[Icom] PRO II relay (hot-switching)

Wed, 07 May 2003 16:52:04 -0700

Hi Greg,

Many thanks for the kind comments about the Icom FAQ site. By raising the
hot-switching issue, you have brought up a very valid point. The best way to
minimise the risk of hot-switching on keying is to use a high-speed relay,
such as a reed, or a solid-state buffer circuit, with current/voltage
ratings sufficiently high to handle the amplifier's keying line.

That said, transceiver ALC action during the brief open interval whilst the
amplifier's input relay is switching from receive to transmit may minimise
the risk of damage to the  input relay contacts - assuming (1) that the PA
is fitted with a reflectometer which reports excess reflected power to the
ALC subsystem, and (2) that the ALC attack time is much shorter than the
relay operating time.

Best 73,
Adam, VA7OJ/AB4OJ
North Vancouver, BC, Canada
http://www.qsl.net/ab4oj/
Icom FAQ site:
http://www.qsl.net/icom/

-----Original Message-----
From: [email protected] [mailto:[email protected]]On
Behalf Of Greg Ordy
Sent: Tuesday, May 06, 2003 19:30
To: [email protected]
Subject: Re: [Icom] PRO II relay

>>> Please visit the following link:

>>> http://www.qsl.net/icom/auxrelay.html

>>> The 756Pro II keying relay is rated at 16V 0.5A. The 12V 100mA spec of
the
>>> AL-80A keying line is within these ratings, but it is prudent to use an
>>> auxiliary keying circuit (relay or solid-state buffer).

    As usual, Adam's page contains a comprehensive discussion
of the issue, including a range of DIY and commercial solutions. Good stuff.

    Let me bring up the "dark side" of using an external interface, which is
"hot switching" the amplifier. This occurs when RF is present on the
relay contacts as they are opening or closing. This is a good way to
prematurely wear out the relay (contacts).

    We can assume that the radio designer created an internal T/R
sequence that does not "hot switch" within the radio itself.
The sequence will first close any relay, then start pumping RF,
and the RF will be removed before the relay returns to the "receive"
mode. All of these events are taking place within a few milliseconds.

    Since external amps are popular, the designer must factor in the
possibility of an external amp into the T/R timing. This is usually done by
adding
a few milliseconds to the delay between the keying relay closing and
the start of the RF.

    Any delay present in an external interface directly reduces the time
interval between the radio relay closure and the start of the RF (at the amp
input).
At the extreme, the device added to protect the radio does so, at the
expense of
damaging the amplifier.

    If you had a lot of spare time, you could measure the radio timing,
the interface timing, and the amplifier timing, and see if there is a
problem.
My own rule of thumb, which has come from some personal experience,
and discussions with others, is that the external interface should
not add more than 5 milliseconds of delay to the timing chain.

    What that says to me is that interfaces that use transistors or
reed relays will work, without problems. Some of the larger, open-frame,
higher power relays can have switching times in the 8 to 15 millisecond
range, and this might very well create a  hot switching problem.

    A CW DIT at 60 WPM is 22.5 milliseconds long. There is
not a lot of switching  time to mess with, if you want to produce a product
that
claims to operate full break-in (QSK) at 60 WPM. By the way, the
high-speed CW ops I know say that none of the popular radios get
the timing "right", so they resort to external keyers where they can
skew the element timing so that they can compensate for the
timing distortion present in the radio. This is at fast CW rates.

    In my own case, I drive an Ameritron AL-1200 without an external
interface
from a 756PRO. I measured the open circuit amplifier keying input
voltage at 11.6 volts, and the closed circuit current at 116 mA.

    In the case of the PROII and the AL-80A, I would not use an interface,
but I might double check the actual voltage and current to make sure
that they are as specified. If they are within the specs presented on
Adam's page, why add another layer of circuitry?

    As a final step, you could check the keying circuit in the amp.
If it directly energizes a relay coil, there should be a reversed diode
over the coil to prevent a voltage spike when the field collapses in
the relay. This spike could exceed the relay rating in the radio.
Any amp without that minimal protection is simply a poor design.

    All of the Ameritron amps I know of that use relays have that
diode. For the last two years or so, most of the larger Ameritron
amps have switched to a new relay circuit that replaces
a triple-pole, double-throw relay with a pair of relays driven by
a transistor circuit. When you "key" these particular amps you
are pulling a transistor emitter to ground.

    Greg Ordy, W8WWV, [email protected]

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