1. Home
  2. Embedded Programming
  3. Power supply in automotive environment

Power supply in automotive environment

Sadly, I'm running up against the limits of my meager mixed-signal design knowledge. I'm trying to put together an embedded device intended for use in an automotive (well, motorcycle) environment, and am trying to figure out how to power the damn thing.

A few of the components I need to use (video overlay chip, MEMS accelerometers, op-amps) require +5V. And since I'm going to to be doing analog sampling and then A/D conversion, I suspect the power has to be fairly clean. None of my signals are going to be above 100Hz, so I might even be wrong on that point.

Anyway, as near as I can tell, my options include powering from the vehicle +12V system via a linear regulator, using a lithium-ion battery along with one of the nifty USB-powered charger chips and a switching regulator to step up to +5V, or just whipping up a 6-cell NiMH or NiCad pack and regulating it down to +5V, then using an external charger to recharge the thing.

The switching option seems like the best combination of operational and design simplicity, but I'm worried about the effect of the 50mV P-P 1MHz power supply ripple on my already poorly-understood analog circuitry. It seems like I should be able to LC notch filter that out, but is this going to mess up the feedback on the power supply chip?

The vehicular +12V supply seems the next simplest (operationally), but I've heard all sorts of nasty things about the quality of that power, and how it's full of noise and spikes, occasionally reverses polarity, etc etc. Is handling this just a matter of putting in appropriately rated caps and a reverse-polarity protection diode, or is there more to it?

Is implementing this going to be so much of a pain in the ass that I should just solder together a bunch of AA NiMH cells and accept the need to swap out battery packs every now and again?

(note: I'm not planning on actually selling this to strangers, so I'm not concerned about UL testing or functioning 100% of the time, just "mostly working".)

-jake

Reply to
Jake McGuire
Loading thread data ...

The one sentence, mostly working answer is Nat Semi Simple switcher for +5v and a 5 volt A/D with an internal 4.096v reference. Pay close attention to data sheet recommendations for reference bypassing and/or filtering.

Put a diode and a fuse in series with the battery, followed by a 20-some volt zener across the regulator input and you should be reasonably safe. I would not guarantee it for mass production though.

Reply to
Jim Stewart

First verify that the supply is 12V, most motorcyles use a 6V supply. If this is the case and your current depands are small a zenner diode and a few capacitors would be my choice.

Reply to
Grant Killey

That was true at one time, but not for about the last 40 or more years.

--
========================================================================
          Michael Kesti            |  "And like, one and one don't make
                                   |   two, one and one make one."
          mkesti@gv.net            |          - The Who, Bargain
Reply to
Michael R. Kesti

It depends on your power requirements.

If it's a few hundred mA or less, I'd use a series diode, a reservoir capacitor and a linear regulator (78M05 etc). You could include a series L to reduce "noise".

The limits of this circuit will likely be dissipation and cooling of the regulator ((12v - 0.7v -5v)* Current)

For more than this, use a monolithic switcher (Linear Tech make some very nice ones and their website has afree simulator for designing with them). You can get wound components for these from many suppliers (e.g. Coilcraft)

Reply to
Gary Pace

You can put an LC notch filter or just a lowpass filter after the regulator, but AFTER the feedback. Do not include the filter inside the feedback loop, or it will likely oscillate. You could also put a linear regulator after the switcher, and if you use a low dropout part, the losses aren't very high. Just remember to set the output voltage of the switcher high enough to give you the headroom required for the linear reg.

On a 12V system, usually you have to be able to handle at least 24V on the input side. Put in a reverse polarity protection diode, and a fuse (or polyswitch), then follow that with a 24V power zener to ground to limit incoming voltage spikes. We used to use a MR2535L part from Motorola which was meant for this specific purpose. A datasheet can be found here:

formatting link

I would think that you should be able to get standard regulators to do what you want, without having to resort to a battery supply.

Mike Anton

Reply to
Michael Anton

As someone else also stated, do not use a bridge, just a single diode in the positive supply line. You should have your circuit referenced to the vehicle ground.

There is no issue with the negative spikes, as the MR2535L is not bidirectional. It looks like a standard zener diode, so since its forward voltage is around a volt, that is what it will clamp your negative spikes to.

Sure, but only if your rectifier bridge is fast enough, and rated for the high instantaneous currents involved. But the method above is better.

Mike Anton

Reply to
Michael Anton

Look at the LM2940. It was designed for vehicular use; it has short-circuit and reverse voltage protection.

Reply to
Alan Holt

ElectronDepot website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.