Car Engine signal interference with cell phone - Page 1
Forum section: electronics / beginners

author: Tybe
author_rank: Contributor
timestamp: August 17, 2012, 02:11:06 pm
content: Hi again.I'm having some trouble with my Car Audio. Signal interference from the car engine. I have connected my cell phone to the car battery (12V). With a non-isolated step-down module in between to get the 3.7 voltage needed (no battery in phone). I'm getting the same interference If I'm using the cigarette output cellphone charger (with battery in phone).I wondered if I could create a circuit with basic electric components to eat up or reflect the Car Engine interference away, so I can get "clean" current into my cell phone. Like a filter.I would be glad if someone could help me with this problem. Or give me an idea of what to try. (I have tried 5 capacitors in paralell but with no sucess). I really want my "car phone" to work nicely and give me clean audio signals:)Thanks

author: jahonen
author_rank: Super Contributor
timestamp: August 17, 2012, 02:22:38 pm
content: You probably have a ground loop where alternator ripple current flows. Those things are sometimes a bitch to debug. One possible "quick-and-dirty" solution for this problem would be to use suitable isolating dc-dc-module (something similar than Dave used in his USB power supply) in power feed to your cell phone. Another possibility is to isolate the audio side but good quality audio transformers are somewhat expensive, so I suggest isolating the power feed.Regards,Janne

author: Tybe
author_rank: Contributor
timestamp: August 17, 2012, 02:31:30 pm
content: Thanks for Your input. I'll check out Daves video when I getting home from work later today and see if I can learn something:)

author: SeanB
author_rank: Super Contributor
timestamp: August 17, 2012, 03:50:58 pm
content: Most of the time you can reduce the noise a lot by using a LC filter in the power line. Go to almost any car audio place and buy one, or just use a old transformer and place the secondary winding in the line to the regulator, and a 2200uF 25V capacitor after it as the filter. Insulate the unused primary winding, and almost any transformer that provided between 6 and 15VAC will work. Brute force approach, but it almost always works. as well make sure you take a fused feed direct from the battery ( a 5A fuse in an automotive holder right there as well) or from a switched supply like the accessory lead for the radio, and a ground wire to the chassis of the car.

author: Tybe
author_rank: Contributor
timestamp: August 17, 2012, 04:30:50 pm
content: Quote from: SeanB on August 17, 2012, 03:50:58 pmOk. so.. Car Battery - Fuse - Transformator - Regulator - Capacitor - Cell Phone? I really like this idea. Are the transformer used as kind of an inductor then?Does the transformer need to have the primary windings intact?And one more question. I connect the Transformer into the + line, but what about the capacitor same?Thanks

author: SeanB
author_rank: Super Contributor
timestamp: August 17, 2012, 06:42:22 pm
content: Battery, fuse, choke ( transformer secondary) capacitor then regulator. Positive of capacitor is connected to choke, and negative is connected to ground lead to car chassis. Primary winding is not used, but must not be shorted, and must be insulated and left open.

author: Tybe
author_rank: Contributor
timestamp: August 17, 2012, 06:44:23 pm
content: Thanks a bunch Sean

author: IanB
author_rank: Super Contributor
timestamp: August 17, 2012, 10:45:18 pm
content: Here's a quick diagram:

author: Tybe
author_rank: Contributor
timestamp: August 19, 2012, 12:29:15 am
content: Now I cant get it wrong. Thanks again IanB!

author: Tybe
author_rank: Contributor
timestamp: September 25, 2012, 07:30:32 pm
content: I didnt get rid of the sound interfrenece with capacitors and transformer. (Maby it was too much noise to fade out).Anyway for all forumsearchers that may have the same problem, I solved this way;I bought this module from Jameco.https://www.jameco.com/webapp/wcs/stores/servlet/ProductDisplay?langId=-1&storeId=10001&productId=2105973&catalogId=10001&krypto=9x3mj8umRTrLLwWAuw16PjffkH%2F79uyMiOaQ%2BN0RrVWJqLPaQyg7zz0j7Hj3XUQ%2FK7S3lyBVaU03%0D%0AiH5zu7Kc5pa6N8Y1ylaC&ddkey=https:StoreCatalogDrillDownViewI also bought a project box. It is very satisfying to finally get rid of that engine-sound in the speakers and having the phone always powered by the car battery!

author: jahonen
author_rank: Super Contributor
timestamp: September 25, 2012, 08:07:32 pm
content: Hi Tybe,nice to know that you got the problem solved. It was probably the ground loop I suspected initially. Simple LC filtering does not cure that since the offending noise voltage is produced in the signal wire ground. While I'm not a great supporter of isolation, sometimes it just works and gets the job done.Regards,Janne

author: Tybe
author_rank: Contributor
timestamp: September 25, 2012, 08:14:07 pm
content: Thanks for Your concern Janne:)It was actually Your input that lead me to get this dc-dc module, so thanks again:)Have a nice evening;)

author: poptones
author_rank: Frequent Contributor
timestamp: September 25, 2012, 08:56:07 pm
content: Oh, I see. It's not just powering it, it's because you have the phone connected to the stereo via audio jacks.If you pull the radio and attach power and ground leads at the back of the radio you'll be able to get rid of the box. The problem is caused by pulling the ground from a place so far distant from the radio ground.

author: Tybe
author_rank: Contributor
timestamp: September 25, 2012, 09:43:27 pm
content: Im not so sure about that, but it would have been fun to try tho. But I have to say. If a put the cell charger into the phone from the cigarette outlet I get the same interference as the wires from the car battery.By the way... I like the box :p Its also stabilize the voltage at 12.0V witch I think is nice. Not a big deal, but I like it.

author: Tybe
author_rank: Contributor
timestamp: September 25, 2012, 09:50:03 pm
content: Quote from: poptones on September 25, 2012, 08:56:07 pmIf thats the case, very nice information. Thanks

author: poptones
author_rank: Frequent Contributor
timestamp: September 25, 2012, 10:06:29 pm
content: The "auxilary outlet" (what used to be called a cigar lighter) usually has its own high current power run from the battery. And it's typically grounded to the chassis nearby. Taking power and ground from the head unit usually works.