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

**rando** · 08-23-2004, 10:21 PM

I assume the yellow switch is just your manual kill switch.

So what happens to your your fakeups relay when you use the green switch? It doesn't look like it will be energized in that case. I think you need to move the yellow diode back behind the yellow junction point to fix that problem.

As for the LED, you need to make sure it has the current handling capability to support the CNTX turn-on and your fakeups relay coil. Also, you might want to put a shunt diode across the relay coil.

In general, fuses are to protect your wires, not your devices. Your fuses should be sized to support a load no greater than the maximum current carrying capacity of your wires. Everytime you branch a circuit to a smaller wire size, you should install another fuse. The 5V regulator is a special case. Since it is not internally overload or short circuit protected, it should have a fuse at the output. The regulator is rated at 6A max so a fuse around that size is probably appropriate. If your regulator is damaged before your output fuse blows, a fuse at the input might provide some additional protection. I believe that regulator claims ~80% efficiency. So at 6A output it would pull ~7.5A at the input.

Maybe someone else will chime in with other ideas.

**Rob Withey** · 08-24-2004, 02:47 AM

For the LED to be in series with the CNTX turn-on line, you'll need to be aware of several things.

1) The current draw of the turn-on signal + the current draw of the relay coil (assuming you move the diode before the yellow junction) must be approximately equal to the required LED current.
2) The LED will drop approx 2.2v from your supply, leaving the rest for the CNTX and relay coil. Ensure that the CNTX and relay coil are happy with this lower voltage.

It may be easier to use a PNP transistor to drive the LED. Base to the accessory line, Emitter to the battery line, Collector to the LED (+resistor) down to ground. You'll need a resistor at the base of course.

**Gibson99** · 08-24-2004, 07:53 PM

i never thought an led could be so complicated. i think i'll just use a little incandescent lamp in paralell from my switch to ground instead of an led. a lighted automotive toggle/rocker switch ought to do it. sure, an incandescent will make some heat, but won't care about the resistance of the trigger of the CNX or fake ups relay.

rando - i guess i hadn't thought about the fake ups fully... i drew it that way on purpose thinking that i would not want the normal 10-15 minute delay from the cnx, but rather an immediate shutdown once i turn off the manual switch... but then again, if the fake ups is "on battery instead of mains" then it'll shut down as soon as the ups software finished loading on boot. duh. besides - the carnetix will still be giving power anyway, so yeah i need to move it back on the other side of the diode.

yes, the yellow switch is manual kill so i can turn the car on and leave the pc off.

and thanks for the education on fuses. this is a great forum!

if i'm still awake when i get home tonight (still at work at 7pm and it looks like i won't be leaving before 9...

my shift was supposed to end at 5!!) i'll move the diode and fake ups relay trigger, and add some fuses. i'll also change up my green switch and light to something easier.

oh yeah - you guys both mentioned a diode across teh fake ups relay's coil. any particular type of diode? it should "point" towards ground, right? most of the car alarms i install have all built-in relays for everything, so it's rare for me to use a diode anywhere in an install nowadays.

**rando** · 08-25-2004, 12:44 AM

The diode across the relay should run in the opposite direction -- you don't want it to short circuit your ACC. Here is what happens. When the relay is energized, the coil retains energy via it's generated magnetic field. When you cut that circuit the magnetic field around the coil collapses. This causes a voltage to be generated in the opposite direction. This voltage can be several thousand volts -- more than enough to damage any active devices hanging on your ACC rail. The diode is there to shunt that voltage and to force the energy to harmlessly dissipate within the diode and the coil windings. Since the collapsing field can produce very little current, the diode isn't harmed.

**Gibson99** · 08-25-2004, 08:00 AM

ok, i think i did everything right... the image above should be updated, but in case you're lazy like me

and don't want to scroll up, here it is again:

**rando** · 08-25-2004, 10:50 AM

OK, it looks better.

You should replace the 10A yellow fuse with something a bit smaller. That PSU is 6A output MAX. I don't know if it will pop the fuse before it takes damage from a short or overload. Maybe a 5A fuse would be better?'

Now that you have the lamp wired appropriately, you can replace it with a LED and resistor in series. The rest of the green circuit would remain unchanged.

**DjBac** · 09-11-2004, 09:54 AM

I need a 5V regulated power supply for an audixy (usb), usb hub and lapto slot loatd dvd-rom...
I've found something, but I have a question...
What should I use a +5V output only or a +5v/-5V output???
+5V output is rated @ 12A
and +5V@10A/-5V@2A

**Gibson99** · 09-11-2004, 09:59 AM

Originally Posted by DjBac

I need a 5V regulated power supply for an audixy (usb), usb hub and lapto slot loatd dvd-rom...
I've found something, but I have a question...
What should I use a +5V output only or a +5v/-5V output???
+5V output is rated @ 12A
and +5V@10A/-5V@2A

most usb and other 5v devices don't need a -5v rail.

can you link us to that 12 amp 5v supply?

**DjBac** · 09-11-2004, 10:01 AM

So I only need a +5V output and ground to the chassis of the car????

http://www.astrodyne.com/astro/dept_dcdc.asp?dept_id=6

**Gibson99** · 09-11-2004, 10:18 AM

Originally Posted by DjBac

So I only need a +5V output and ground to the chassis of the car????

http://www.astrodyne.com/astro/dept_dcdc.asp?dept_id=6

basically.
something you may want to consider though - the spec sheets on those power supplies show that they work fron 9-18vdc input. that may not survive crank on a lot of cars. notice how the carnetix works from 7.5-18vdc... and it does survive crank.