[R-390] killer and mica caps

[David Wise]

I've used the LND150 N-channel depletion-mode MOSFET as a current limiter before.  It's darn handy - and almost the only one of its kind.  It can block 500V, Idss is a couple mA, and it's very fast.



The impedance seen by the filter and its trim cap is the parallel combination of pentode V501's plate resistance and choke L505's impedance.  Both are high, so the additional kilohm or so of Rds(on) is insignificant.



The filter input coil is low-resistance, so tacking a diode across will not save it.



The R648/URR works around the problem by elevating the filter input coil to B+.  I don't like it.



Regards,

Dave Wise

________________________________
From: Larry H <dinlarh at att.net>
Sent: Thursday, October 13, 2016 5:47 PM
To: David Wise
Subject: Re: [R-390] killer and mica caps

Hi Dave,  This certainly has possibilities. I had no idea that a high voltage N-channel depletion mode (normally-on) transistor DMOS FET existed with this speed.  I'm not sure what the 1k ohm resistance would do to the signal path.

I was just thinking that since the filter inputs are grounded, a diode clamp might work.

Regards, Larry

________________________________
From: David Wise <David_Wise at Phoenix.com>
To: Larry H <dinlarh at att.net>
Sent: Thursday, October 13, 2016 2:08 PM
Subject: Re: [R-390] killer and mica caps

How about an LND150 current limiter?  Acts instantly.

Dave Wise
________________________________________

From: R-390 <r-390-bounces at mailman.qth.net<mailto:r-390-bounces at mailman.qth.net>> on behalf of Larry H <dinlarh at att.net<mailto:dinlarh at att.net>>
Sent: Monday, October 10, 2016 8:53 PM
To: k6fsb at arrl.net<mailto:k6fsb at arrl.net>; R-390 Forum
Subject: Re: [R-390] killer and mica caps

Sorry Renee, There are no fuses that I know of that would open before the filter would be damaged.

Chris, no need to hide, this is a good learning tool.

I also wanted to define what I meant by test the cap before installing.  Apply the rated voltage (not higher than) and look for leakage with a sensitive amp meter (around 50 micro amps full scale) in series with a 10k ohm resistor.  If there is any current flowing at all, do not use it for C553.

As for measuring the distortion, in this situation, you would need to be able to measure the IMD of the IF unit.  This takes special equipment, but in any case, this is a sensitive cap for minimizing IMD.

Clearing 'self healing' event possibilities can not be done today, as was pointed out, they normally occur because of contamination (and other issues) in the manufacturing process, and it takes a varying amount of time for insulation penetration/breakdown to occur.  However, you could test them as the manufacture does, with a recording scope monitoring for current spikes while measuring for leakage for an extended time.  If you apply twice the rated voltage and you see no leakage, that does not mean that an event will not occur in a few months or a couple years.

I am all for finding a longer life cap for C553.  Does anyone know of any tests done by independent testers?  I'm not too keen on believing this kind of data provided by a manufacturer.

Regards, Larry
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