PI shielding

[green]

Hi green,
seems me the TX_bipolar is not balanced respect to 0V.
Do you tried to swap the scope probes?

Including schematic of bipolar Tx. Used components in spice, circuit components similar but not same number, used what I had. M1 and M4 turn on for one polarity, M2 and M3 for the other polarity. When M1 and M4 turn off (a)goes to M1 avalanche volts(580V), (b)goes to about 1.2 volts below -battery and coil decays to zero volts across Tx coil. When M2 and M3 turn off (b)goes to M2 avalanche volts(580V), (a)goes to about 1.2 volts below -battery and coil decays to zero volts across Tx coil. Both scope channels decay to the same voltage(zero coil volts)between 200 and 300 volts. Thinking the M1 and M2 Coss might be different is the reason the two polarities don't decay to the same voltage. Not sure and don't know if it matters. Rx signal isn't the same both polarities and I wonder why.

[Tinkerer]

Hi Tinkerer,
it could try with a dehumidifier in a close room in order to have a repeatable conditions of temperature and relative humidity; it's just an idea.

Altair, thank you for the feedback.

Yes, a dehumidifier works for lowering the relative humidity.
Under normal circumstances I don't like a lot of static in my lab, so I use a humidifier to boost humidity to about 40 - 45%. Sometimes I have to evaporate one gallon of water in 24 hours to maintain this level.

[Tinkerer]

Hi Tinkerer
Good thread. I have wondered why the hand can be detected. Have no idea why. Tried some things this morning. Could get the PVC pipe to pickup a small piece of paper but couldn't get it to light the led with scope ground connected to circuit common or not connected(led lights when signal exceeds threshold). With a 6us sample delay, led lights if scope ground lead is connected to circuit common when I move my hand near the coil. Doesn't if scope ground lead isn't connected. With a 10us sample delay, doesn't light with scope ground lead connected or not. At times have thought hand is acting as a target but when Rx signal is the same with hand over either figure 8 coil(should be opposite if it's a target)I'm thinking it's not a target.

Hi Mr. Green,

Did you try what it does if you ground one hand on scope ground or analog ground or battery or power ground while approaching the other hand to the coil?
Is your coil shielded? What kind of shielding? connected where?

I suspect that the hand has a very short TC.

[Tinkerer]

Including schematic of bipolar Tx. Used components in spice, circuit components similar but not same number, used what I had. M1 and M4 turn on for one polarity, M2 and M3 for the other polarity. When M1 and M4 turn off (a)goes to M1 avalanche volts(580V), (b)goes to about 1.2 volts below -battery and coil decays to zero volts across Tx coil. When M2 and M3 turn off (b)goes to M2 avalanche volts(580V), (a)goes to about 1.2 volts below -battery and coil decays to zero volts across Tx coil. Both scope channels decay to the same voltage(zero coil volts)between 200 and 300 volts. Thinking the M1 and M2 Coss might be different is the reason the two polarities don't decay to the same voltage. Not sure and don't know if it matters. Rx signal isn't the same both polarities and I wonder why.

It took me a long time to find the reason why the 2 channels are of different amplitude. I will try to write and post my take on it.

[green]

Hi Mr. Green,

Did you try what it does if you ground one hand on scope ground or analog ground or battery or power ground while approaching the other hand to the coil?
Is your coil shielded? What kind of shielding? connected where?

I suspect that the hand has a very short TC.

You are correct. Touching circuit ground with one hand and taking other hand near the coil acts the same as connecting circuit ground to scope ground and taking hand near the coil.

Coil is shielded with graphite paint. Including picture of the 133mm fig8. I strip insulation off a piece of #24 gauge stranded wire(7strands of #32)and place it across one side of coil and then paint that side. Repeat other side. Haven't been painting the edge. Seems to work but maybe there is a better way?

Still wonder if hand has a TC.

[Tinkerer]

You are correct. Touching circuit ground with one hand and taking other hand near the coil acts the same as connecting circuit ground to scope ground and taking hand near the coil.

Coil is shielded with graphite paint. Including picture of the 133mm fig8. I strip insulation off a piece of #24 gauge stranded wire(7strands of #32)and place it across one side of coil and then paint that side. Repeat other side. Haven't been painting the edge. Seems to work but maybe there is a better way?

Still wonder if hand has a TC.

This picture is worth a thousand words.


So, what can I say about the shielding?


Disclaimer: Before I get into a lengthy diatribe, I want to make clear that what I say may be unscientific, irrelevant or even completely wrong. I make many mistakes every day. Quite often I say something and the next day I find myself wondering: "Why did I say that? That seems to be wrong".
So, if something does not seem right, don't hesitate to question it. Don't hesitate to question my statements. Ask for clarification and show me where I am wrong. I am grateful for it.


Back to the picture.


Looks like a pancake wound coil assembly. This produces less interwire capacitance. Good point for high self resonant frequency SRF.
The shielding is relatively low conductive, but any shielding current has a short path to the drain wires. Good
With the pancake wind, there is a relatively large shielding surface. This is mitigated by more distance between most of the shielding and the coil wires.
Capacitance is very much influenced by distance. This calculator is very useful for understanding the role that distance plays with capacitance, inductance and dielectric. http://www.mogami.com/e/cad/electrical.html


Insulated, tinned, fine stranded wire works good. The type of dielectric as well as the thickness of the insulation play an important role for SRF.
Air is a good dielectric, but we do not want to leave open air spacing between the TX coil wires, because with strong current pulses, the wires move with the pulses and produce HF noise.
I try to keep a minimum of 1/8 of an inch or 3mm of distance between the coil wires and the shield, more is better. PVC foam works fine for the flat surface. For interwire spacing I use PE fishing line and glass micro balloons set in low viscosity Epoxy when using solid magnet wire.
Enough for today.
Next time we may talk about the cable. The cable plays an important role for getting the plus and minus signals of a bipolar system equal.

[green]

Disclaimer: Before I get into a lengthy diatribe, I want to make clear that what I say may be unscientific, irrelevant or even completely wrong. I make many mistakes every day. Quite often I say something and the next day I find myself wondering: "Why did I say that? That seems to be wrong".
So, if something does not seem right, don't hesitate to question it. Don't hesitate to question my statements. Ask for clarification and show me where I am wrong. I am grateful for it.

I would hope everyone would feel the same way. I know I do, I'm just learning and make mistakes all the time. There is a lot of good information on the forum and some bad. I know I'm responsible for some of the bad, hopefully some of the good.
One thing worse than being wrong is not knowing it.

[green]

How to measure resistance of painted graphite shield? Is it better to measure resistance from the ground wire to different places on the coil or point to point resistance on the coil(distance between the probes?)? How much variation should I expect?

[6666]

Next time we may talk about the cable. The cable plays an important role for getting the plus and minus signals of a bipolar system equal.

The bipolar info sounds interesting, thanks.

[Altair]

I would hope everyone would feel the same way. I know I do, I'm just learning and make mistakes all the time. There is a lot of good information on the forum and some bad. I know I'm responsible for some of the bad, hopefully some of the good.
One thing worse than being wrong is not knowing it.

Do mistake is the way to learn! We are humans, me for first (to do mistakes)!

[green]

Does graphite shield act as a target? If I painted a 2, 4 and 8 inch square piece of poster board with graphite paint could they be detected? Would adding another layer of paint make it easier? Still trying to understand why bringing the hand near the coil causes a response.

[Qiaozhi]

Still trying to understand why bringing the hand near the coil causes a response.

It's a capacitive effect, which is what the electrostatic shield is intended to block. A Faraday shield stops false signals due to changes in the ground to coil capacitance, and also attenuates external electromagnetic interference. Of course, the shield itself must not be detectable as a target.

[green]

It's a capacitive effect, which is what the electrostatic shield is intended to block. A Faraday shield stops false signals due to changes in the ground to coil capacitance, and also attenuates external electromagnetic interference. Of course, the shield itself must not be detectable as a target.

Thanks.

Have a sheet of poster board 10 inches wide painted with graphite paint that I have used for shielding. Don't know why I never thought of recording TC. Tried this morning, recorded with target at each end of figure8 coil to see if showed difference(maybe measurement error). Charted same decay slope(.7us TC). Think I'll try a 4 and 8 inch square, maybe painted 1, 2 and 3 times to see if more paintings increases TC to learn something. Anyone have other suggestions?

[green]

If anyone is interested in how decay chart reply #28 was done. I recorded a no target, target end A, target end B and another no target with a DSO. Copied data to Excel and charted.

[Tinkerer]

How to measure resistance of painted graphite shield? Is it better to measure resistance from the ground wire to different places on the coil or point to point resistance on the coil(distance between the probes?)? How much variation should I expect?

If I understood right, The correct way to measure is "per square". For example: 2 each, good contact areas of 1" length, separated by 1", should give the resistance per square (inch).
The difficulty is to have a good contact on the surface of a graphite paint.

When we embed 2 drain wires into the graphite coating we do not necessarily have a perfect contact area, but over a certain length of wire, using stranded wire, we achieve the best we can.