zebra50

Potting JFETs?
« on: May 17, 2011, 04:11:43 AM »
Hi!

Before I go and find out the hard way, are there any issues I should be aware of when potting JFETs?

I have a little high impedance circuit that I would like to hide from prying eyes, but I'm a little concerned about leakage. I wondered if there are special compounds for this?

Cheers!

Stewart
Ribbon microphone services
http://www.xaudia.com
Microphone blog


ruairioflaherty

Re: Potting JFETs?
« Reply #1 on: May 17, 2011, 05:31:33 AM »
Hey Stewart,

It might be worth sending a mail to Fred Forssell.  He's a nice bloke, helpful to the DIY community and has been potting jfet modules for many years.

Cheers,
Ruairi


shabtek

Re: Potting JFETs?
« Reply #2 on: May 17, 2011, 07:09:29 AM »
there are more than a few potting compounds too.can you sand off the part numbers or add some useless extra parts--gotta give the next generation something to keep 'em out of trouble
"really fine players do not use stomp boxes or master volume, they match the amp to the room and turn it up to 11.  Stevie Ray, BB King, Albert King, Duane Allman, Dicky Betts, Louis Armstrong"
   -CJ

RuudNL

Re: Potting JFETs?
« Reply #3 on: May 17, 2011, 08:09:10 AM »
I once used a spraycan with black paint to cover (part of) the PCB.
That will teach them to steal my design!  ;D
There is a solution for every problem!

http://www.vansteenisaudio.nl

mikeyB

Re: Potting JFETs?
« Reply #4 on: May 19, 2011, 07:28:29 PM »
Hi Stewart - long ago used  pcb track cleaning block - this is like a rubber/sand composite - found it ideal for removing identifiers from dips and transistors and cap values too
 ;)
making better music!

zebra50

Re: Potting JFETs?
« Reply #5 on: May 20, 2011, 03:22:10 AM »
Thanks guys for all the ideas & suggestions.

I actually potted one up yesterday with 'normal' epoxy. The circuit still works fine, so looks like I'm OK. The epoxy also helps secure some of the wiring, so I think I'm good to go.

Cheers!

Stewart
Ribbon microphone services
http://www.xaudia.com
Microphone blog

ruairioflaherty

Re: Potting JFETs?
« Reply #6 on: May 20, 2011, 03:27:04 AM »
Stewart,

I think a problem with potting is the propensity for the circuit to cook itself.  If this is a circuit that dissipates heat it might be worth doing a few stress tests before you release to market.  Didn't John Hardy have issues in the distant past with some potting compound or other (not on his current product?

Of course if it's a low wattage mic circuit or something you'll be just fine.

Cheers,
Ruairi

Re: Potting JFETs?
« Reply #7 on: May 20, 2011, 07:18:20 PM »
Potting compounds usually have a higher expansion rate as things heat up, compared to the expansion rate of electronic components. If an epoxy is hard enough, and expands at a greater rate than the component that is embedded in it, it can pull the component apart. Way back in 1981 I had a rash of failures with the 990 when I was potting it in a clear epoxy. One or another of the eleven 1N914B diodes would become open-circuit as the module heated up and things expanded. I changed diode suppliers and I also changed the heat-curing approach, using a lower curing temperature for a longer time period to build up less stress. This solved the problem.

Shortly after I began manufacturing the "A" and "C" versions of the 990 in an aluminum potting shell around 1988, I began using a soft silicone encapsulant. It has an expansion rate that is even higher than the typical epoxies, but it is so much softer that it does not build up the stress that a hard epoxy can do.

Encapsulants have a specification known as "dielectric constant". If you fill the space between components with an encapsulant instead of air, it can change the performance. This is because the capacitance between adjacent component leads will increase if the dielectric constant is higher. Some circuits will not be significantly affected by the change, but others will be.

Encapsulants can help with heat transfer, depending on the thermal conductivity of the encapsulant. They can make the assembly stronger and more reliable. They can hide things.

John Hardy
The John Hardy Co.
www.johnhardyco.com
« Last Edit: May 20, 2011, 07:51:59 PM by John Hardy »

ruairioflaherty

Re: Potting JFETs?
« Reply #8 on: May 20, 2011, 07:32:55 PM »
Excellent info John thanks.

Re: Potting JFETs?
« Reply #9 on: May 20, 2011, 07:41:29 PM »
Speaking rather unscientifically and empirically, from experience (and not including Forssell or Hardy opamps!) reading Mr Hardy's post makes me wonder if potting issues could explain the terrible reputation some opamps have with regards to reliability? Not mentioning names, of course... A friend of mine has a certain console and it seems to need a new opamp every month... A certain OEM I was involved with potted all their gain blocks and continued to do so, despite frightening failure rates (like about 50% of everything that left the factory!)... This was a huge reason why I chose to disassociate myself with said OEM.

If you're talking discrete parts, the issue you'll have with just scratching off component printing is that someone can desolder the part and run it through one of those danged component analysers. It won't tell you the exact part number, but you'll be able to narrow it down to a handful of parts with similar Hfe / Vbe etc. Not an authentic 'rip-off', but potentially near enough to most people's ears if they trace the whole circuit and can make it stable. One might dare to suggest that some of the opamps available from boutique OEMs could have their origins in reverse-engineered famous designs...

Justin  
« Last Edit: May 20, 2011, 07:43:46 PM by thermionic »
Prepare yourself. You are about to become the voice of Interplanetary Parliament.


zebra50

Re: Potting JFETs?
« Reply #10 on: May 21, 2011, 03:35:58 AM »
Some great info & thoughts! Thanks!

In this case it is a simple little P48 mic circuit, with just a few discretes. Heat is not a problem, but the dielectric issue may well be - I'm still a little concerned about parasitic capacitance and other weirdness from the potting.
Ribbon microphone services
http://www.xaudia.com
Microphone blog

Gus

Re: Potting JFETs?
« Reply #11 on: May 21, 2011, 12:05:18 PM »
You are going to hide a microphone circuit?

zebra50

Re: Potting JFETs?
« Reply #12 on: May 21, 2011, 12:29:42 PM »
Hi Gus,

It's an experimental circuit for a ribbon mic. I'm sending it off for evaluation, and for various reasons would prefer to keep it hidden at the moment. It is not super-high-Z like a condenser mic.

I may be barking up the wrong tree.
Ribbon microphone services
http://www.xaudia.com
Microphone blog

Gus

Re: Potting JFETs?
« Reply #13 on: May 21, 2011, 01:32:50 PM »
I would maybe solder a closed metal box around the circuit and mark the solder and/or use the stickers that show if the seal has been broken

As others have posted epoxy potting can have issues


zebra50

Re: Potting JFETs?
« Reply #14 on: May 21, 2011, 03:39:20 PM »
Gus, that's a great idea. It would also provide additional shielding, which would be welcome.
Ribbon microphone services
http://www.xaudia.com
Microphone blog


 

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