Don & John,
When a relay is de-energized the voltage developed from the collapse of the magnetic field can exceed the source voltage by many times.
In the past it was the convention to put a reverse polarity diode across the relay coil to absorb the energy when the relay is de-energized.
Since Elecraft dose not use the reverse polarity diodes in any of their kits I assumed this function was built into the relay driver chips. If not, perhaps a higher voltage rating for C13 and other capacitors like it is called for? Or could the relay driver U2 be defective?
73,
Bill - K6WLM
Date: Fri, 03 Apr 2009 10:29:59 -0400
From: Don Wilhelm <
[hidden email]>
Subject: Re: [Elecraft] K2: KPA 100 Capacitor Specification?
To:
[hidden email]
Cc:
[hidden email]
Message-ID: <
[hidden email]>
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John,
I have never found one of those caps shorted, and I believe the DC
rating on them is 50 volts, so they should have an adequate safety
margin since they are used in a 12 volt DC circuit.
I have trouble imagining a condition that would cause them to fail, but
then I recall that your 'shipboard' operation is unusual and may
introduce 'perils' that I cannot imagine.
There would be no harm in replacing those capacitors with 100 volt caps
*if* you can get them to fit physically. You may have to solder them
flat against the bottom of the board. The other problem is that 100
volt caps may be difficult to procure with 0.1 inch lead spacing and you
may have to get creative to substitute caps with 0.2 inch lead spacing.
73,
Don W3FPR
[hidden email] wrote:
> Felow Elecrafters,
> I have just replaced the second-to-fail .01uF cap on the KPA/100 low band
> filter relay circuitry of #5087. In this case, C13. Failure is a partial
> short which allows that relay pair (k3 and k4) to close even though U2 hasn't so
> ordered. Symptom that this has happened is inability to tune down SWR and
> power runaway. This would make sense because in this condition - particularly
> this one - since I don't have a 160 meter setup - the low band filters are
> triggered for 160 plus whatever other band I thought I was going to work, two
> filters for the price of one - not a bargain.
>
> This radio has suffered some hard-knocks before I understood how to deal
> with ESD and I feel that I owe
> Elecraft far more than they owe me, But.........
>
> Should I replace the relay coil caps with ones with better specs? If so,
> what should I use?
>
> 73 AI4TO John Ferguson M/V Arcadian
>
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