Looking for 2nd opinion: +12v & +5v only, no need for AT/ATX or neg rails

Gibson99 · 08-19-2004, 06:57 PM

well i tried to search, but terms like that are being blocked as "common used words" or it actually interprets the NOT as an operand, not a regular word.

anyway here's what i've got: a SBC mainboard that, instead of an AT or ATX power connector, has a connector identical to the one you'd see on the back of a cdrom or hdd. yup - it actually only needs +5v to run, but you get better quality sound if you give it +12v as well. so at home i'm testing it with an old 45W AT power supply, with the AT leads just dangling. for the car i want to go the direct DC-DC route for space and efficiency, but i'm not sure what's out there, and would like to build my own p/s instead of shucking out $150+ for a p/s that (imho) just has more parts to break and aren't needed.

I was thinking of getting the ITPS and feeding my +12v rail for my HD, CDROM and mobo directly from the ITPS, and then making some sort of regulator or transformer to drop the regulated +12v down to regulated +5v for the mobo and drives.

so my question is, do you think i can do it this way? i can switch it all externally with relays and switches, and can do a fake UPS no sweat (got 4 serial ports to play with!) what would you recommend as a way to drop the 12 down to 5 in a way that won't overload anything?

rando · 08-19-2004, 11:17 PM

12V: TI PT5071
5V: TI PT6653D

Or

Just build the +12 and +5v portions of Mastero's Sproggy (MK3.5, MK4, or MK4.5).

It doesn't get much easier.

Mastero · 08-20-2004, 03:08 AM

Email me and i can design you what you require ... it shall be small and you shall have a full kit to build your own ....

Mastero

rando · 08-20-2004, 08:40 AM

Having Mastero design a kit for you is quite possibly an even easier solution.

I stand corrected.

Gibson99 · 08-20-2004, 08:08 PM

cool, thanks mastero. i'll email you in a little bit once i come up with my requirements. it probably wouldn't be hard to put the relays onboard, which would not only make the install smaller (very important in a mazda mx5!), it would be simpler to install, too.

after reading more and more on the forums, it sounds like the istp (or whatever) will not work for me, since most people say it doesn't survive engine cranking, and doesn't run (most of the time) with the engine off. but the sproggy DOES. and that's what i want.

thanks again mastero.

Rob Withey · 08-21-2004, 03:23 AM

Look on my site (Hardware page). I have exactly the same situation, and built a 5v/12v power supply specifically for the em586. Based around a PT5071 and a PT6625, the entire power supply sits over the motherboard keeping your system footprint down.

These regulators even have a switch-on pin that removes the need for relays on the input.

Gibson99 · 08-21-2004, 07:49 PM

Quote: Originally Posted by Rob Withey

Look on my site (Hardware page). I have exactly the same situation, and built a 5v/12v power supply specifically for the em586. Based around a PT5071 and a PT6625, the entire power supply sits over the motherboard keeping your system footprint down.

These regulators even have a switch-on pin that removes the need for relays on the input.

so you have the voltage from your car going straight to your powersupply with no external regulation? very interesting indeed. however, i will be hanging a lot more hardware off mine than what you're running - a big 3.5" hdd, a cdrom, pcmcia wifi, usb touchscreen, and eventually GPS (likely also usb). plus my cpu, though an amd, is running at 300 mhz - more juice.

Rob Withey · 08-22-2004, 04:33 AM

Quote: Originally Posted by Gibson99

so you have the voltage from your car going straight to your powersupply with no external regulation?

Correct.

Quote: Originally Posted by Gibson99

very interesting indeed. however, i will be hanging a lot more hardware off mine than what you're running - a big 3.5" hdd, a cdrom, pcmcia wifi, usb touchscreen, and eventually GPS (likely also usb). plus my cpu, though an amd, is running at 300 mhz - more juice.

The power supply on my site should be good for 5A at 5V and 1.5A at 12V. You may need some heatsinks/forced airflow. I have a heatsink on and a fan sitting right over the 5V regulator.

Gibson99 · 08-22-2004, 11:11 AM

well after doing a little more searching and reading, i think the carnetix is what i need... except i will need to mod the cnx to also have a 5v rail. i think i can just build the 5v side of rob's mk2 p/s, which would be fed from the 12v output of the cnx. and conveniently, the cnx has an unused wire on its 6 pin output - the white one. i can probably put the +5v out into this wire. i wonder if it'll all fit inside the cnx casing... <evil grin>

rando · 08-22-2004, 11:17 AM

The carnetix is a good choice for a high power 12V regulator.

Rob's 5V supply (PT66xx based) can be fed directly from your battery...just like the carnetix. There is no advantage in powering it from the Carnetix. In fact, you'd just be wasting a good chunk of your Carnetix output in doing so. Rob's supply has a turn-on wire that can be fed from the carnetix so that they come on/off together.

Gibson99 · 08-22-2004, 11:44 AM

Quote: Originally Posted by rando

Rob's supply has a turn-on wire that can be fed from the carnetix so that they come on/off together.

duh, that makes sense. sometimes i overlook the most obvious things and make things more complicated than they need to be.

i guess i could simply tap the turn-on in the 5v section from the car's ACC lead inside the cnx, and the 5v out on the white wire on the cnx. no need to tap into anything or steal current from anywhere. the simpler the better!

Rob: what size (watts) is the 15kilohm resistor in your p/s? just something small like 1/4W just to drop the voltage down to whatever your PTxxxxs are looking for on that pin? also, if i'm reading your schematic right, i'll br bridging several pins on the PT6625 - 4-6, 7-10, and 11-13. pins 1 and 3 are the only ones with a single wire connected to them. right?

if you can't tell, i'm not that great at reading schematics, and my knowledge of electronic theory is greatly lacking. your PCB doesn't make any sense to me at all without components on it. but that's just because i'm not that familiar with circuits - more just components, such as installing radios, dvd players, alarms etc into cars, and basic home electrical wiring.

rando · 08-22-2004, 01:17 PM

A 1/4W resistor will be more than enough. The 15k & 5V zener path regulates the STBY voltage to a safe 5V level. Assuming you provide 16V max to that pin, your 15k resistor would drop 10V. P = V*V/R = 733microW.

Those pins are internally bridged on the PT6625. They correspond to input, ground, output. You should hook them all up to improve maximum current handling. Pin 1 is an output sense/feedback signal. Pin 3 is the standby (on/off) pin.

Gibson99 · 08-22-2004, 01:46 PM

i just read the datasheet on the 6625 here: http://www-s.ti.com/sc/ds/pt6625.pdf

i'm confused. on page 5 of that pdf, it says that if you have an open circuit (which i interpret as no connection) on pin3, then the regulator will supply a regulated output whenever there's a valid Vin. i thought an open circuit was what you had when you turned off the ignition.

but then there's a little table below that paragraph...

Code:

Table 1 Inhibit Control Thresholds
Parameter     Min     Max
Enable(VIH)    1V     5V
Disable(VIL)    –0.1V     0.3V

to me, that table means that if you put voltage on pin3 it turns on the output, and no voltage (open circuit?) turns it off.

obviously since putting 5v(max) into pin3 turns it on, and no voltage to pin3 turns it off according to rob's schematic, then my definition of "open circuit" is different from TI's.

maybe they mean open circuit as a direct short to ground. since my ignition switch does not ground anything when it's off (and i've never seen one that does) should i add a DPST relay before the 15k resistor to force it to ground out when i turn off the key?

edit: ok, my green is showing... i was only looking at the upper half of rob's schematic and didn't see the 4v7 diode or 4k7 resistor between ground and pin3. durrrr someone please smack me. :P

and after you smack me, would you mind telling me what the heck a 4v7 and 4k7 are? i know zip about diodes except that they restrict current from flowing one way, and my guess at the 4k7 is a 4007 ohm resistor, but that seems an oddball impedance to me.

Rob Withey · 08-22-2004, 03:35 PM

Quote: Originally Posted by Gibson99

would you mind telling me what the heck a 4v7 and 4k7 are? i know zip about diodes except that they restrict current from flowing one way, and my guess at the 4k7 is a 4007 ohm resistor, but that seems an oddball impedance to me.

4v7 is a 4.7v zener diode.
4k7 is a 4700 ohm resistor.

Low wattage is fine throughout for the enable pin.