[Elecraft] K3 external qro keying line

[David Cutter]

Agreed.  It's good to bring out these things so we are aware of ALL the 
ramifications of doing things, you never know who's monitoring this channel.

There are some fast, small, non-vacuum relays that can be speeded up with a 
little circuitry.

73

David
G3UNA

----- Original Message ----- 
From: "Don Wilhelm" <w3fpr at embarqmail.com>
To: "David Cutter" <d.cutter at ntlworld.com>
Cc: <elecraft at mailman.qth.net>
Sent: Sunday, February 15, 2009 1:37 PM
Subject: Re: [Elecraft] K3 external qro keying line


> David,
>
> The back emf pulse from a relay occurs when the coil circuit is opened, 
> and its polarity is opposite that used to operate the relay.  A properly 
> connected diode should 'swallow' the back emf, and a capacitor in parallel 
> with the coil should reduce the amplitude of the pulse.
> Any delay introduced by the diode and capacitor will be on the trailing 
> edge and will not  normally introduce significant delay on the leading 
> edge.
>
> Anyone adopting this scheme would not be operating QSK, so any additional 
> delays introduced will not be significant.
> Those desiring QSK operation should consider fast relays (vacuum relays) 
> or solid state switching devices for their amplifier switching.
>
> 73,
> Don W3FPR
>
>
> David Cutter wrote:
>> That's a good thought and provides instant isolation, but have some 
>> thought about the back emf off the coil - it could exceed the rating of 
>> the keying transistor.  I measured 800V on a very fast scope many years 
>> ago from a reed relay.  Just sticking a diode across the coil will slow 
>> it down a lot, so, some experimentation with zeners and resistors will 
>> get the situation optimised.  I once used a separate keying transistor 
>> and relay and found that a small cap from the base to the collector did 
>> the job quite well by slowing the off rise time, so the transistor 
>> absorbed a bit more power but gave good control.
>>
>> 73
>>
>> David
>> G3UNA
>>
>>
>> ----- Original Message ----- 
>> From: "Don Wilhelm" <w3fpr at embarqmail.com>
>> To: <elecraft at mailman.qth.net>
>> Sent: Thursday, February 12, 2009 4:00 PM
>> Subject: Re: [Elecraft] K3 external qro keying line
>>
>>
>>
>>> Terry,
>>>
>>> For those who prefer not to touch the K3 with a soldering iron, an
>>> external 12 volt relay is a good solution.  Obtain the power from the
>>> K3  12VDC OUT jack and use KEYOUT  to key the relay.  It accomplishes
>>> the same thing and retains the ability to use QSK in the future without
>>> 'undoing' the internal relay.
>>>
>>> 73,
>>> Don W3FPR
>>>
>>> w8zn at comcast.net wrote:
>>>
>>>> I've seen numerous posts about the PTT output line on the K3 and using
>>>> older amps. Yaesu's, Kenwood's, Icom's all share the same issue.
>>>>
>>>>
>>>>
>>>> Unless you are running QSK and need instant keying, I would suggest
>>>> adding a small 12v relay inside the K3 that the solid state device
>>>> keys and presents the user with a set of dry, isolated contacts that
>>>> can handle positive, negative and up to 250v DC or AC. I have added
>>>> this to several K3's to protect the switching transistor from eventual
>>>> destruction. Once again, if you are using QSK, solid state due to it's
>>>> fast switching time is your only option.
>>>>
>>>>
>>>>
>>>> Yes, it means touching your K3 with a hot soldering iron, but it's a
>>>> "pay me now or pay me later" thing, sooner or later it will blow the
>>>> switch if you are hooking up older or HB amps that use other than low
>>>> current positive keying voltages.
>>>>
>>>>
>>>>
>>>> Terry
>>>>
>>>>
>>>>
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