GREAT feedback, guys. I really appreciate it. Keep 'em coming.
Consider putting a bridge rectifier on the input side rather than a diode?
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
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?
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.
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.
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.
I'll see what I can do. :-)
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.
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.
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
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 thought about doing some kind of over/under protection using a discrete transistor... If you have a cheap and easy IC to suggest, that would be interesting.
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
the full wave bridge is to be placed at the input of the switching supply along with two 68,000uf 50v caps
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.
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?
I really don't know. I expect many of the cheap ones out there are built to generate revenue, and the well-being of the load is the onus of the end device's engineers and the user. Things that stay in the car over the long haul (your stereo and the engine computer) are probably fortified with vitamins. Just an uneducated guess.
At any rate, my goal is a PSU as robust as possible that can be built by anyone with experience soldering, while keeping the cost, size, and power draw down to a minimum.