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I would like to know if a faraday shield is used on a coil for a P.I. detector? And if so what is the method used for putting one on a coil and material used? Thanks Tom
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Re: Faraday Shield
Tom,
I noticed there hasn't been a reply to your question so I will try to help a little. I asked a similar question of Eric Foster on the following forum;
http://www.findmall.com/metal/school1/config.pl?#1519
I have taken the liberty to copy his response and post it here for you."...
a Faraday shield is a must, otherwise false signals will result from capacitive effects with the ground. A paint shield is good, fine pitch ribbon cable is better as is 0.1mm lead tape."
I don't know how often he monitors this site, so you might want to ask the question again as to the best method of application on the other site also.
I hope this helps.
Reg -
Re: Faraday Shield
Thanks Reg, I'm still not sure what type of paint or ribbon or lead tape exactly and if it shares a common connection with the coil windings? Thanks TomComment
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Re: Faraday Shield
Hi Tom,
Just seen your question. The lead tape is 1 inch wide and comes with an adhesive backing. It can be wound helically round the coil winding leaving a small gap between start and finish, so as not to get a shorting ring. A wire is soldered to one end of the tape and this in turn connected to the braid (ground) of the coax. You have to be quick in soldering the wire to the lead as it melts very quickly and will leave you with a hole. I only know of one source for the tape in the UK and it is quite expensive per roll, and heavy for shipping. If you just want to make one or two coils, email me with coil diameters and I can give you a price for a sufficient length.
Eric.Comment
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Re: Faraday Shield
Tom,
Eric's technique is the best. As for the conductive paint, I have used Television Tube Coat by GC electronics. A coating is sprayed on inside of the coil housing (top and bottom),one end of a thin stranded bare wire is then glued (using conductive glue) or taped to each of the coated housings and the other attached to the shield as Eric mentioned or if you are not using coax, to the lead that goes to the power supply. There should be a little spacing between the windings and the coating for best results. One technique for spacing is to get some rubber hose like that used on vacuum lines on a car, split it and put small sections around the windings for spacers. The conductive coating has been a common technique used by several of the VLF manufacturers.
RegComment
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Re: Faraday Shield
Hi Reg and Tom,
One of the side effects of the lead tape, when wound directly on the coil, is that it adds a fair bit of capacitance to the coil circuit. Depending on how short a sample delay you are using, sometimes this is a problem - sometimes not. To reduce this capacitance, I have used a helical polythene wire wrap over the winding and then the lead tape on top of this. Any residual overshoot can be compensated for, by using small non-ferrous washers, chosen to have the same time constant as the overshoot. These are cemented to the coil former adjacent to the coil. By careful positioning, you can end up with a nice flat response.
Eric.Comment
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Re: Faraday Shield
Thanks Reg for the information, I'll give them a try. I'm sorry I didn't get back sooner , the end of school is near and I'm covered up with work and kids events. Thanks TomComment
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