Guys,
Listed below are all the replies I received on vacuum relays,
plus the web links.
Many thanks and 73,
Ken W2DTC
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I use a Kilovac (or Jennings) vacuum relay at moderately high speed CW to do
TR switching here. I think the only caution would be to avoid doing any hot
switching. Assuming cold switching, a relay such as the Kilovac HC-1 or
Jennings RJ1A I know from personal experience to work just fine. They have
no problem, assuming their vacuum is good, handling 1500W of RF. They do
cost about $35 each, however.
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Depends on the anode supply potential and on the RF circulating current
[Q x I-anode]. Surplus vacuum relays are more reasonably priced than new
ones.
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I ran a 3CX3000 amp that used HC2 Kilovac relays for bandswitching and had
absolutely no problems over a 5 year period. Good way to go but expensive,
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Solenoid Failures
The solenoid in a vacuum relay is simply an electromagnet. There are several
factors which can cause undesirable dynamic characteristics of any
electromagnet, notably (1) heating of the solenoid, (2) friction, and (3)
coercive force of the material used in the armature. I'll address each one of
these factors singularly, however, any or all may apply to a specific
electromagnet at any given time.
1. Heating effects: Heating of the solenoid and armature may be attributed
to the applied current or the operating ambient, or both. Heating has a
twofold effect on the electromagnet. (1) Heating causes an increase in the
solenoid resistance which results in a reduction of the ampere-turns when
a fixed voltage is used to activate the solenoid. (2) Heating also reduces
the permeability of the iron-core armature, causing a reduction in the induced
flux. Both these effects result in a reduction of the force produced to move
the
armature and when they are cumulative the net effect can become substantial.
2. Friction effects: Friction associated with the movement of the armature
may cause variable effects on the operation of the electromagnet. At the
start of travel of the armature the initial force is minimum while the
static forces are maximum. In a similar manner, when the solenoid current is
removed, the spring which is used to cause a restoring force to be applied to
the
armature, must overcome the static friction forces. In either case, the
armature may
fail to close fully (solenoid current applied) or may fail to open fully
(solenoid current removed).
3. Coercive force: Coercive force of the magnetic material used for the
plunger may cause sufficient "residual" magnetism to remain which causes
sticking when the solenoid current is removed. This effect is minimized in
the design of the armature by the use of "soft" magnetic materials and by
including a small air gap at the end-of-travel of the energized position of
the armature. The effects of coercive force generally can be overcome by
reversing the applied field. Some electromagnet/solenoid applications are
specifically designed to apply the field current in one direction (to close
the armature) and (momentarily) in the reverse direction to open the
armature.
Contact Failures
Relay manufactures design the contacts of their relays for specific
applications. Relays generally carry an application rating such as
'Make & Break Load Switching', No Load Switching', Make Only
Load Switching', etc. The selection of the proper relay for a specific
application is crucial to the life of the relay and it's performance in
the circuit. Relays rated as 'Make and Break Load Switching' depend
upon current flow through the BOTH the normally closed AND normally
open contacts in order to maintain a specified minimum
contact resistance over the specified life of the relay. Use of such a
rated relay in a circuit where no current flows through either the NC or
NO contacts can cause contact resistance to increase over time and the relay
will eventually
fail to provide circuit continuity through the contacts. This is sometimes
misinterpreted to be a sticking relay armature problem.
Use of such a relay in an QSK amp where "hot switching" is prevented by
circuit design but also no current flows through the NC contacts (on receive)
will
eventually cause these symptoms to occur, usually identified as "no signal
or agreatly attenuated signal on receive". Rapping the relay may cause the
symptom to disappear (for awhile) and is often interpreted as a sticking
solenoid
versus a contaminated NC contact. A better choice for such an application
would be
arelay whose contacts are rated as 'No Load Switching'.
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http://www.ciitech.com/brands.kilovac.tech.support.asp?sel_item_1=37
http://www.mgs4u.com/relay.htm
http://www.fairradio.com/vacure.htm
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