Simple Automotive Project Proposal Pdf
Maricel Fergason
I'm seeing quite a few car project ideas come up. I have my own in the works and have been studying PSU design principles that take into account the unique and challenging environment that 12v electronics have to deal with. So, I have attached a switching power supply design based on posts and app notes I've read. I've taken the best ideas I found and put them together in hopes that it'll be nearly bullet-proof.
I'm not an accomplished engineer, so I'm hoping someone out there with some experience can throw in their two cents. I'm hoping to put together a prototype, put one in my car, and (continuously) run some kind of simple yet persistent sketch, where I can tell if it crashes or reboots. Like a software clock or something. I plan to leave it running until it stops, so there's a baseline for how reliable it can be expected to be. Maybe I can put one on my Ninja, too ... that's a really hostile power environment. Any car should be better than that.
Battery and fuse -- obvious. The diode (D1) protects against reverse connection, and negative spikes. The 10-ohm resistor (R1) is a current-limiting device in case of high-voltage positive spikes or a fault in the PSU. D1 will also ensure the caps inside don't discharge externally during starter cranks or spikes.
The transient voltage suppressor (D2) will clamp the input voltage to 20v. This is designed to catch alternator load dumps. The previous resistor will prevent this from turning into a short if the car is jumped at 24v, but I don't know if the fuse, resistor, or TVS will survive. I'd like to test that.
Output shorts and current limiting should be handled by the regulator -- it claims to handle such faults in a graceful manner. However, I would not oppose a 500mA fuse (ala USB power entry circuits) on the connection between this and the load. Good suggestion.
I am completely unqualified to answer that question. I just know that mechanics occasionally make mistakes and was encouraging you (and anyone else following along) to spend a few moments considering the unexpected.
It looks like R1 (the current limiter) should be 47 ohms instead of 10. With a 1W resistor, a short-term 24v jump-start should be perfectly safe. The TVS (essentially a 20v zener diode) will conduct and clamp the output to 20v, but with a real-world average of 26v input, that's still only around 600mW through R1. It touches 1W just under 28v, which would be asking too much in my opinion, so a running 24v vehicle could damage the resistor unless the fuse blows first. (This is assuming a similar curve on nominal vs. charging voltage in 24v vehicles as compared to 12v vehicles, but I have no experience with that.) Using a 68 ohm resistor would move the 1W threshold to around 29v. The trade-off here is a slightly longer charge cycle for the filter cap (C1), but with a 13.8v input, the output still reaches 9v within about 30ms at 47 ohms, and 45ms at 68 ohms.
I seem to have over-sized L1 (the input choke) quite a bit. Even at 10uH, there's very little fluctuation after the L1/C1 filter. This should keep the switching regulator happy enough. With a 24v, 40Hz AC input, I get about 300mV ripple (mind, this is before the regulator). At 400Hz, it's around 50mV. At 4k, the simulator doesn't show anything meaningful. It's well out of my reach to do proper noise analysis, though.
I'm using Circuit Simulator Applet for my tests. You can import my test circuit and play around with it. The voltage input is variable (set to -30 to +30), and can be wiggled around in real-time to test the filter.
@CB: If you'd like to move this around, feel free. I want to get this right so folks can grab a parts list, build it, and concentrate on their project instead of having to figure out how to reinvent this particular wheel.
While I've never designed a switched mode power supply, I thought it was possible to design them so that they tolerated an input voltage up to the breakdown voltage of the switch. In that case, if you target 24V or 30V or whatever as your peak input load, there'd be no normal situation where your crowbar overvoltage protection was required.
If this is intended to be a generally robust supply it would be worth providing enough output protection for either output line to be shorted to ground, 12V or the other output line. If using a fused protection, you would earn lots of brownie points by using resettable (or even self-resetting) breakers rather than a disposable fuse.
Just wondering, we plug sensitive electronics like GPS, ipods, etc into our lighter sockets all the time and they don't fry left and right. Do they have this type of circuitry built it or are our boards just sensitive to spikes and such?
three good ideas to include
1 25 A 400 piv full wave bridge on the input makes this able to be used in positive or negative ground vehicles or ones with reversed positive on the power outlet
2 on the output stage use the following 7812 + 12v reg ti makes these with a 125v input rating use that version its rated at 1.5a so use something like a poly fuse ptc -Protection/PTC-Resettable-Fuses/_/N-5g3e?Keyword=ptc&FS=True in the output also make sure to place a 4700uf cap between output and ground and a 480uf cap between input and ground they should both be rated at 100 v
3 do the same circuit but use the 5v version of the same reg as listed above this gives you stable 12 and 5 volt dc psu ill look in my design library for a power management ic that will provide a soft on function that allows the system to come up easy to avoid damage as well as over under voltage protection to prevent turn on if either is present
this should be a circuit that can be applied to direct vehicle power or to the front end of a dc switching supply be advised that the full wave bridge is to be placed at the input of the switching supply along with two 68,000uf 50v caps for the primary stabilization of the input voltage the input should have a 25A 32 volt atc fuse before the bridge and a 50A 32 volt atc fuse after the primary stabilization circuit this fuse is there to protect the secondary systems
also a 50a 1000 piv diode should be placed in the positive leg from the bridge to prevent power from returning back through the input as there wil be enough stored energy in the primary stabilization system to possibly weld with if it shorted out
this suggestion is from experience as i have built Mobil dc psu for Mobil server systems with as much as 4000A at 12 and 5 volts on the outputs and 9,000 A on the +3.3 volt line those however had a 480 volt 3 phase primary but were able to be run on 480 volts dc in emergencies and they did do that in two situations
Hmm... I didn't think about positive grounds. As devil's advocate though, I do wonder if there's any danger of being too accommodating? I.e., if the PSU works under potentially accidental conditions, it may mask an improper hookup until the load starts interfacing with external circuits. Maybe that's the user's problem. Another possible solution is to add a bi-color LED. Green = polarity OK; Red = polarity reversed. Anyone with a positive ground is likely to know this beforehand, and can either flip the LED, accept red as their indicator, or leave it out of their build entirely. I'll have to see what options exist for ultra low current bi-color indicators (or just use two in reverse parallel.) Further thoughts?
Fair point.. Ideally, I would like to be well below the maximum input range of the switching chip to allow room for things like inductor-induced swings (is that even an issue here?) and temperature derating. I'm sure I can find a suitable (cheap, easy-to-use, minimal external part count) replacement IC with a higher rating though -- I think this one is safe to 40v anyway, so there's already wiggle room. It's certainly possible just to use a higher clamping voltage on the TVS and reduce the margin on the input at the users' discretion, so maybe clamping at 28v would be a better choice. Personally, I don't think I would ever consider 24v jumping as "normal", but that's my definition, not everyone else's. Incidentally, I wonder how the average user's existing car electronics would handle this situation. Opinions welcome.
I think I might plan to have and test some various options, including the inbuilt output protection from the IC alone, and intentionally abuse it to see the reaction in each case. Then, offer some part numbers (fuse, breakers..) along with test results, and leave that up to the user based on their needs. In a perfect world, the output fuse will be nothing but a fail-safe, where the IC can and does handle 99% of faults, and the fuse only blows if something is really very wrong.
Can you explain why you would recommend a linear regulator? Is it just for simplicity? Using a power-management controller would help if there's no need for the project to be running 24/7, but otherwise, wouldn't it be prudent to use the most efficient conversion possible?
ill look in my design library for a power management ic that will provide a soft on function that allows the system to come up easy to avoid damage as well as over under voltage protection to prevent turn on if either is present
I was thinking 100v, too. 4700uF seems like a lot, though. 68,000uF seems like gross overkill. Keep in mind -- the design goal (at least for this iteration) is to deliver less than an amp at 5v. Probably much less, typically. That in mind, is there a good reason to go this high, or were you thinking of something larger scale? It would significantly complicate the requirements for the input stage, I think, to build this large without justification. That could be my ignorance, though, so please elaborate if I'm mistaken.
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