K6JRF's Page
formerly W6FZC
ESSB Audio Techniques Page

(Oct 23, 2010)
This Technical Note details how to eliminate RFI problems w/ CMOS and LCD electronic ckts. The case here is a LCD AC Line Voltage Monitor.

LCD Electronics in High RF Environments

I guess that we all have RFI problems with electronic ckts at one time or another. It goes with the territory. This note shows how I eliminated some major RFI problems w/ a LCD AC Line Voltage Monitor.

Recently, I decided that monitoring my 240Vac line voltage would be handy b/c of the heavy load that is placed on this service in my shack. The FT-2000, YO-301, TS-711 and FT-1500 Linear amp are all connected to the 240Vac line. When all are operating, the line sags a bit so a line monitor would show me what happens. Easy enough you would think!

LCD AC Line Voltage Monitor When I surveyed what's available, I couldn't find a reasonably priced 240Vac LM. Lots of 120Vac LM but "zero" for 240Vac. I did find some very reasonably priced "electronic" 240Vac LM from here . Scrolling through the store shows some interesting voltage/current monitoring ckts that are about $10 delivered. Plus they are small w/ a blue back lighted display. Just the thing! The picture shows how it was mounted into the front 'facade' panel at my station.

RS DPDT Toggle Switch I decided to add a "switch" so it could be turned-off when not in use. So I found a DPDT toggle switch at Radio Shack, PN 275-691 for apx $2.99.

After the LCD AC Monitor arrived, it was connected to a standard "zip-cord" line plug, the readings varied when transmitting. Based on where it is located, it is in a very high RF environment and reads anything from 100Vac to 390Vac.

A 0.01uF, 1KV disc capacitor was added close to the input along with three (3) clamp-on ferrite chokes. All of this helped but there was still over 40Vac variance in the readings.

Click on the image to enlarge
LCD AC Line Voltage Monitor back-side view
Next I went to a shielded cable (two (2) leads wrapped in a shield) where the shield was terminated at the LCD VM end at the "-" (minus) end as shown in the picture. This helped to improve the "variance" when subjected to high RF environment but still wasn't "perfect"!

So I constructed a "copper-house", using 4 mil copper tape, to act as a shield. The ground was soldered to the top of the house to ensure that it acts as an RF shield. The RF currents are now "common-mode" to the ckt so there's no interference. The "hole" is for readout voltage adjustment. Copper tape can be obtained here . Recommend that you get the 8" width tape.
LCD AC Line Voltage Monitor back-side view
Glad to say it works perfectly! Even w/ the AL-1500 amp w/ 1500 watts output and the LCD VM only 3" away, there's no RFI. Here's how it looks mounted in my studio.
LCD VM shown in my station

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