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A neighbor of my remote station replaced
his old bi-metal thermostats with electronic LCD thermostats. The
thermostat has nice features like WiFi connection
for remote control. But unfortunately the thermostat increased
wide band RF interference abt 30 dB over the atmospheric noise in my station on 3.5MHz
meter band. The distance to the neighbor is abt 100 meters.
The product is manufactured in China with a product code of MTWECB63A with misleading FCC and CE markings. Marketing name in Finland is Oval WiFi but the products comes from Estonia with this sticker on the the cardboard box. |
Front view |
Side view |
Rear view (click the photo to enlarge) |
| The measurements showed that the product
does not comply EMC standards. I investigated the the flyback power
supply circuit and found that the Y capacitor over the isolation transformer
is missing. Here are the results of
emission measurements before and after the modification.
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Conducted emission with a LISN (Line Impedance
Stabilization Network)
before (red) and after (green). Improvement is abt 30dB. |
Near field radiated emission comprison with a short whip antenna
before (red) and after (green). Improvement is abt 30dB. |
Conducted emission measurement at spot frequencies
with a receiver before (red) and after
(green), blue line is the CISPR 22 Class B limit.
The improvement is abt 30dB. |
The modification, a 4.7 nF Y2 approved capacitor
added across the isolation transformer of the flyback converter.
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The flyback converter board before modification
Click photo to enlarge |
The flyback converter board with added 4n7 Y capacitor
Click photo to enlarge |
.
Use only X1/Y2 approved capacitors. I used SMD size 1812 Murata GA343DR7GD472KW01L
Be careful to keep clearance marked in yellow. The green marked pad is not a problem because there is only 12V to the primary ground. Be aware of these thermostats! There are very similar
thermostats
on the market and I believe they are using the same
bad flyback converter circuit.
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Further improvement
Additional 47n/400VAC X-capacitor installed between N and L to filter the noise coming from rectifier current peaks. |
self-healing capacitor installed between N and L. |
Red original, green with 4n7 Y-capacitor and yellow with additional
47n X-capacitor.
Total improvement more than 40dB on low frequencies.
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Update 2023-04-18 OH7SV
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