LC Low Pass on USB car Charger? - Page 1
Forum section: electronics / projects

author: rwgast_lowlevellogicdesin
author_rank: Frequent Contributor
timestamp: September 21, 2015, 03:58:58 am
content: So I have one of those things you plug in to a cigarette lighter and it has a flexible tube and phone mount, it also has two usb charging ports. Anyways Ive tested several chargers and whenever I plug my phone in with an an audio Aux cable I get alternator whine. Im assuming the cigarette lighter has a crappy ground or is in a ground loop. I busted a charger open an also noticed theres a switching regulator inside of it.Most people would try to elimnate this noise by A fixing the grounding issue (which I just dont want to waste time with) or B adding some audio transformers that usually dont work and change the audio. Wouldnt the best idea be to add a 1Mhz LP inline with the USB cable? Ive read alot of people have this problem im surprised I havent seen anyone sell a product to filter the usb chargers output....

author: ConKbot
author_rank: Super Contributor
timestamp: September 21, 2015, 12:12:52 pm
content: Is it a constant whine or does it vary with engine RPM? If it varies with the engine, then it is alternator whine because of poor grounding, and probably not much to do with the internal dc-dc.And for alternator whine, a filter on v+ won't help much. Plug your phone into the aux, and slowly plug in the charger so the ground makes contact first, the whine will start as soon as the ground contacts hit.To kill the whine, you need the radio and power outlet to be grounded to the same spot. First make sure your engine ground straps are in good condition. Simply using an ohm meter from block to chassis won't be enough, physically find the ground strap and make sure it's not half way corroded away. Same with battery to chassis. If those are ok, then I'd string a 16-14 or so awg wire from the radio to the outlet to the chassis, just splicing it into the ground wires (if they exist) as you go. No need to remove or disconnect the old wires.

author: DmitryL
author_rank: Regular Contributor
timestamp: September 21, 2015, 01:04:19 pm
content: Quote from: rwgast_lowlevellogicdesin on September 21, 2015, 03:58:58 amSounds like a true ground loop issue. Simple filters won't help here. IMHO the best approach is to make a charger based on isolated DC-DC converter (comparing to a simple BUCK one with a common ground). The reason that no one makes it by default it's the higher price.

author: rwgast_lowlevellogicdesin
author_rank: Frequent Contributor
timestamp: September 21, 2015, 02:22:36 pm
content: Ya its a ground loop for sure, Ive been doing stereos long enough to know what going on. It whines based on the alternators RPM. I really didnt want to take the console out to ground the cig lighter properly. At this point my transmission is going to need to be replaced so im not tyring to do to much to this car, not knowing if I will keep it or how long it will last. I have have a jetta, but my mother bought a new pilot and gave me her old chevy HHR for free I guess she never had the transmission fluid changed in the 100k miles she drove it.... I didnt know that and about a month after having it the thing started choking to get from first to second.I understand why an isolated buck convert would work, but why wouldn't adding an LC filter inline with the charger work? Usually I would have ripped the dash out fixed the ground put a PI or something in there etc etc, but like I said idk if im gonna pay 3g's to fix this car. This is why im looking for solutions that dont have to do with me dissembling the thing. I wanted to originally planned on throwing a bunch of antennas and SDR equipment in it but ehhh well see what happens with the thing. Looking for the quick fix right now. The front console does not look easy to remove!

author: rwgast_lowlevellogicdesin
author_rank: Frequent Contributor
timestamp: September 21, 2015, 03:53:10 pm
content: I was thinking even a bypass cap would clean up the AC component but then it would need to be connected to a proper ground, is this the same reason a LP LC filter wouldnt work inline with the usb? What about freight, im pretty sure the frequency doesnt go over about 500KHz is this to low for a snap on ring to filter?

author: DanielS
author_rank: Frequent Contributor
timestamp: September 23, 2015, 01:50:36 am
content: The reason why filtering won't work is because unless you tie the amplifier's ground to the 12V socket ground, the ground loop between the 12V socket, whatever it ties to for power/ground, the battery or wherever it becomes common with the circuit that feeds the amplifier, the amplifier and the analog cable will directly inject noise in the amplifier's input.You could possibly add a common-mode filter on the 12V socket but since the frequencies you want to filter out extend down to 100Hz or less, you would need some very large chokes and capacitors. Much simpler and cheaper to use isolated DC-DC.

author: DanielS
author_rank: Frequent Contributor
timestamp: September 24, 2015, 09:26:29 pm
content: Quote from: blueskull on September 23, 2015, 06:07:55 amAny isolated DC-DC converter design should work since that would make the output ground referenced to whatever the output gets connected to.

author: NiHaoMike
author_rank: Super Contributor
timestamp: September 25, 2015, 07:04:08 am
content: You can also look into audio differential line receivers. Or you can try connecting the input of the charger right at the amplifier power terminals.