Morex Cubid PSU Protection Circuit? (Would this work better?)

chiark · 06-09-2003, 03:59 PM

Well, the 12V of the Morex PSU is not regulated by all accounts. This isn't a good thing for the life of drives, or mother boards...

Regulators, even switchmode, have an inherent dropout of around 1.3V, which knackers us when the engine is off.

For Flyback topology, the only thing I can find suitably cheap is from national semiconductor, and that can't provide enough current in flyback (2A is around the max)

So how about combining a tanking circuit, regulator and relay to make a real bodge in the best of British tradition

The idea is that when the engine is off, the feed goes directly to the PSU. When on, it's regulated... When cranking, the supply is tanked...

Is this any good, or am I smoking something?

Cheers,
Nick.

Telek · 06-09-2003, 06:26 PM

This has been discussed before, I don't see anything wrong with it, however you can probably put the diode on the left side of the relay so that you only need one instead of two. Otherwise it's a cheap hack but it'll work.

Out of curiosity, what do you need to power that needs more than 2A at 12V? What we suggested before was to run the PSU off of unregulated +12V main from the car, and just use a flyback regulator on the +12V *output* from the PSU, which should be able to remain easily under 2A. You don't need to regulate before the PSU because all voltages other than the +12V output are regulated.

maxcosity · 06-09-2003, 08:42 PM

Telek: I think he wants to protect his PSU.

What do you need 12v out from your PSU for? I can't think of anything (except maybe an LCD but I don't think you'd want that powered from your computer's PSU - would you?)

chiark · 06-10-2003, 04:17 AM

Thanks for the replies guys

Yes, I want to protect everything, basically

If the PSU is happy with 14-15V shoved up it, then that'd be great! Is this absolutely known, as I've read somewhere on here (I've searched

) that there's been a problem with a PSU dieing?

I'm running an Epia 800, slimline DVD and 2.5" hard disc, so nothing really takes much 12v.

If the PSU will "survive" being directly connected to the rather harsh electrical environment of a car, then I really am better off just building a flyback circuit for the 12v stage out of an LM2577-12 or LM2587-12?

Apologies for raking over old ground, but I couldn't find an absolute answer. Perhaps this should be added to a FAQ?

Thanks,
Nick.

Telek · 06-10-2003, 02:33 PM

As I said, there is no need to protect the PSU itself, as the components are rated up to 18V or something like that. It's only the +12V output that is a problem. Other people have used these power supplies directly attached to their car mains, and the only issue is the unregulated +12V output. So there is no issue with the PSU dying.

You are better off just building a flyback circuit for the 12V output using an LM2587-12.

maxcosity · 06-10-2003, 11:07 PM

That is really cool, I have the same PSU and have been worried to death about this issue (see my other post with diagrams). I did read another fella had the same case/psu and had his running for 6+ months now with no problems (except the shutoff when cranking because he didn't build a tank circuit).

So as I asked above, what do you need the 12v output on the PSU for? The motherboard is 3v right? And the disks/drives are 5v right? The only thing I can think of is an LCD but most ppl run that separately...

chiark · 06-11-2003, 03:18 AM

maxcosity, it's a very very good question. I guess I should try dropping the 12v lines and see if it works - but I believe that the EPIA requires some current at 12v...

Telek · 06-11-2003, 08:41 PM

Well 3.5" hard drives and 5.25" devices all use +12V. Additionally most fans in your system will also require +12V, but you can either (a) find 5V ones, or (b) just run them directly off the car input, you don't need to regulate the voltage input for fans.

If you take a look at my power calculator, it'll give you a good idea what needs how much : http://www.mymp3car.com/MP3Car/PSUpower.asp

If you're using a P4 then the mobo will take power from the +12V rail to supply the current, otherwise most components are either +2.5V or +3.3V. I know that some computers require -12V for the RS232 ports, otherwise I can't think of anything else that actually needs +12V.

chiark · 06-12-2003, 04:58 AM

Hmm... hmmmmm...

I definitely need to drop the 12v lines and see if it all still works