USB power failing on car stereo - Page 1
Forum section: electronics / projects

author: brainwash
author_rank: Frequent Contributor
timestamp: September 07, 2013, 10:24:38 pm
content: I'm trying to troubleshoot my old car stereo and solve a problem that it had since quite a few years: plugging in a power-hungry device (>100mA) resets the stereo or it even keeps it in reset until the load is removed.Text description of the power circuit:The 12V line is going through two 3W resistors in series, one is 4.5 ohms and the other is 5 ohms.It then goes through an unknown part (B I 240, 3 pin DIL), could be a transistor. Pin one is the 12V in, pin 2 is the 12V out, pin 3 is ground.The 12V goes to an LM7805CV and the resulting 5V is fed into the USB port. There are no other 5V loads.First issue is that the resistors seem horribly abused, there is some black stuff under them that looks like glue but I haven't seen that type of 'glue' anywhere else inside the unit. The sheathing is cracked on both of them and they have blisters.Before the first power resistor the 12V is clean, after it it has some periodic 1V dip (5-10ms forgot to measure). After the second resistor the value is already below 11V so there must be something loading the line or the resistors are completely gone.When a load is applied the 12V (10V) line dips to 6V and the unit resets. Without a load the 5V output seems very noisy, as seen in the screenshot.The strange thing is that I was not able to find why the reset happens, no traces go from this supply to anywhere else inside the unit.I have several questions, they might get answered or I might solve it somehow in the meantime:- what is that 3-pin part (BI 240, RS)?- what are the resistors for? Are they taking a part of the linear drop? Or crude current limiting?- is it worth bypassing everything and just using the 7805 as it is? The 12V input is already switched by the unit, it goes live only when the unit is on- is there anything wrong with the overall design? It is a chinese unit after all but it seems pretty well built otherwise and has been running for 5-6 years

author: brainwash
author_rank: Frequent Contributor
timestamp: September 08, 2013, 12:23:26 am
content: Please ignore the first circuit description, it's late here.The power resistors provide current limiting (?) for at least two power circuits: 5V for the USB and ~9V for the CDROM drive. Both are activated by some digital signals.The BI240 is most likely an NPN transistor / darlington with pins 1=C,2=E,3=B. Collector is connected to 12V, base is held high (12V) by a 100k resistor. The emitter goes to the 7805 input. A small NPN transistor (NF BH?) is pulling down the base through a 1k resistor, when triggered by a digital signal.I've noticed that in some cases the base of the big transistor is at 10V instead of 0V [when active] so it might mean that the little transistor is dying (not able to pull down 0.1mA).Still not sure what causes the reset, too late to investigate today.

author: Andreas
author_rank: Super Contributor
timestamp: September 08, 2013, 06:44:08 am
content: The resistors look not sound.The upper seems to be broken.The lower seems to be burnt.With best regardsAndreas

author: SeanB
author_rank: Super Contributor
timestamp: September 08, 2013, 10:47:57 am
content: Replace both resistors and the 7805, and mount the new one on a heatsink like the metal frame next to it. Use 5W ceramic resistors in place of the 1W ones. Short wires to link the 3 pins of the regulator to the PCB and a little thermal compound between the regulator and frame and it will do 100ma reliably. If you want to be able to do 500mA then feed the regulator input from 12V via a 3R3 5W resistor, and use a 100n chip capacitor soldered across the pins of the regulator input and output to the centre common to bypass it.

author: peter.mitchell
author_rank: Super Contributor
timestamp: September 08, 2013, 11:59:52 am
content: while you're at it you might want to stick a 5.6v zener across the output.

author: brainwash
author_rank: Frequent Contributor
timestamp: September 08, 2013, 12:04:03 pm
content: Here's the schematic.I thought the big transistor isn't fully driven but the 9V rail is identical and has almost the same voltages while providing a stable 1A load current.BTW, the 9V rail has a different topology, it is a transistor follower circuit with a zener.So far things point out to a broken 7805 which would be the happiest situation. Even with no load there is a significant voltage drop compared to the 9V circuit.Also, it seems underdesigned as: 12V going through a 10 ohm resistor limits the current to 1.2A. The power dissipation at that current would be 14.4W, way above the 6W provided by the resistors. With a 0.5A load it's 2.5W split between the two resistors.Edit: just made some calculations and it seems that with no load it still draws 140mA, judging by the resistor drop.Please correct me if my calculations are wrong.@SeanB: I will mount another 7805 and measure the dissipation. I'm afraid there is not enough space inside as the CD reader is taking most of the inner room. I also don't have ceramic resistors at hand, though I do have quite a lot of switching supplies.

author: brainwash
author_rank: Frequent Contributor
timestamp: September 08, 2013, 01:55:35 pm
content: It was indeed the 7805 that was broken, though it still measured 4.95V at output.I've finally found what caused the reset: from the 5V line some chips inside the unit were fed through a couple of small 1117 regulators, so that was the "invisible" load causing voltage drop on the resistors.Clearly the resistors have to be replaced, at 0.5A on the USB line plus ~0.14A for the miscellaneous chips they were dropping about 3V each which means more than 10W were dissipated on them.Really crappy thermal design especially since this goes into an enclosed automotive unit. Similar to that Rigol PSU, it seems they are outsourcing PSU designs to first-year students. I'm really wondering how this goes past the design stage.I'm wondering if it's better to replace the resistors with a 317 dropping 12V to 9V and strapped to the chassis.@peter.mitchell: good idea, I'll do that since if the regulator fails high it might drag everything with it.

author: SeanB
author_rank: Super Contributor
timestamp: September 08, 2013, 02:04:41 pm
content: It passed QC in China, which consisted of powering it on and no smoke coming out. If it had advanced QC then they connected a speaker and played music through it. Note the speakers in the test setup were salvaged from a old PC from the tip outside, so as long as they made a noise that did not sound too bad or caught fire it passed.

author: brainwash
author_rank: Frequent Contributor
timestamp: September 08, 2013, 02:08:22 pm
content: I meant design not QC. This thing had A2DP back when we were struggling to implement it for OEM and the specs were not finished. And there were only a handful of phones with A2DP, running Windows Mobile. Not to mention that the complete bluetooth functionality works almost perfectly and certainly better than any OEM unit I've used.

author: SeanB
author_rank: Super Contributor
timestamp: September 08, 2013, 02:13:45 pm
content: Design made something that worked somewhat, or copied a design that worked, but did not do a correct finish by looking at layout and thermals. Then the final check was done in a nice cool lab with the covers off, and a cheap low power USB device ( which had an onboard 3V3 regulator and so was not concerned much with dropout voltage) that was the memory. There it worked, and during production test the thermal mass of the regulator was enough to keep it running. Does not work in a car dash where it can get to over 70C very easily.

author: brainwash
author_rank: Frequent Contributor
timestamp: September 08, 2013, 02:17:40 pm
content: Well, after running a 0.5A USB load for 15 seconds the resistors were both at at least 100C. While idle and in open air they sit at around 50C. I have to figure out a way to replace these with something else, preferably passive since this line is always powered.

author: brainwash
author_rank: Frequent Contributor
timestamp: September 08, 2013, 05:11:20 pm
content: I added a 7808 that was lingering in my parts drawer and mounted it to the case. That drops 3-6.4V and the remaining 3V are done by the 7805. The resistors were replaced with a piece of wire. Running a stress test on the bench to see what fails.

author: SeanB
author_rank: Super Contributor
timestamp: September 08, 2013, 05:23:50 pm
content: That will work, just replace the original 7805 anyway, it will have suffered from all the thermal cycling.

author: brainwash
author_rank: Frequent Contributor
timestamp: September 08, 2013, 05:59:32 pm
content: Yes, I forgot to mention that I replaced it. It should have a longer life dropping only 3V instead of almost 10V. So far the 7808 heatsink is not going higher than 80C and inside the car it should be less since it's mounted in a metal frame and there is some airflow by design.I'll do a detailed write-up on this, the teardown and various design decisions, like the removable faceplate 'protection'.

author: mmagin
author_rank: Frequent Contributor
timestamp: September 08, 2013, 06:03:39 pm
content: The 12 volt supply for the rest of the stereo goes through those resistors, or just the supply to the 5 volt regulator?In the first case, that's just crazy.In the second case, sounds like a clever if cheap way to reduce the dissipation in the 7805

author: brainwash
author_rank: Frequent Contributor
timestamp: September 08, 2013, 06:16:29 pm
content: Just for the 7805 and I think they have a dual purpose, intended or not. First off is the reduced dissipation on the 7805 and the second one as a crude current limiter. I have to see if the mod gets rid of resets when cranking with a discharged battery.