Boosting Cars

Brian R. Smith

Issue 1, July 2017

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No... we’re not talking about stealing them! Brian developed a method for keeping his stereo cranking, during those pesky voltage drops when the engine starts up.

While some modern cars have already figured this out, if you have an older car, or a classic without all the bells and whistles, then you’ve probably encountered this problem. It’s also a common issue on many boats. It didn’t matter so much when we were only tuned into the FM radio, but nowadays with Bluetooth®  connections and the like, it can take up to a few minutes for the stereo to figure out what it’s doing. Brian decided to tackle this problem head-on, by developing a great system to stop his stereo from restarting. So we caught up with him to learn more.

This is definitely one of those pesky problems. It’s not life threatening, but it’s annoying! What made you start thinking about this project, and how did you first tackle it?

Yeah, I was annoyed that my car stereo would power-off and take 20 seconds to boot every time I started (or restarted) my car’s engine. I’ve had a car stereo that didn’t restart when the engine was cranking, so I knew it wasn’t an impossible task.

Once I knew that the ignition switch was intentionally cutting power to the ACC (accessory) circuit during cranking, I started with the obvious solution of a two-diode OR setup to provide the stereo with a power-on signal even during cranking – which fixed nothing. Further investigation showed the voltage sag (down to 7.5V, as recorded on a digital scope) was also a problem, and one that was much more difficult to solve.

How much research went into developing the current design?

I’ve never built any kind of switching power supply before, so I ended up reading three to four chapters of a book called “Switching Power Supplies: A to Z” by Sanjaya Manikala (2006), a bunch of different switching controller datasheets, and learning to use LTSpice while trying to find or construct a suitably beefy voltage booster. The stereo can draw 6 amps (@12V) at high volume, so the booster may be pulling 10 amps (@7.5V) in the worst case. I also wanted it to shut down, bypass itself, and consume minimal power when it was not needed - which is 99.9999% of the time.

Completed circuit
The completed circuit - case included.

The datasheet for the LT1270A provided the basic booster circuit, which I simulated in LTSpice to make sure it could turn on, and to make sure it could begin boosting fast enough that the stereo would never see an input less than ~10.5V. It’s up and running in less than 3 milliseconds, which is good enough.

Definitely a solid foundation. You have gone to the effort of having a custom PCB made. Have you developed PCBs before and were there any challenges?

I've been ordering small boards from OSH Park for years. EAGLE has a free version for hobbyists, and OSH Park imports EAGLE board files directly, making it a great option for beginners like me. Having made half a dozen PCBs already, it wasn’t difficult and the results look surprisingly professional for a hobbyist. I’d never made my own boards before that – the equipment and chemicals were daunting.

Outsourcing PCBs to those who are set up for it definitely provides a more refined result. I’m sure many of us have the skills, but in a DIY scenario the PCB manufacturers will often trump even the most meticulous DIY efforts. What challenges did your PCB design face?

The booster needs to handle currents larger than I’ve dealt with before, so I attempted to keep the high-current components close to each other, with short fat traces between them. The book I read, “Switching Power Supplies: A to Z”, has an entire chapter on the subject of PCB layout, which was helpful. The other concern is avoiding any induced currents in signal lines under the largish inductor – by keeping signal lines away from it. Toroidal inductors are supposed to have low leakage anyway, but I don’t have the experience to predict if it will matter or not.

The booster only needs to run for short intervals – half a second, typically – but I’ve tried to specify all the components, traces, and heatsink, so they could handle full load continuously. It’s overkill, but I don’t know where I can safely trim the specifications.

battery voltage drop
The trace showing the battery voltage drop, and the resulting stable output from the unit.

It’s always wise to keep the high-current routes as short as possible, with the less heat the better; on occasion, circuit tracks have been known to make a surprise performance as a fuse! Did anything unexpected happen while testing your design in the car?

One of my tiny surface-mount logic-level MOSFETs released the magical black smoke and shorted, leaving the stereo permanently on. Fortunately, this was just before arriving home, so I could immediately pull the dash apart and disconnect it. I failed to put input protection on any of the four MOSFETs which, in retrospect, was a naive mistake. Just because it works in LTSpice doesn’t mean it’s ready for the real world, and automotive electronics have to resist some nasty surges and spikes.

Oh no, the dreaded smoke release! Sometimes it provides the best insight into circuit design (and its faults). If you were to make another version, what would you change?

After submitting to DIYODE Magazine, I’ve switched it over entirely to through-hole components (the board grew only slightly longer), and added input protection for all MOSFETs. It should survive longer, and doesn’t have any of my dodgy surface-mount soldering. I should probably investigate automotive-grade components, but I won’t... until it breaks.

Well you know that old saying; “if it ain’t broke...”. What are you working on currently?

Right now I’m trying to make my home alarm send SMS messages.

That sounds like a great project. We’d love to hear about it once it’s complete! 

Reading & Resources:

Brian R. Smith

Brian R. Smith

Semi-Retired Software Engineer from NZ.