Relay Interface System for HF Amplifiers



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Relay Cleaning

Relay transfer Timing


This page deals with the entire system,
including radio RF output.

In radio systems, especially when multiple
devices are connected, the sequence of events is critical. Some radios have inherent RF output to external relay
control timing issues. The result of improper timing is often a very wide
leading edge “pop” or “click”. The pop or click repeats at
every relay closure. Slow VOX or semi-break in causes less frequent pops than
QSK or fast VOX operation.

Required RF and relay Sequence

The system sequence is:

  1. Key closure, PTT line, or voice start (VOX) at radio
  2. Unwanted delays in CW keying, PTT, VOX, and radio circuits
  3. Amplifier relay or accessory (like front-end saver) signal activation
  4. Desired RF hold-off delay in the radio that is slightly greater
    than longest possible relay pull-in delay for all switched devices
  5. RF finally appears
  6. Desired transmission is stopped by operator. Signal, keying, or voice stops
  7. RF stops (must include fall time of RF envelope)
  8. Desired delay for complete signal fall to zero power before
    external relays and system
    release and revert to receive mode

Some radios are terrible for RF sequencing.
There are some radios that actually output RF

telling external devices RF is going to appear. There are other radios that
continue to output RF after they tell external devices to go back to receive.
This is a radio design problem! It is bad radio circuit engineering.


Time Delays

Most small to medium relays for amplifier switching require 5-15 milliseconds
to transfer and stop contact “bouncing”,
with small enclosed and vacuum relays running around 5mS. Typical larger open-frame
relays are in the 10-20mS time range.

  • Relay closing time delay increases greatly as
    voltage across the coil is reduced. Decrease in speed can be
    time/3 as minimum operating voltage is approached. This
    means a 5 mS closing time transfer might increase to 15 mS or more if relay
    coil voltage is low.


  • Relay opening time is delayed by excessively high
    relay holding current and addition of backpulse-canceling diodes. The slowing
    is often as much as 200%, but can be even more. Maximum relay speed requires
    a design that eliminates backpulse-canceling diodes. The diodes can be
    eliminated without danger or harm if we are careful with layout and

PIN Diode switches can close and open in a few milliseconds or less.

Some very small reed relays rival PIN diodes in time. There are some low-power reed relays that operate in less than 1mS!

Common Errors

The most common errors are:

  • Lack of high operating voltage across the relay coil
    on relay pull-in. This is caused by excessive voltage drop in switching components or
    low relay supply voltage
  • Use of relays to buffer or isolate amplifiers
  • Use of accessories that add time delay to relay

Measuring Time Delays

Measurement of time delay requires a dual channel oscilloscope with separate
trigger input, and a low frequency signal generator.

I normally use this keying test setup:

  • The level of the generator controls weight or duty cycle (ratio of
    on-to-off time).
  • The trigger is to the scope external trigger. The level threshold of
    trigger sets scope sweep starting time.
  • The scope probes can be moved to any points to compare timing or sequence
  • A pull down on the transistor base is not necessary with modern devices. 




What to Watch

The radio must have enough RF output delay AFTER the
radio low-amplifier control signal appears so the amplifier relay does not
truncate leading edge of RF

The radio or amplifier must hold the relay long enough
so the falling edge is not truncated.

Problem Areas

  • Any device added between the control line output
    of the radio and the amplifier
  • Any device that switches on the output of the
  • Front end savers and amplifier relay buffers are
    a particular problem, often causing excessive control line delay times

W8JI Control Line Interface

Some systems have multiple control lines from different devices that must be
“keyed” in parallel. With direct parallel connections, one device can
load the system and stop other devices from operating properly. This usually
happens when a device is off but still connected. Simple isolation diodes work
to prevent that, but the voltage and current at the radio is the highest voltage
of any device and current is the sum of all the device currents. Even worse, the
radio output control has to discharge the RF bypass capacitors of each connected

My 160 station consists of a solid-state homebrew amplifier with 16
MRF-150’s, an AL1200, an 8877 amplifier and either an FT1000D or an Orion. I
almost never listen through the FT1000 or Orion when working CW, so I also have
at least two outboard receivers that must be muted. 

My homebrew 8877 amplifier uses a high-voltage relay system, the solid state
amplifier has 5-volt TTL level switching, the TX antenna sampling line uses a
28-volt vacuum relay, and the receivers have 5 volt muting lines that mute when
pulled to ground. The Ten-Tec Orion requires an isolation relay on the receive
antenna port, and I occasionally switch other devices when transmitting.

All of this is driven by a microprocessor controlled antenna switching system
that prevents changing antenna directions or antenna while transmitting. 

Any attempt to directly parallel devices would be a

The circuit below interfaces my radio to multiple amplifiers, receivers, and antenna
control systems. Multiple devices can be connected in parallel, such as 28 volt
relay control lines and 5 volt control lines, as long as they are all positive
voltage. This system pulls within 1 volt of ground.   

It is necessary to be careful with some radios. There is no standard
interface voltage. TenTec, for example, uses interface systems that have a
threshold around 1 volt. TenTec’s handshake lines require buffering with a small very fast reed
relay, many transistors and IC’s will not pull low enough to activate the TT handshake
line.  If you need relays of any type between the amplifier keying system
and the radio, always be sure you test the entire system for proper relay sequencing.   

RadioAmp goes to radio’s amplifier keying control output.

EXTcon goes to computer or other keying device that only handles low-voltage

D2-D6 go to devices that require

+5V sets the threshold of EXTcon trigger. EXTcon open circuit voltage is also
limited to less than 5 volts. 

The radio is exposed to the highest voltage from the keyed loads, but the
current is limited to roughly V/500 where V is highest voltage device.  A
28 volt amplifier would require the radio to key about 14mA peak current during
relay transition, although average current would be lower. The exact current
depends on beta of Q2 and Q4, and if EXTcon is used. A device like Q6 can be
added to the radio line to limit radio voltage to 5 volts or less regardless of
amplifier relay voltages.