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Switching B+ -- how to do it safely?




 
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Author Topic: Switching B+ -- how to do it safely?  (Read 616 times)
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K8DI
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« on: January 06, 2019, 08:10:41 PM »

Hello all --

So I am working on layout for a permanent chassis for this transmitter I've been working on. I'm  wondering how to deal with T/R switching -- seems like many/most schematics I see just show a generic switch in the B+ or the cathode ground. Somehow grounding the cathode makes people feel safe, but that cathode is still at B+ before you ground it. Normal switches you can easily buy seem to be 250 volts AC at best, except for a few way expensive ones. Those 250v switches have way lower DC ratings, too.  So how are you turning off the thing?

Second, while thinking on this, it occurs to me I could use a high voltage MOSFET, or an IGBT transistor,to do it, but I have no idea how well that will work to ground the cathode through one, with RF involved. I know it'd work fine to control it from a DC perspective, and allow me to use any old switch to do T/R duty; operating the FET or IGBT with the same 12 volts I use to run the antenna relay.

Thoughts? Experiences doing this?

Ed/KB8TWH
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Ed, K8DI (fmr. KB8TWH)
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« Reply #1 on: January 06, 2019, 08:24:11 PM »



Which transmitter?

Anyhow, T/R relays are for switching the antenna from the transmitter to the receiver...
I think you are speaking about "keying" the transmitter.

The transmitter is in an enclosed chassis, presumably with interlock switches that will
KILL any B+ if the covers are removed. One should ALWAYS use a "shorting stick" before
sticking one's body into ANY transmitter or PS, even if it is PRESUMED to be off and unplugged.

So, when the rig is on, there may well be B+ on the plate.
Many rigs are "biased off" to take the quiescent current down, and to make the key up
cleaner and faster. CW rigs that do QSK have even more difficult requirements for keying.

Cathode keying is one method...

                   _-_-bear

PS keying methods are covered in the ARRL handbooks of yesteryear, and the BIll Orr "West Coast"
handbook...
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K1JJ
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« Reply #2 on: January 06, 2019, 08:55:00 PM »

Somehow grounding the cathode makes people feel safe, but that cathode is still at B+ before you ground it. Normal switches you can easily buy seem to be 250 volts AC at best, except for a few way expensive ones. Those 250v switches have way lower DC ratings, too.  So how are you turning off the thing?
Ed/KB8TWH


Hi Ed,

If I'm reading you right....The cathode does not rise to B+, rather a few hundred volts at most when floating. Put a 50K  2 watt resistor in series with the cathode to ground and the tube will be cut off. Short the resistor with a set of contacts on key up and the tube will conduct normally.  Any relay rated at 120 AC contacts will work fine.

There was a recent thread here where we went into detail about this method.

I key the cathode of my tubes AND I also step-start key the primary 240 VAC of the HV transformer as a safety factor. IE, HV is in shack only on key up.  Though you could leave the HV on and just key the cathode as described.

Here is a typical rig with cathode keying for both the finals and modulator tubes  - look for the 50K resistors:

http://amwindow.org/tech/htm/813/813.htm

Tom, K1JJ


T



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K8DI
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« Reply #3 on: January 06, 2019, 09:07:18 PM »

If I'm reading you right....The cathode does not rise to B+, rather a few hundred volts at most when floating. Put a 50K  2 watt resistor in series with the cathode to ground and the tube will be cut off. Short the resistor with a set of contacts on key up and the tube will conduct normally.  Any relay rated at 120 AC contacts will work fine.

I key the cathode of my tubes AND I also key the primary 240 VAC of the HV transformer as a safety factor. IE, HV is in shack only on key up.  Though you could leave the HV on and just key the cathode as described.

Here is a typical rig with cathode keying for both the finals and modulator tubes  - look for the 50K resistors:

http://amwindow.org/tech/htm/813/813.htm

Tom, K1JJ


T





My transmitter is a toy compared to the 813 rigs being discussed around here lately. Maybe in a few years or after I find some spare cash to buy a few hundred pounds of iron...  But 600 or more volts on the B+ will still ruin my day, so I am trying to make sure to key the thing in a safe manner. What you're saying is that the cathode won't be at the B+ when ungrounded, which is not what seems instinctive to me, but I don't have experience with this.

I'll defer to your and other's experience and just add a relay to key it.

Ed/KB8TWH
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K1JJ
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« Reply #4 on: January 06, 2019, 09:17:19 PM »

Ed,

Yes, that's correct.  The HV drops mostly across the plate to cathode which is a vacuum.   Be sure to add the resistor. A smaller rig with an indirectly heated cathode should work the same.

The 50K is a cathode biasing resistor that drops a positive voltage across it which becomes a relative negative voltage at the grid. The tube goes way far into cutoff until it is keyed/shorted and the tube conducts normally.

You don't really want the cathode to float which can generate instability, but it is not at a lethal voltage at the cathode.  It may give you a jolt, however... remember the cathode keyed Novice rigs and the shocks off the CW key?   Wink

BTW, use .001 caps to bypass the cathode to ground for RF.

T
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Frank / WA1GFZ says when he's working near high voltage, as a warning he sings this song by Jay and the Americans: "Come a little bit closer, you're my kind of man, so big and so strong, come a little bit closer, I'm all alone and the night is so long."
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« Reply #5 on: January 06, 2019, 09:56:49 PM »

On my rig I switch the HV primary since I don't like the idea of having B+ on while the rig is in standby. You could use a solid state zero crossing switching relay, probably much easier on the iron. I use a big multipole AC contactor, it is overkill and very loud when it hits (something like 30 amp rated, and this is only a 150ish watt transmitter I'm building, at the moment that power supply and switching setup is just running a single 807 rf deck) but it works good.
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« Reply #6 on: January 06, 2019, 10:20:34 PM »


You're keying the AC mains (primary) of your HV transformer every time you push the button to talk?
Big surge... you have to charge the caps each time too.

I guess you can get away with this on a single 807 rig, but doing this on a "real" kW class power supply
is a bit of a different matter...
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K6JEK
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« Reply #7 on: January 07, 2019, 03:58:14 PM »


You're keying the AC mains (primary) of your HV transformer every time you push the button to talk?
Big surge... you have to charge the caps each time too.

I guess you can get away with this on a single 807 rig, but doing this on a "real" kW class power supply
is a bit of a different matter...
I  once worked on a kw-class transmitter that switched the 240 AC with a step-start when keyed and only put the bleeder across the caps with power off. It also switched the HV to the transmitter with a separate relay. The caps didn't discharge on key off, only on power off. The transmitter did not need the  extra regulation a bleeder provides.  The HV was metered at all times so you could see how long it took to go down. The step-start assured a gentle re-entry on key down, especially gentle when the caps were still charged from the last time. This arrangement kept the surges to a minimum while still being safe. This was not a CW transmitter.

I rather liked this set-up but  I think most of the benefit could be achieved with just the step-start. That would minimize the cap surge without all the other fancy stuff, maybe even use a zero-crossing solid-state relay to take even more of the surge away.


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KK4YY
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« Reply #8 on: January 07, 2019, 06:11:36 PM »

In the 813 linear amplifier that I built, I use a step-start relay at power-up and keep the HV on during stand-by while the tubes are biased to cut-off. The bias voltage is removed to go to transmit.

The T/R control signal from the exciter (a contact closure) goes to an opto-isolator relay board inside the amp that controls all internal switching. There isn't anything more than 12vdc coming out from the amp. Alternately, I could have sent a 12vdc signal from the exciter to the opto-isolator in the amplifier (hence, nothing coming out from the amp). I considered this, but opted for the contact closure.

I don't know why people are worried about having the HV on during stand-by, but apparently not bothered by it during transmit. Any old boatanchor receiver has at least 300 volts running constantly, not to mention the old CRT television sets we all had in our homes, and fell asleep in front of, with tens of thousands of volts buzzing away inside. Shocked

I'm more concerned with the surge caused by hitting the transformer primary with AC while a big filter cap bank is discharged. By keeping the HV on, that surge happens only once per operating session - and with a step-start resistor the surge is small anyway.

But, the way you choose to do it depends on many factors. There is no one 'right' way for all circumstances. Just make it safe (without big ventilation holes like on the back of those old TV sets). Roll Eyes

Don
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