Inverter Causing Interference

[ParkingLotLust]

Awhile ago I picked up this inverter, and it does the job fine, but I find that whenever its plugged into a vehicle and turned on, I lose 90% of radio reception - only the local stations come in, and even then they are static-y. If I turn the inverter off I get all my stations back. Also, if I plug an AC adapter into the inverter, plug my iPhone into the adapter, and listen to music through the car's aux-in jack, I can hear a high-pitched static-y whine sound through the speakers, that stops when I turn the inverter off.

Ive attached two pictures of the inverter. Let me start by apologizing for the quality of them; I know they arent the best pictures, but they should give an idea of what type of inverter it is. Under the heatsink on the top are two TO220 devices, but I cant get to them to read them (Id have to desolder them to get to the screw on the other side of the heat sink).

I was hoping to add some sort of filter in both the DC power and generated-AC power lines. Where should I begin?

Thanks!

 

[mneary]

What's the power rating? Does it interfere with FM or just AM? Does it have a DC cable you can insert a filter on?

 

[ParkingLotLust]

Its rated for 100w continuous

As far as I know, just FM, but Ive never tried with AM - Ill get back to you on that one

And yes, in the first picture on the left, and the second picture on the right; that connector goes to a cable that plugs into the cigarette lighter

 

[JimB]

On the DC supply from the car to the inverter, I would try a Pi filter.
An inductor in series with the +ve line and capacitors either side of the inductor to the -ve line.
Make the capacitors 0.1uF, maybe add a 1uF in parallel with them.
The inductor, try stripping down a small mains transformer (10VA) and rewinding with as much 16 or 18 gauge enamelled wire as will fit.

On the AC side, try something similar but make the capacitors 0.01uF and make the inductor by winding 22 gauge enamelled wire on a couple of inches of ferrite rod (as used for the antenna in an AM radio).

JimB

 

[RCinFLA]

Modified sinewave (more appropriately modified square-wave) inverters are notoriously noisey.

They have a DC-DC converter, usually around 25-50 kHz, that boost 12vdc to 155 vdc. Then there is an MOSFET H-bridge that chops the 155 vdc to 0vdc, +155 vdc, 0 vdc, -155 vdc, at the A.C. plug outlet. The duty cycle is about 2.5/25 at zero, 7.5/25 at +155 vdc, 5/25 at zero, 7.5/25 at -155 vdc, and 2.5/25 at zero for the A.C. cycle. This yields the 120 vrms. A.C.

For 220 vac European inverters the boosted D.C. is about 285 vdc.

You can not put too large a capacitor directly across the A.C. output as it will cause the H-bridge MOSFET's to get hot and potentially blow out.

If you want an inverter with lower noise, get a true sinewave inverter. They are not that much difference in price anymore. Not a 100% guaranty but much better usually. The difference is boosted D.C. is higher voltage (above peak of sinewave) and H-bridge chops at high freq in pulse width modulation that matches instantaneous sinewave voltage. There is a Pi-filter on output to filter PWM waveform leaving just sinewave.

 

[ParkingLotLust]

On the DC supply from the car to the inverter, I would try a Pi filter.
An inductor in series with the +ve line and capacitors either side of the inductor to the -ve line.
Make the capacitors 0.1uF, maybe add a 1uF in parallel with them.
The inductor, try stripping down a small mains transformer (10VA) and rewinding with as much 16 or 18 gauge enamelled wire as will fit.

On the AC side, try something similar but make the capacitors 0.01uF and make the inductor by winding 22 gauge enamelled wire on a couple of inches of ferrite rod (as used for the antenna in an AM radio).

JimB

What value inductor should I be looking for? I dont think I have any small mains transformers around, so if I have to go buy one, I may as well buy an inductor. If my (max) power output is 100w @ 120v, my current at the output should be ~.833A. Lets say its 50% efficient. That means I should be drawing ~1.66A at full load, so lets round that to 2A draw from the 12v source under full load. Is this correct? Im just trying to find a current rating for the inductor I will need, and neither the inverter nor Crappy Tire's site list any current ratings.

Also, do I need special caps for the 120v output, or are normal ceramic disc caps fine?

Thanks for your help so far!

EDIT scratch those "calculations" - apparently the english specs rubbed off of the inverter but the french specs that I found say that it draws a max of 8A (!!) at 12v. Does this sound right? Will I really need to find an inductor rated for 8A?

 

[audioguru]

8A times 12V is an input power of only 96W. It probably draws 10A from 12V when it has a 100W load.
The inductor must be able to pass it.

 

[ParkingLotLust]

What value of inductor do I need? I did a quick DigiKey search for 10A+ inductors, with 1mH or higher inductance (read somewhere to get the highest inductance you can afford) and the cheapest inductor is 22 dollars, and isnt in stock. Perhaps I would be better off rewinding a transformer? Or would that even be able to pass 10A?

 

[bountyhunter]

One of the three fluorescent ceiling lights they put in my kitchen puts out so much EMI it would wipe out channel 2 on the TV that is 50 feet away from it. Digital TV doesn't have that problem anymore.

Even if you filter the lines, the unit will still directly radiate some noise.

 

[ParkingLotLust]

If I have the unit plugged into the trunk of the car, it still takes out the radio. But if I plug it into a different vehicle and set it on the seat of the car the radio works fine. This lead me to believe the noise is being sent back through the DC power line, not being radiated to the antenna.

 

[JimB]

If I have the unit plugged into the trunk of the car, it still takes out the radio. But if I plug it into a different vehicle and set it on the seat of the car the radio works fine. This lead me to believe the noise is being sent back through the DC power line, not being radiated to the antenna.

That is a quite reasonable assumption, conducted noise on the DC supply is often a problem especially "alternator whine" from the cars alternator.
The usual fix for alternator whine is a filter like I have just described for the DC supply.

For the inductor, it sounds like you dont have a well stocked "junk box". I would not advocate spending $22 on an inductor from Digikey or anyone else.
It may be worth going to a car accessory shop and see if they have a filter to cure alternator whine. I think you need one which has an inductor not just a simple capacitor.

JimB

 

[ParkingLotLust]

Heh, I have a junk box stocked full of all sorts of connectors, resistors, capacitors, ICs, but alas, no inductors. As for buying something to fix alternator whine, the stores that Ive been to massively overcharge on just about everything (ie. well above MSRP), but I will have a peek the next time in the area. Also, while I wouldnt buy a $22 inductor from DigiKey (I can find electronics parts either at school or from shops downtown), I use DigiKey as a specs reference, and to give me a ballpark price range.

What value of inductor should I be looking for?

 

[mneary]

Here's a Large Toroid Inductor-The Electronic Goldmine but it's probably only good for 8-10A. It would be tedious, but you can get 2x the current rating by removing the wire and rewinding the same wire as a pair (bifiliar).

This Ferrite Core 10uH Inductor-The Electronic Goldmine should be good. Don't worry too much that they aren't sure how much current it'll handle. I looked through various Vishay data sheets and would guess that it's at least 20A since the Dale IH-15 10µH coil already is rated 20A.

10µH is probably a nice value to start with in the 12V line (each conductor). That, with a couple of low ESR capacitors in a hash filter might make a beginning.

 

[JimB]

What value of inductor should I be looking for?

In terms of uH, mH or just plain Henries I have no idea, I just know what works, usually!

JimB

 

[bountyhunter]

You want as much inductance as you can get out of the selected core without saturating it. If the DC current gets high enough to saturate it, the inductance basically goes away.

BTW: a powdered iron core has a more gradual saturation, a ferrite core's inductance more like drops off a cliff.

If this is very HF noise, you could probably kill it with an air core inductor like winding AWG14 wire onto a toilet paper tube.

 

[ParkingLotLust]

Well, if FM signals are 88-108MHz Id expect the noise (or some harmonic of it) to be somewhere around there. Ill see what I can get downtown when I get back to Toronto. Thanks for the ideas guys!