Removing RF noise from any inverter? how?

Submitted: Friday, Sep 26, 2003 at 16:34
ThreadID: 7455 Views:8991 Replies:4 FollowUps:3
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RF noise suppression inverter any tricks.
Is there anyone who would practically know how to remove the noise coming form an square or sine wave inverter?
I purchased 1 month ago a 12v--> 230 volts inverter from Jaycar-(5066 model). Excellent inverter which power my laptop without a problem, recharge my battery.
But this inverter has a bad hum on 8022KHz (VKS737 main channel) and 50xx KHz frequency.
I have spoken to many electronic technicians, but I can see that 99% of them are totally guessing the answers.
( ferrite beads around the 12 volts cables is a good solution but the tech do NOT know how I should install it - this is only one solution)

Does anyone knows how to remove the RF coming from the chassis of the inverter and radiating RF around from 8500 to 5000 KHz?

I have tested the inverter 50 metres away, connected to a totally different battery, not electrically connected in any way to my battery system. The hum is still there and has stroung.
A practical solution would be appreciated
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Reply By: Member - Bradley- Friday, Sep 26, 2003 at 16:49

Friday, Sep 26, 2003 at 16:49
Sadly it is probably a design or build fault with the inverter itself, try getting another from jaycar and see if it is the same, and see if they will let you try a different model. The rf chokes are usually only effective on small general interference but while you are at jaycar grab one and place it around the output cable and if the gap in the choke allows, wrap it around a few times to increase the effectivness. It would be a shame to have to go to the trouble of wrapping the entire unit in sheilding mesh and earthing both the case and the vehicle as you have already determined thet it is a very strong signal by using the other power source etc. Hope this helps some.Life is short- but there's always time for a yarda.
AnswerID: 32123

Reply By: Eric - Friday, Sep 26, 2003 at 22:08

Friday, Sep 26, 2003 at 22:08
The output of most inverters contians a lot of harmonics, that is probably what is giving you the problem. The best way of loosing the harmonics is to use a transformer and capacitors designed to resonate at 50 cycles. the case of the inverter is best earthed and the lead from the battery run in shielded wire. The down side of the transformer is the weight, it could be 5 kilo. Eric.
AnswerID: 32148

Follow Up By: Guy - Saturday, Sep 27, 2003 at 10:58

Saturday, Sep 27, 2003 at 10:58
Eric, I tried connecting the chassis of the inverter to the chassis of the car and it actually increases the hum.
So, i heard somewhere putting a resistor and capacitor of 47 Microfarads between the chassis of the inverter and the car chassis and the hum diminished to about 5% on the 8022 Khz but is still strong on the 5000 KHz.
Why do you think the capacitor reduces the hum a that frequency?

What's the principle of harmonic behind that hum?

What about if I put a mesh_copper_bread around the cables and instead of connecting it to the earth, I connect it to the earth through a 47 Microfarad cap?
Let me know what you think?
FollowupID: 22956

Reply By: Eric - Saturday, Sep 27, 2003 at 21:37

Saturday, Sep 27, 2003 at 21:37

When you connected the inverter chassis to the car chassis two things could have happened the voltage could rise because the voltage drop in the negative lead would be reduced and the car chassis not being earthed may have acted like an antenna. To find out if the problem is the dc side or the ac side you could try placeing the inverter and a spare battery in a metal box and bring the ac out through a small hole. If it is harmonics on the ac, the noise will still be there. Dont run it for more than a few minutes as the inverter may over heat, Eric.
AnswerID: 32193

Follow Up By: Guy - Monday, Sep 29, 2003 at 11:43

Monday, Sep 29, 2003 at 11:43
Thanks Eric.
what you suggest is exactly what I checked out first.
The RF is coming from the case of the inverter and probably from the 12 volts leads.
As I explained in my first post , the battery was NOT connected to my car and the inverter was just OFF with no output leads, 50 metres from my car. As soon as I turned ON the inverter the hum came.
FollowupID: 23068

Reply By: Member - Bob - Sunday, Sep 28, 2003 at 20:43

Sunday, Sep 28, 2003 at 20:43
I did a google search using 'inverter interference'. There is stacks of information. It seems that EMI is inherent to inverter design and is related to harmonics of the inverter's oscillator frequency (as stated above). Twisting the dc input wires has been suggested. And using an LC filter on the output side. But basically I think it will come down to avoiding simultaneous use of the radio and the inverter.Bob
AnswerID: 32249

Follow Up By: Guy - Monday, Sep 29, 2003 at 11:56

Monday, Sep 29, 2003 at 11:56
Bob, thanks for your answer. Before posting I searched the web. I've been successful in reducing the inverter RF interference from 60% to about 5% by using a 47 microfarad capacitor.
During my web search I never found any article which refer directly or indirectly at the usage of a capacitor to reduce an RF hum in the HF band? I would like to know why this capacitior trick is not mentioned and if other tricks also have been ommitted or not mentioned on the web.
What I understand from RF interference: the "hum" at HF 8000KHz is very different from a RF inteference in ELF ( 0.01 KHz to 2 KHz),
VLF, LF produced by the harmonic of a car distributor/generator or main A.C. electrical lines.
Shielding the cabling is one good solution .

Connecting the shielding "directly" to ground is probably not the solution.
I tried and it actually increases the hum noise level.

At 8000 KHz harmonic RF will travel on the outside of any cable including shielding cable.

Connecting the shield to ground via a ceramic capacitor is a more successful in my case because it dimishes the "hum" on the receiver by nearly 50% because I believe there opposition of phase.
Ferrite beads seems to also work but not as well as capacitors. This is the same principle of opposition of phase or phase shift.
Twisting the power cable, like in telephone wires, is good but still not as efficient as a capacitor.

Does anyone know any other technic to reduce RF which are known to work?
Websites? Books?
Practical experience, even if these personal or hearsay experiences sound totally crazy?
FollowupID: 23070

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