# Best method to eliminate alternator whine from a 12V line?

I've purchased a Miccus Mini-Jack Rx BlueTooth audio receiver. This device will accept a bluetooth signal from my phone and output it to a headphone jack that I plug into the AUX-In on my car stereo. Works great.

Current setup

I do get a little alternator whine when I have the USB charge cable plugged into the device.

I was thinking of purchasing a DC-to-DC power convertor like this one and install it inline with a new 12V power line from the fuse box and take the 5v output straight to a female USB plug.

Is this the best route? I've read quite a bit about people installing capacitors between the 12+ and the ground as well to help dampen the noise.

• This is a good read on dealing with noise: elektrologi.ee.itb.ac.id/wp-content/uploads/2014/02/iap125.pdf – Guy Sirton Jun 16 '14 at 18:03
• You should probably be aware that some brick-DC-DC converters, like the one you linked, actually have a minimum load. If you have no load, they can output excessively high voltages, and actually damage themselves. – Connor Wolf Jun 17 '14 at 10:36
• @ConnorWolf I'm very much an amateur at this, I'm not sure I understand what you mean, can you explain that a bit more? This is the one I ordered. The line coming in should always be ~12v switched on with the ignition of the car. – ProfessionalAmateur Jun 17 '14 at 14:15
• @ConnorWolf After reading this article, I think my minimum load is 10Ω based on the following formula R = V\I. In my case 5V\.5A for my output channel. Is that correct? – ProfessionalAmateur Jun 17 '14 at 15:44
• Your minimum load is 60 mA, which is 0.06A. Ohms law tells us $\frac{V}{I}=R$, so $\frac{5}{0.060}=83Ω$. I don't know where you got the 0.5A number from. – Connor Wolf Jun 17 '14 at 23:33

Three issues. First - the isolation in the DC2DC should be OK to maintain an RF earth through the system. There is a slight doubt that if it is too good then it may impair the antenna performance. Second - the DC2DC is only 1 watt rated so is this going to be enough - probably enough for the BT receiver but don't plan on any other devices that might take more than 1 watt.

Another thought is that due to inevitable leakage capacitance inside the DC2DC, it might not solve the original problem. You've got to consider that although it is isolating (because it uses a transformer) the inter winding capacitance (which provides an RF earth) might also propagate the original problem.

Having said all that it's a cheap enough unit to try and throw away if you are not happy with it.

One more thing about the DC2DC - I don't think it is a regulating type - if you look at the datasheet on page 1 the 12 volt version only has an operating input voltage range of 10.8V to 13.2V and if your lighter socket can produce 16 volts there's going to be a problem. I think you should consider a better device.

If your BT receiver only requires 5V I'd consider a linear voltage regulator but choose one that is peak rated at up to 60 volts because now and then these are the sort of input voltage spikes you can get on a car wiring system. Same applies to any regulator type including any DC2DC.

A linear voltage reg will probably do a better job getting rid of the whine if it isn't earth related but imposed across + and - wires.

• Thanks. I have emailed Miccus to get the power requirements for the BT receiver so I can match it more precisely, I think 1 watt should be enough and this is all I plan to use this line for. Mean Well does make another converter with a wider input range. A bit more  but this should address your third concern? I will need to read up on linear voltage regulators, I have no experience with them. Appreciate the input. – ProfessionalAmateur Jun 16 '14 at 18:01
• Hey this worked out great. I fried the first one not checking the +/- lines in the car before I hooked them up (white for a ground... fooled me). The only thing that occurs now is when I turn the key off, the BT unit changes the song on my phone. I think there is a little power glitch/spike when I turn it off. Would a capacitor or a diode on the + line when control this spike? – ProfessionalAmateur Jul 1 '14 at 20:59
• Hard to say dude but try it and keep thinking good thoughts. Glad it worked out. – Andy aka Jul 1 '14 at 21:02
• @ProfessionalAmateur: You could use a Shottky diode on the positive input side, and a capacitor of maybe 47uF to 470uF (start small, keep on increasing if the smaller value doesn't work, and make sure it is rated for at least 16V!) after the diode (between the positive and negative input lines of the converter). You could try an inductor/choke instead of the diode, or just try with a capacitor alone. – Edin Fifić Apr 3 '20 at 20:35