Relay Timing System for HF Amplifers
Related pages at
The text below covers electronic circuit to
sequence and speed separate relays (like dual vacuum relays) and describes how
to sequence mechanically ganged contacts like those in open frame relays.
This page deals with the actual amplifier
Relay Timing and Sequencing
All energy available from the operating output device in an RF power
amplifier has to go somewhere!
Amplifiers must have a load connected to the output anytime drive is
applied to the input. Without a load, the amplifier can be damaged by arcing or
excessive internal voltages or currents. If there is no antenna connected, the energy builds up in the tank
circuit or other energy storage system in the amplifier until something
absorbs the excess energy that would have made it to the antenna.
In cases of unstable amplifiers, such as those using
non-neutralized tubes with high internal feed-through capacitance (i.e. 572B’s
or 811A’s), the output relay must be closed before any idling current is
applied. (See stability) To
ensure safe operation, the amplifier has to be brought “on line” in a specific sequence
in comparison to RF and bias.
The normal safe timeline of engagement (turn on) relay system sequencing is:
- The operator “keys” the radio (zero time)
- Amplifier output contact fully transfers (let’s call this time “a”,
and it is around 1-10mS depending on relay type)
- The amplifier input contact fully or partially transfers (time
“a” plus 1-3mS=time “b”)
- Amplifier operating bias or cathode return path contact closes, applying
normal operating bias (time “b” plus 0-2mS=time
- Radio output delay, RF appears at the output of the amplifier (time
“c” plus 0-10mS)
This entire sequence can take up to 15mS with large open-frame relays.
The release sequence must be:
- Transmitter ceases output of RF
- Bias drops
- Input relay drops
- Output relay drops
The time sequence looks like this:
This window can tighten or expand in time, but the order has to
remain the same sequence.
If this process is not followed the relay, bandswitch, capacitors, or other tank
components can be damaged from intermittent arcing!
Even though all radios should have a transmitter inhibit delay, many radio
designs do not include such delays. The radio output delay primarily prevents
shutting down of the radio during leading edges while relays transfer, and
annoying broadband clicks or spits at the start of VOX closure or each transmitting cycle.
Some radios have incorporated delays, but incorrectly applied the delays. For
example, some early Kenwood radios actually delayed the turn-on and turn-off of
amplifier relays, forcing amplifiers to “hot-switch”. Such radios are
often disastrous to amplifier component life.
Increasing Relay Speed with proper sequencing/ Dual Relay System
Relays can be made faster by operating them from a higher-than-normal supply
voltage, and using external current limiting to provide a constant current at
the relay’s rating. For a single relay, omit everything associated with RL2. R3
becomes a jumper.
A two-relay circuit would look like this:
R1 1k 1-watt
R2 1k 1/4-watt
R3 100-ohm 1/4-watt
R4 found by the following, where Irl1 is the antenna relay current:
Power rating of R4 is:
where I is RL1 current and R
is R4 resistance.
C2 input time delay cap, normally 10-50uFd 10v
D2 3-4V small zener (.25w-1w)
Q1,Q2 NPN power transistor. Vceo rating must exceed Vcc supply voltage, plus
safety margin. Imax must exceed relay current ten times
or more. Watch dissipation! Use hfe above 40 for best current stability.
- Vcc supply must exceed RL1 relay voltage by 2-3 times.
approximately equal RL2 rated voltage. This voltage must be obtained from a
reasonably stable supply voltage +-20% regulation no load to full load. Do not use a large dropping resistor, or you
will unintentionally speed up the input relay RL2.
R2 sets current through RL1 (antenna relay). The adjustment range of R2
is from zero current to a maximum where only RL1 resistance limits relay
current. R2 must be adjusted for rated current through RL1.
R3 should be set for proper
delay of RL2 (input relay).
D1 adds delay to release of RL1 (antenna relay). If release of antenna is too slow, add small
value of series resistance until RL1 opens just after RL2 opens.
The above circuit has the following features:
- Control V open circuit will never exceed +12V
- No back-pulse or transient is generated
- Delays are easily tailored to any relay
- Extremely high or dangerous voltages are not present.
- Speed is faster than systems using dropping resistors for speedup
- Relay speed is maximized without exceeding rated relay current
How It Works
Relay pull-in is always slowed by the inductance of the armature coils. Since
RL1 is fed by a voltage higher than normal, current will reach pull-in strength
much sooner than with normal supply voltages. Q1 is a constant current source,
adjusted by R2. This circuit prevents RL1 from being overheated, or having
excessive steady-state operating current. Q1 also limits the voltage appearing
After RL1 (antenna relay) current reaches a value that allows contact
transfer, Q2 will conduct. This applies current to RL2, the input relay. It
pulls in last, time delay is set by R4, R3, and C2.
Upon release of Control_TX, RL2 releases with only internal relay delays. RL1
is held on by the current loop through D1 as the field tries to collapse. This
causes RL1 to have a longer release time than RL2, so the antenna remains
No back-pulse voltages make it to the exciter, because of the circuit
This is a simple, safe, adjustable relay system.
Mechanically Ganged Relay Contacts
Large multiple-contact open-frame relays using a common armature can be
mechanically sequenced by slightly bending each contact carrier, the associated
normally open contact, or both.
By carefully pushing on the armature center:
- The output contact should very obviously close first
- The input contact should close second
- The cathode or bias contact should close last, but nearly at the same time
as the input contact
1.) Reference the pictorial above, use needle nose pliers to bend the unused
normally OPEN contact at the amplifier tank circuit connection UPWARDS. Do this
by positioning one jaw on the top of the bakelite frame and the other under the
outer tip of “A” and squeezing until “A” is just slightly
bent upwards towards the moving contact. DO THIS
ONLY ON THE OUTPUT CONTACT! This will force the TANK CIRCUIT of
the amplifier to connect to the antenna BEFORE the input circuit
connects. This change prevents arcs in tank components
caused by improper relay timing.
2.) Locate the unused normally closed contact for bias switching. Using the
same technique above, bend “B” (the normally closed contact) upwards
until B no longer contacts the moving contact. This
change puts 1/3 more contact pressure on the amplifier bypass contacts, and
decreases receiver dropout when the amplifier is on
When properly modified, you should see the output contact
make slightly before any other contact on the relay when the armature is
manually closed. You should see the UNUSED bias or cathode switching normally
closed contact have a slight gap when the relay is NOT energized.
Separate relays offer a unique problem. First, some relays can actually be
dangerous to use.
NEVER USE REED RELAYS, UNLESS YOU ARE ABSOLUTELY
NEVER USE COMBINATIONS OF SINGLE THROW
DUAL REED RELAYS OR SEPARATE SINGLE-THROW RELAYS
Relays must have an absolutely fail-proof positive transfer between open and
closed poles. The normally closed poles should also be series-connected so the
path is through both normally-closed contacts. This will prevent any possibility
of accidental connection between amplifier output and input in the event of a
relay or component failure.