The AM Forum

Hello All!

     I have a bunch of solid state relays that I had planned on using in an upcomming TX project. They came off some industrial production machines. I can program them to come on, and off.

     Will they "behave funny" around high RF fields?

Tnx,

I replaced the plate relay in my Johnson Desk KW with a pair of SS units and I used a pair of small RF chokes with .01 bypass caps on either side in the control leads.  Prior to this RF did get into the control causing some interesting effects.  After RF bypassing they have operated flawlessly for 6 years now.

I have a homebrew amp that also uses a pair of SS relays to control the plate transformer and these do not have any RF bypassing on the control lines to the relays but they are DC controlled instead of AC controlled (like the Desk KW) so that might be the reason for the different behavior.

Be careful with solid state relays. They all leak a little current when in the off state. A couple ma into  a high voltage supple could store some energy.

The Harris HT series FM broadcast transmitters are full of them. The ones that are driven by a small DC voltage, no bypass or capacitors or anything. They just sit there and work. They use them to control fans, control and bias supplies. They also control contactors used for the plate supplies. BE (Broadcast Electronics) builds up their own unique versions for their transmitters; just like BE having to be different.

If they do, you'll be the first one to know ;D

It's possible, you'll have to try them to really know.

Fred

Indeed. I was going to say I found this out a long time ago farting around with a 14kv supply for a tesla project. Shut everything down, hit it with a jesus stick, was gonna grab something and move it and noticed the hv meter with about 900v sitting on it. Not good.

New ones feature zero crossing switching, which can help with that annoying ker-thunk
when switching a plate transformer on.

Pete

Many of us like that big old clunk when the transmitter comes on. I think one of the class E guys is using a 12 pound wood splitting maul hitting  a kettle drum actuated by an air piston, controlled by his push to talk, so he can feel more manly when his signal comes on the air ;D ;D

Knocks the crap outta a DX-60 mode switch though  ;D

If that's not reassuring, I don't know what is. :-)

In the 80's and 90's, I designed and built a lot of industrial controls using those SSRs. The majority of those controls are installed all over the USA and Canada in packaging machinery to control the heat in the crimpers that seal things like candy bar wrappers and potato chip bags.

I usually used Crydom, probably because of the price and that Mouser Electronics had them. I never heard of one failing; but if one had failed shorted, the results would have been fairly catastrophic.

However, I did make liberal use of MOVs and snubbers to protect them, just because they were solid state. And if there was RF in the picture, I would automatically add RF chokes made from ferrite beads, probably made from 73 material.

I would sure love to get rid of all the big relays in my system. Unfortunately, power is power whether it's class E power or tube power, and switching a lot of power takes a big relay.

The combination of all of the relays involved in going from receive to transmit is incredibly loud  :P

I am really trying to figure out how to make the relays quieter, or possibly substitute solid state relays for some circuits.  Being rather paranoid, there's nothing like a mechanical relay from a safety standpoint  ;)

The antenna change over relay alone is one of the major offenders.  It really makes quite a loud snap when it engages.

MOVs have a limited life span and die in two modes leaky or kabooom
SSRs are great as long as you can live with a little leakage.
They are used in many industrial controls and last longer than clacking relays.
Many newer aircraft use SSRs but I don't think you will find any around a weapon interface.

Frank,
         A motor control board that I used to repair here used a MOV across the 120v input line after a 1A circuit breaker for "surge protection". It was pretty funny, they would come in for repairs with the MOV burned to a crispy critter, the board all charred from it, sometimes even burned through, and still be working fine!! It used to drive the field techs nuts!!

We can't use them in airplanes for that reason.
I have a number of Cubic power supplies with burner spots on power supply boards due to MOVs

Another type of relay we used to use (rarely) was Durakool mercury displacement contactors. They last far longer than ordinary mechanical relays. No leakage, and they don't need a heat sink like many SSRs do.
I forget how much noise they made when they operated.

I use SS relays(dc) to switch my ole DX-100 from tx to standby using my D-104 PTT. regards  John,K1DEU

We used to use them. They are pretty quiet, just a slight little snap / squishy. If you werent listening for them, you wouldn't even notice it. And they seem to last forever. The ones we used to use had a 1 second hold in delay.

Solid state relays rated at 150 amps are available off the shelf. Clacking relays mechanical relays are fun but not absolutely necessary.

I was thinking of using them in my B&W 5100 transmitters until this leakage issue
came up.

Pete

3-8 million cycles at rated amperage on those mercury plungers according to spec sheets. I have a couple that came on a henry plasma gen I picked up to reconfigure a while back. Even 100amp rated one's are very quiet. Although I prefer a nice NEMA size 2 motor starter that hits so hard, it shakes the cabinet. Let's everyone know there's a strap that just keyed up. :P :P

Pete,

Unless your HV supply has perfect capacitors (and that excludes electrolytic caps) AND no bleed resistor or other load the small leakage current through the SS relay is never going to develop much of a charge.  My Desk KW supply volt meter stays on zero until it is in transmit condition.

WHAT! No loud clack of relays going from RX to TX.  That's unAmerican  ;D  

Steve.  I love the sound of your relays.  AND it's a 5.1 stereo experience when they are going off all around you.   ;D

Al

The sound is neat the first couple of times  :D  Then my ears actually start ringing because its so loud :o

I could record it, and then mix it back in with one of those digital recorder boards that cost about $20.00.  Each time the transmitter goes from receive to transmit, I could fire the trigger and play the sound in to the mixer  ;D  I bet someone has already done this, somewhere!

I have a plugin for Reaper, Audition and other compliant sound recording software suits that allows just that.  You can have it triggered by another tone.

I set it up for a relay clack when it gets a spike from the mic..  I use a FET Follower D104, and it has a small keyup spike.  That spike is what triggers the plugin, and then it plays a 2 second .mp3 file.

A couple of the CBers have also done this. 

Matter of fact, RF Limited has a programmable key up or key off board.  I don't know what the time is on it, but I'd say for a simple relay sound, it would work FB.


--Shane
KD6VXI

Everyone likes (and comments on) the sound of the fans spooling up when I key the 4X1 rig. The radio room is small enough that the mic picks it up.

We dont need no stinkin relay clack! !   ;D  ;D

I like the guys that when they don't speak, you hear nothing...  Silence on Slop, just silenced carrier on AM.

Then the noise gate gets broken with speech and all hell breaks loose, sounds like the blower noise from a 208-10 Collins.

And no, I really don't like it, but it's an interesting effect.


--Shane
KD6VXI

Same here.  I once used a Dow-Key relay, but the Gates burnt it to a solid block of charcoal before I addressed the relay sequencing problem.  So I pulled out this buzzardly pre-WWII antenna change-over relay I picked up at a hamfest probably decades ago, n.o.s., still in the original box.  The coil is 110V, and the thing is constructed like a transformer core.  Engages with a clank, plus the coil buzzes from the a.c.  It worked well, with plenty of margin, but the noise was unbearable.

I fixed it by mounting it inside a hinged wooden box I had on hand, and mounting the whole thing on the wall.  The relay is suspended on springs, and the springs are fixed to the box using rubber grommets. When the door is closed, the buzzing is inaudible, and the "clank" is reduced to a benign "thump" that is barely audible at the operating position.

why not run it off DC. Most 110VAC relays work well at 28VDC and they don't buzz

It is true that MOVs don't last forever. But that's no reason not to use them. :-)

I've installed hundreds of MOVs--large and small--in the past 30+ years. I am absolutely convinced they are worth using.

Last place I worked, I installed MOVs on many solid-state devices that used to regularly fail during a lightning storm. It reduced those failures to zero.

The only time ever I saw MOVs go up in flames ("kaboom" mode) was when the end-user applied 480 volts to a 120 volt circuit -- after he installed the biggest slow-blow fuse he could find. And somehow, the MOV (along with the vaporized circuit board traces) still protected the circuit.

There used to be a ham in Freehold NJ who installed MOVs on all his appliances in the house.

House burned down after an MOV set the guts of a TV on fire after an electrical tormenta.

Moral: enclose MOVs.