The AM Forum

Hello all,
I am having issues with RF getting into SSR's that I have installed in my transmitter.
They are 375vac @50a Crydom units that are DC controlled by 12 volts.
I have two of them on the main input (one for each leg of the 240)for the filaments and one each for low and high power taps on the primary of the plate transformer.
On 40 meters, the rf gets into the main relays and causes them to be stuck on (shorted).
I put some .01 caps across the control leads and it helped but I still get a stutter once and a while in the plate transformer.
Should I try some RF chokes in the control leads?
If so, what values?
Any input would be appreciated.
Thanks,
Don W9BHI
 

hi Don ... I am assuming the control power leads on the SSR are low volts dc .... I wud recommend adding a .005 to ground (chassis) from either control lead with shortest length leads possible  .... .01 across is a little on the high side and whatever discharges it (hv on switch ?) might have a current spike to deal with ... add a 10 ohm resistor in series with the .01 to limit this .... if the problem persists then add the series rfc (s)  ... 1 mH should be plenty ...73

You might want to also put some ferrite beads or a ring with multiple turns through it on the power coming in.  If your rfi is coming in on mains power (likely) then decoupling the control leads won't completely eliminate the problem.

--Shane
KD6VXI

All good ideas, but don't forget that these potted assemblies have little to no shielding other than the metal side that is intended to be screwed to a metal surface.  If all else fails, mount the unit in a box and bypass everything going into and out of the box.

Joe-GMS 

yep. in a closed box with bypassed feedthrough filters.. Not saying you can't do something else but a friend of mine with a plate modulated 250TH tried several fixes and stopped short of the enclosed box before going back to to clackers. He said they "went nuts". same kind of trouble I guess.

Yesterday I posted in the Hints and Kinks section about a nickle impregnated spray used for RF shielding. I've used this product for several years and it works quite well. Two or three light coats - dries fast.

I will put some .001 from the control input terminals to ground as well as some 1Mhy rf chokes in series with both control leads and see what happens.
Should I put a .001 across the control terminals also?
Thanks for all of the responses.
Don

Besides the RFI issue, keep in mind how a SSR works.

As I recall, most are zero crossing activated when the control voltage enables them. The back to back SCR's or single triac than turns on for the rest of the 1/2 cycle, and after zero crossing, they turn on again as long as the control voltage is on.

How do you turn off a Traic or SCR?

1.) the anode/cathode voltage swaps polarity after the gate trigger pulse is applied
2.) the on state conducting current goes below some minimum holding current. Using a large SSR such as a 50 amp jobbie might cause trouble with a low current load.

A way around these issues is to have a pulse train of gate pulses whenever the control input is on. Since you usually get one pulse immediately following zero crossing, the dam thing can turn off mid 1/2 cycle if 1, or 2 above occurs.

If the load is inductive, or if the power supply has a capacitor input filter, then all kinds of trouble can occur even without any RF. One simple tactic is to put a resistor across the switched AC. Something like a 7 watt incandescent lamp might be enough to make it behave, and also makes a nice vintage dial indicator of power being ON.

Jim
Wd5JKO

These are not zero crossing type.
According to some SSR manufacturers, zero crossing types are not good for switching transformers on and off.
See TE Connectivity app notes on zero crossing SSR's.
Don