4148 (your are unlikely to get enough voltage to damage anything with such a large capacitor)
That is capacitive coupling.
The wires around the wire have both inductive and capacitive coupling but the inductive coupling is very weak with low current in the HV wire, while there is a very high voltage for capacitive coupling, so the capacitive coupling dominates.

#### WBahn

Joined Mar 31, 2012
30,243
I agree with what some others have remarked -- it's unclear what it is you are trying to accomplish. My first thought was that you wanted the LED to flash at the spark rate, but then you mentioned 470 ms for the expected on time of the LED. If this is connected to one of the spark plug wires on a four-cycle engine, then anything over about 250 rpm would result in the LED being on continuously. Is THAT what you want, instead?

#### wayneh

Joined Sep 9, 2010
17,498
The wires around the wire have both inductive and capacitive coupling but the inductive coupling is very weak with low current in the HV wire, while there is a very high voltage for capacitive coupling, so the capacitive coupling dominates.
Ah, got it. I was totally overlooking the sensor and focused only on the circuit.

FWIW, here's an LED flasher circuit I've used with a 9V supply. The input (upper left) comes from a 4017 counter running at ~1Hz. Bottom is ground. I wanted just a very short pulse to save power. The capacity of C4 determines the length of the flash, with R5 limiting the peak current. R17 drains the cap between pulses.

#### Peaches41

Joined Dec 15, 2016
70
Here was my original thought on this circuit:

Goal: Construct a circuit that will capacitance couple the HT spark plug wire to send a signal to a PIC microcontroller.

Before using a microcontroller I wanted to use an led to make sure my calculations were correct before moving on to adding the microcontroller and subtracting the R3 resistor and LED.

Steps taken:
1) As a reference, use 30,000 volts on HT wire
2) Use 5.1v zener and resistor R1 to step down voltage to 5.1 volts and .01A
3) Use 470uF cap, and 1000 ohm resistor (R2) as a time base of 470mS (using 1 rpm as time between spark plug firing) to trigger microcontroller. Time of 470uS is just a sample so I can see that the circuit functions as desired at 1 rpm.
4) R3 and LED are used as my load and to visually see that the circuit functioned as desired.
5) Diode 1N4148 is to block the discharge of the cap from going to the ht wire and only going to the led.
My thought was if this worked with the LED that I would remove R3 and the led, and add a schmitt trigger.

There may be a much simpler way to do this but this was my initial thought.

Is it easier to just use a voltage divider to step the voltage down to 5v and send that to the microcontroller?
What if the voltage from the HT varies due to the plug gap size and say 40KV is available how should I deal with that?

I am open to all suggestions, but really want the solution with the least components and least complexity.

I hope this clarifies what I was trying to do here.

I am trying to learn something new.

Thanks,
Peaches

#### wayneh

Joined Sep 9, 2010
17,498
Ah, that’s a horse of another color. The zener is a good idea in that case. I don’t think you’d need the big capacitor at all.

#### Sensacell

Joined Jun 19, 2012
3,475
The flaw here is your assumption of "0.01A"

This current can only come from the spark plug wire, via the very small (a few pF) capacitance of your pickup electrode.
If you could get 0.01 A flowing in that capacitor, it would mean that you had so much capacitance loading the spark plug wire that it would not have enough voltage to spark.

What you could achieve is a couple of microamps, hardly enough to illuminate an LED.
Neon lamps work because they are high impedance devices, a few microamps will make them glow, because it takes around 90 volts for them to light up. LED's are low impedance devices.

What would work is connecting an FET to your circuit, use that to switch an LED, powered from a battery.
The gate of an FET is a better impedance match for the circuit.

Ignition voltages are great for blowing up electronics, you would have to carefully design this to prevent the thing from getting blown up.

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#### DickCappels

Joined Aug 21, 2008
10,230
Resistive voltage dividers: resistors good for 40 KV are very large and they are expensive. Your capacitive pickup is a good silution.