[Amps] B- switching.

[craxd <craxd1@ezwv.com>]

Angel,
You don't really need a vacuum relay, I used (model# 
EC12-NC/NO/DT-12-0-5-BD) from a company named Ross Engineering. They 
make open frame relays with a switching capability of  10Kv @ 450 mA on 
their smallest size. This will de rate to 5Kv @ 900 mA, or 2.5 Kv @ 1.8 
amperes. There's also sealed reed relays available from Ross with a 
switching ability of 5-15Kv @ 3 amperes. The first relay mentioned was 
used to switch the B- . You can use vacuum relays if you want to pay the 
cost though. The only time the primary would be opened is with a HV 
safety switch being thrown, and the capacitor bank being discharged 
quickly after the primary opened. On lower secondary voltages, I have 
switched the secondary ac feeding the rectifiers with good results. This 
being anywhere from 1Kv to 1.5Kv. Ac will not arc near as bad as DC due 
to the cycling current through 0 volts extinguishing the arc. The 
contact rating has to be correct to do this as HV relays are different 
than the DPDT relays in use at other places. The one from Ross has is a 
solenoid type with a large air gap and rods for the contacts. These full 
round surfaces act to kill out arcing from a sharp point. The shorting 
contact is a round washer. The air gap is about 5/8" to 3/4" which also 
helps kill the arcing. Most DPDT relays with 10 amp contacts only have 
about a 1/16" to 1/8" gap at the most. In doing this, a HV relay is a 
must. The screen voltage can be switched with a DPDT relay and10 amp 
contacts with no problems.

Will Matney


Hi

Will Matney wrote:


> > In any amps I've made, I did kill the B- lead for the HV until the 
> > antenna relay pulled in even though it was biased to cutoff on idle.
>  

How do you do this? Switching several Kilovolts must surely be done with
an expensive vacuum relay?

You certainly cannot switch the HV transformer primary on and off every
time you cycle between xmit and receive, because of the HV capacitor
current inrush. 

73

Angel Vilaseca HB9SLV


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