I'm not quite following this, Bryan. Unless I've dropped a factor somewhere
(always a possibility!) or misunderstood the circuit (also a possibility), a
.02uF capacitance charged to 120V has a stored charge of only 2.4
microcoulombs, and a stored energy of only 144 microjoules. If the
capacitance discharges in 1 uS, an implausibly short time IMHO, the peak
current pulse is 2.4A, but is only 1 uS long. When one considers the
inductance of the interconnecting cable and the residual resistance of the
cable and relay contacts, I'd estimate the actual peak current is likely to
be only about 10% of that, or 200-300mA
More to the point, however, the tiny amount of energy stored in the .02 uF
capacitance seems to me to be less than what would be required to weld
relay contacts together. The current pulse only occurs when the relay is
closing, and there is no metal plasma formed when the contacts close.
Furthermore, there is no arc formed when the contacts open, because the .02
uF capacitance is fully discharged then. I'm not doubting that a transceiver
keying relay can be damaged when the amp relay draws too much current at too
high a voltage, but I'd guess the damage might be the result of an inductive
kick when the contacts open caused by the relay coil inductance rather than
energy stored in the .02 capacitance.
73,
Jim W8ZR
> -----Original Message-----
> From: Amps [mailto:amps-bounces@contesting.com] On Behalf Of Bryan
Swadener
> Sent: Saturday, October 19, 2013 1:17 PM
> To: amps@contesting.com
> Subject: Re: [Amps] Keying Kenwood TL922 with Yaesu FT1000D (both old
guys)
>
> Jean,
>
> The TL922 T/R control has the same problem that the
> Heathkit SB-220 has... whatever drives the "RL CONT"
> input must discharge 0.02 uF (C27 + C61). While the
> two relay coils' DC resistance permits only a small
> amount of current to flow, the instantaneous
> discharge current of C27+C61 results in a HUGE
> current spike of several Amperes. THAT is what
> welds tiny 1A driving relay contacts and shorts most
> driver transistor junctions. Rich AG6K has suggested
> using a small resistance in series to limit discharge
> current. According to the FT1000 user manual on
> page 67, it can be optioned to drive up to
> 220 VDC @ 300 mA.
>
> On the other hand, if you want something that is
> FAST (QSK), look at what Jim W7RY offers.
>
> vy 73,
> Bryan WA7PRC
>
>
>
> ________________________________
>
> Date: Fri, 18 Oct 2013 23:14:20 +0200
> From: Jean ON4AEF
> Subject: [Amps] Keying Kenwood TL922 with Yaesu FT1000D (both old guys)
>
> Hi guys;
>
> Anyone could me put in the right way for next coming contest.
> Witch connection plug I best use on the back of the FT1000D for keying
this old TL922.
> I saw that the Tl922 keying system has been modified so it could be keyed
by means of a
> low power transistor.
> The keynig system of the FT1000D could not be damage by this setup !
> I useual do not handle with amplifiers in my ham carreer so poor
expirience around hi..
>
> Kindly regards and thanks to all I look forward
>
> Jean ON4AEF.
>
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