How safe is running something designed for a car 12V on a computer?

Nazo

Nazo

2[H]4U
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Apr 2, 2002
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I hooked up a cigarette lighter plug on my PC primarily just so I had something nice and simple to plug my portable DVD player into (this comes in handy because it has a video in and can act as a very poor quality monitor in an emergency or something, but, mainly I'm just lazy and wanted somewhere handy to plug it in.) Well, I have no idea how much current it draws when hooked up to 12V, but, since the manual says it absolutely will not charge the battery when hooked up to a cigarette lighter, I imagine it's nowhere near to the 13.5 watts it runs at on the 9V adaptor that came with it for charging. I feel pretty confident that this particular device uses a very minimal amount of power really, but, it gave me a thought that now I have a cigarette lighter plug hanging out of my computer and theoretically I might be able to use it with other things as well. I don't know how much power is left over after all other devices in my PC are considered, but, I do suppose it's more than the highest universal adaptor I've seen for a car so far (800mA max @ 9V max.) From what little I've seen about car cigarette lighter plugs, they are considered unreliable and it sounds like they aren't expected to be able to provide a lot of power, so I'm thinking that maybe a lot of things designed for them really won't use much power. It looks like it's causing no noticable effect since I can open up my temperature/etc monitoring program and watch the fan speeds (latest bios, or maybe the mb itself, no longer has a 12V sensor) and, as before I plugged in the player, the fans stay almost rock solid on the same speed with no more variance than before (I think the most I've ever seen them drift was around 50 RPM. This PSU is amazing IMO.)

Also, just to be sure, there's not any chance that they could do damage to the system, right? I'm pretty sure there isn't, but, better to ask and make sure than to find out the hard way. The thing to bear in mind is that I'm talking about simply plugging in a standard car adaptor style plug, which goes to a molex connection.
 
Nazo said:
Also, just to be sure, there's not any chance that they could do damage to the system, right?
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Actually, car power is pretty dirty, and you get some very nasty overspikes, particularly when starting the car.

A good power adaptor can clip and filter to a safe level...but most are not designed for running sensitive electronic circuits. At the very least, I'd keep the thing unplugged when you're starting or stopping the car.
 
Well, one time I plugged in my laptop based GPS navigation system into the cig lighter socket, there was an arc and smoke. (somehow the power leads in the plug had twisted and shorted). The car fuse burnt out and the auxillary electronics stuff in my car wouldn't work (radio, etc). Car was fine when I replaced the burnt fuse and I repaired the GPS cigarette plug.

So at least put a replaceable fuse in series with that cig lighter/molex connection so you don't accidently burn out your PSU. Cheaper to replace a fuse than to replace a burnt out PSU.
 
Hodiscuss, good point. I may still do that, but, the only thing to bear in mind is that the PSU has a safety cutoff if you accidentally short. I have had the unfortunate bad luck to find this out a few times in the past due to sheer carelessness. Oh, and I too have had an incident where a metal part came loose and caused a short in a real car. Screwed up the elctrical system of my old accord that way. The cd player never worked quite the same after that and the speakers sometimes would pop and such (probably messed something up, but, I don't really know what.) One thing to bear in mind is that it's a lot easier to have such a thing happen when you have a little hole in the dashboard to plug things into versus a little hanging thing that I have to lift up and plug the plug into. Anyway, I think you can damage PC components that way, despite the safety shutoff, so I'll at the least be careful, and I may well go with the idea of putting a fuse in there after all since it does make good sense.

masher said:
Actually, car power is pretty dirty, and you get some very nasty overspikes, particularly when starting the car.

A good power adaptor can clip and filter to a safe level...but most are not designed for running sensitive electronic circuits. At the very least, I'd keep the thing unplugged when you're starting or stopping the car.
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Backwards. I'm running car devices on the PC. I know they have to handle a lot of trouble from a real car while the PC runs somewhere darned close to 12.0V (too lazy to test exactly how close mine holds right now.) That does make me tend to think that there may be a bunch of components that play around with the power a bit and I just wanted to be sure it couldn't be a problem on a more sensitive PC.


Anyway, to be honest, I ask this question to be sure, but, really, I don't know that I WILL ever plug anything else in there. Only thing I can think of off the top of my head that I might even want to plug in would be maybe the very bright emergency light I have, which would be quite handy for seeing inside of a case. I'll bet it uses even less power than my portable player, and I doubt I'll ever really do even that. But, just in case I should ever want to, I wanted to ask. You never know, I might someday want to hook up a mp3 player or something.
 
I would suggest a few things if you want to put a cigarette lighter socket on the front of your computer.

1) A fuse, with a rating that depends on the size of your power supply. I would suggest no more than 1A.
2) Power diodes coming from the 12V rail and going to GND. In case something happens in the device you're plugging in, you don't want to be putting more than 12V back into the PSU or trying to pull GND lower.
3) Put a decent-sized cap across the load. It'll help avoid current surge when you plug something in.

All together, it should look something like this:
Code: 
12V----~------->|-----+--- to load
    (fuse) (diode)    |
                      = (cap)
                      |
GND--|<---------------+------from load
Now, you'll get somewhat less than 12V going to the load, but that's okay, since just about everything you plug in will have its own internal regulator dropping the voltage down to whatever the load needs, regardless of the incoming voltage.

 
That sounds pretty reasonable. I'll try to do that later. BTW, I tested the voltage from the PSU (I said I was too lazy, but, that was before I realized I had a handy way of testing sitting right next to me without opening the case.) It's about 12.3V. A tad higher than I like for a PSU going on about how stable and great it is, but, still way within acceptable ranges. Who knows, that might drop the voltage to the plug to about 12.0V and I'd be sorely dissapointed if any device made to plug into a cigarette lighter couldn't handle that.

Since I'm a little more amaturish than you, can you be more specific about what you mean by "a decent sized cap"? Also, I'll be honest, I'm not sure what polarity it should be. The negative end on the voltage line, or the other way? Well, on the good side, I explicitely tested to see if it made a noticable jump when I plugged the thing in, and if it did it was at least so quick it didn't register on the monitoring program.
 
Nazo said:
Since I'm a little more amaturish than you, can you be more specific about what you mean by "a decent sized cap"?
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Well, you'll get lots of opinions. As for me, I say "the bigger, the better!". Of course, I have a few 6600uF caps sitting around that I pulled from busted PSUs. I guess it depends on what kind of load you're getting. Something like an electric motor will have a lot more inrush current, while just about everything else won't have significant (to your PSU) inrush current.

In other words, I don't know. :p And if you're not running a high-inrush-current device, you will be able to do entirely without.
Also, I'll be honest, I'm not sure what polarity it should be. The negative end on the voltage line, or the other way? Well, on the good side, I explicitely tested to see if it made a noticable jump when I plugged the thing in, and if it did it was at least so quick it didn't register on the monitoring program.
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The negative leg gets attached to the wire headed towards ground, and the positive gets hooked up to the slightly-less-than-12V line.

 
I have an old chunk of a board that was a power supply for some sort of device (couldn't say what device anymore since it's been hacked off for a long time, but, I'd guess an old radio or something.) It has a couple of gigantic capacitors rated at 200V, 220uF. Does this sound sufficient? I know it's not even on the same scale as 6600uF, but, they are made to take a serious beating at least. I may be able to find higher if I keep digging through my box of old junk, but, these are all I see easily at hand so far.

Anyway, thanks for all the info. I like the idea of adding a little insurance to the thing considering that my computer's total value is somewhere in the area of $1K.
 
A Instead of a cap, he'd get more protection by hanging a zener diode in parallel, to guard against a opposite-polarity overvoltages. You may think the series diode does this, but a little thought should convince you otherwise.
 
Well, I was going to ask you what that was, but, I saw it on wikipedia. Sounds like something I'd have to buy though. Lol, I was hoping to do all this for free.
 
You can buy it all from an electronics supply shop such as Jameco or Mouser. The components should cost you all of 30 cents or so...the shipping will be the big expense ;p
 
There's a radioshack here, I just didn't want to have to go to such lengths since I thought I had everything on hand. Lol, just worried things can get out of hand if I'm not careful for one thing. Anyway, from what I see there, it looks like if I were to go the zener diode route, I'd have to choose one with a cutout voltage appropriate to the application. Any idea which specifically I should be looking for? Sorry, I just don't entirely understand how these things work, despite reading it on wikipedia. Or, a more accurate question would be, what specifically do I look for since I've never bought one before?
 
Get one with a breakdown voltage in the 12.5 - 13v range. In parallel across the output leads, it'll clip output voltage to 12.5v and (accidental) reverse voltages to zero.

Also, instead of a fuse for a current limiter, I'd recommend a thermistor instead. Again, its a part that should only be pennies....though Radio Shack will probably charge you a few bucks for one.
 
Ok, I think I understand about the zener diode, but, you lost me on thermistor. Wikipedia didn't help either. Lol, I just don't see how it replaces a fuse.

And yeah, radioshack is a total ripoff, but, unfortunately, it works out about the same when you include shipping. Plus I can drive down and pick things up rather than waiting who knows how long.
 
Lol, I just don't see how it replaces a fuse...
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You can't see it, because it doesn't...I had a brain fart, and meant a PPTC (thermoelectric) fuse...not a thermister. Think of it as a little resettable solid-state fuse. But if you can't find one at Radio Shack, a normal one will be fine.
 
Oh, that makes sense. I see that it's supposed to be a type of thermistor. Hmm, wikipedia says that the way it maintain's it's temperature is to allow some bit of current to pass through. Wouldn't that still be harmful? I like the idea of a replacable fuse since that just simply blows and that's that. I even have a few on hand, though I haven't found a 1A fuse yet. But, if the PPTC passes such a minimal amount through that it's basically completely harmless, then I suppose that would be less troublesome theoretically (though I plan to never see a situation that could cause a fuse to blow if I can help it at all.)
 
Nazo said:
Sounds like something I'd have to buy though. Lol, I was hoping to do all this for free.
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Thinking outside the box..
An alternative would be NOT to connect to your PSU...

Buy the transformer brick that has switchable variable DC output (even reversible plug polarity) along with the assortment of plugs. You can find this at Radio Shack, Walmart, etc...
 
That kind of wasn't the point. I have an ordinary AC adaptor -- one which allows me to charge the battery when an universal supply would not (requires 1.5A max load it says, and the most I've seen in a universal was 800mA -- and that a car adaptor. Most I've seen for an ordinary household universal adaptor was 500mA.) So why would I pay for a universal adaptor when I have one which won't fry if I plug in the battery?

You may note the section of the forums we're in? Anyway, like I said, I'm not even going to use it all that much, it's just a convenience. Oh, and the cost of a universal adaptor is something along the lines of $15-$20 walmart price, versus the components needed to add to this circuit probably being in the area of $5 radioshack price (aka being ripped off.) Not to mention that many of those parts I have lying around in various boards and such.

Don't say "thinking outside the box" because that's inside the box. Everyone buys ac adaptors. How many set up a little debugging LCD screen that can be hooked up to the video output of a PC with a case designed for carrying (complete with a nice little handle)? All I need is one outlet with one free plug to get by. Mind you, I'll be the first to admit that the screen totally sucks for that purpose, but, it's only there for an emergency. Plus, who knows, I may find some other similar use -- maybe even one that's more useful than this one.
 
masher said:
A Instead of a cap, he'd get more protection by hanging a zener diode in parallel, to guard against a opposite-polarity overvoltages. You may think the series diode does this, but a little thought should convince you otherwise.
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The series diode will protect against the load feeding back overvoltage into the PSU just fine. The capacitor was there to protect the PSU from inrush current on the load, not overvoltage. BTW, the two diodes are the method used on a ot of IC's to shunt ESD to either the power rail or to ground.

 
fat-tony said:
I don't think you'd have any problem just hooking it up, since you can actually buy this: http://www.thinkgeek.com/pcmods/cables/6b35/
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ROFL. Well, I suppose it could have all that stuff in there somewhere where you can't see, but, it does look pretty simplistic. I do find it interesting to note that they mention actually using a cigarette lighter. Considering that it's basically a very small very hot heater, I'd say that draws about as much power as you're ever going to get, and the makers surely would be prepared if it fried PSUs. Anyway, I'll go ahead and go with Mohonori's suggestions because, well, why not, right? It won't take long, and I can do it with junk I have on hand.

EDIT: BTW, I found a couple of 1KuF 35V capacitors on an old printer. That's no 6.6K, but, surely 1K should be quite sufficient so long as I don't plug in something rather a lot more serious than anything I can imagine ever plugging in. (BTW, I don't smoke.)
 
Mohonri said:
The series diode will protect against the load feeding back overvoltage into the PSU just fine.
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It will protect against an overvoltage...but not against the more common occurence of a reverse polarity connection. In that case, the PSU's ground will be at a higher potential than the power rail, so the series diode doesn't prevent current flow.

Furthermore, If you think about it a bit, you'll see that the one zener diode can not only clip a reverse polarity condition to zero...but it also clips a forward polarity to its breakdown voltage. Meaning the series diode isn't even strictly needed in this case.

The capacitor was there to protect the PSU from inrush current on the load
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But a PC PSU is a switching power supply, not a linear converter. So it already has heavy output capacitance to correct overshoot. Even if this output wasn't going to be fuse-limited to a small fraction of the total PSU capacity, extra protection here isn't needed.
 
Nazo said:
ROFL. Well, I suppose it could have all that stuff in there somewhere where you can't see, but, it does look pretty simplistic.
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Bear in mind, this "cigarette lighter" is a hardwired, purely resistive load. So there's no inductive spikes (as you would get from plugging in a small motor or such), no chance of ESD, accidental overvoltages or reverse voltages, etc, etc.

Strictly speaking, you don't _need_ any extra protection. But if you're going to be plugging in random items, a few cents protection can save you the loss of a $100 power supply.
 
masher said:
It will protect against an overvoltage...but not against the more common occurence of a reverse polarity connection.

Furthermore, If you think about it a bit, you'll see that the one zener diode can not only clip a reverse polarity condition to zero...but it also clips a forward polarity to its breakdown voltage.

But a PC PSU is a switching power supply, not a linear converter. So it already has heavy output capacitance to correct overshoot.
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You're correct on all three counts. You could use regular diodes to replicate the reverse polarity protection--just add two more diodes. heck, this is starting to look like a bridge rectifier! :p A zener diode is a more elegant solution, though. And you're right about the capacitor being unnecessary--I just wanted to ensure that whatever the external load is, it wouldn't affect anything feeding the actual computer components.

 
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