12v to 7.5v

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fenton06

2[H]4U
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Mar 28, 2003
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Is there any *small* circuit board that would switch from 12v to 7.5v? I want to be able to power my PSOne LCD Mod from my power supply instead of from a wall outlet. Thanks!
 
correct me if mistaken...but the previous link is for an AC outlet...right? I want something that I can power from my PSU...like a molex connector.

 
Nope, you are not mistaken.
Nobody ever said you had to install the rectifier. :D
Actually the rectifier will drop some of the voltage so leaving it in doesn't hurt you at all. ;)
 
Any chance it will function on 7.0V? If you connect between the 12V and 5V lines on the PSU's output, you get that. That may be far too insufficient, but, I just thought I'd check since not everyone knows of this little trick (it was a lifesaver for me when I got a 120mm fan that was earsplittingly loud at full voltage but at 5V was turning too slowly to sufficiently cool the system.)
 
If you want an easy way to do it, just pick up a car adapter for PSOne and wire that up to 12V from the molex. If you want fully DIY, you'll have to build a circuit to get about the right voltage. The PSOne screen runs from a range of 7.3Volts to 9.0Volts usually. It has two regulators inside, one 7805 and other that I forget the number of.
 
Don't do this... chances are the power ground and video grounds are probably common in the LCD, which will short your 5V rail.

Nazo said:
Any chance it will function on 7.0V? If you connect between the 12V and 5V lines on the PSU's output, you get that. That may be far too insufficient, but, I just thought I'd check since not everyone knows of this little trick (it was a lifesaver for me when I got a 120mm fan that was earsplittingly loud at full voltage but at 5V was turning too slowly to sufficiently cool the system.)
Click to expand...
 
Nazo said:
Any chance it will function on 7.0V? If you connect between the 12V and 5V lines on the PSU's output, you get that. That may be far too insufficient, but, I just thought I'd check since not everyone knows of this little trick (it was a lifesaver for me when I got a 120mm fan that was earsplittingly loud at full voltage but at 5V was turning too slowly to sufficiently cool the system.)
Click to expand...
^^ Most abused and misunderstood mod in power supply history ^^

Works for some low current isolated devices like fans, but has the vast potential to fry major components when used improperly due to the difference (actually lack of) power ground. Can also cause trouble with the regulation of some of today's hi-tech psu's. Only use if you know what you are doing. Just my 2c.
 
Just a note, A PSOne display won't run off 7.0Volts anyway.
 
could I just build a circuit board with enough resistors to lower the voltage to 7.5?
 
if the PSone LCD runs off 7.5-9.0 volts at a reasonably low current, get your hands on a 7808 or 7809 regulator and use that to drop 12V down to 8V or 9V respectively.

Anyone know how much current these things draw?
 
fenton06 said:
could I just build a circuit board with enough resistors to lower the voltage to 7.5?
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No the resistors well have a melt down. But you can use diodes. Each diode well drop almost 1V so put 4 diodes in series. You should end up with about 7.2/8V.
 
ok...any other ideas? There had to be one, if not I'll just wait til next year when I go off to college. (Computer and Electrical Engineering, double majoring)
 
Er, there is one. In fact, two. The diodes or the regulator (diodes sound easier to me.) According to wikipedia, most diodes drop around 0.6-0.7V, so you may need more than 4 though.
 
This may be what you're looking for. It's the first circuit on that page. You will want to just leave out the bridge rectifier part of the circuit (the bottom half), and the 22,000uF capacitor but the rest you can keep (a couple small capacitors, the LM317, a couple resistors, a diode). There's even a chart there to help you choose the correct resistors!

 
fat-tony said:
HOW MUCH CURRENT DOES IT DRAW?
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I've been looking, but haven't found anything yet. I can't imagine it would take more than 5-10 watts.


 
If you're stumpted at circuits more complex than a few resistors, just buy a PSOne car power adapter and wire it up to 12V inside your case, as I mentioned earlier. The tip goes to +12V (yellow wire on a molex), and the prongs on the outside go to ground (black wire, the one next to yellow perfered)
 
tom61 said:
(black wire, the one next to yellow perfered)
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Meh, it doesn't matter which black wire you use. They all go back to the same solder pad in the PSU, and are often on the same solder pad on drives as well.

 
Mohonri said:
Meh, it doesn't matter which black wire you use. They all go back to the same solder pad in the PSU, and are often on the same solder pad on drives as well.

Click to expand...

Granted, which is why I said perfered. Some higher-end power supplies (for labratory use) do have isolated grounds for each line, not sure if any desktop computer do.
 
Mohonri said:
I've been looking, but haven't found anything yet. I can't imagine it would take more than 5-10 watts.
Click to expand...

Current is not in watts, it's measured in amps, the draw of amps is what matters because the PSU has to be able to provide enough for the LCD, along with enough amps for the stuff in your computer.
 
Mohonri said:
I've been looking, but haven't found anything yet. I can't imagine it would take more than 5-10 watts.
Click to expand...

10 watts on 7.5 volts is 1.3 amps. That means you're not going to get very far with any three-terminal linear regulator -- particularly not the LM317, as the input voltage is too close to the output voltage. (Check the data sheet for the dropout voltage vs. output current graph.)

It'll be even worse if you're pushing the input through the unneeded bridge rectifier, since that will narrow the output voltage differential even more.

I think the "Tread" circuit is right out, and that your best bet is to find or build a switching regulator.

Building your own is probably the way to go; I don't see much in the way of cheap choppers. The VI-J0R-EZ fits your needs, but it's $99 and I'm not sure that company does single-unit sales.
 
toymachineman19 said:
Current is not in watts, it's measured in amps, the draw of amps is what matters because the PSU has to be able to provide enough for the LCD, along with enough amps for the stuff in your computer.
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As you can probably gather from the message before me, you can tell all you need to know with the wattage rating since it includes current.

I would tend to say that if your PSU can handle it, probably the car adaptor will work. Cars are supposed to be able to work with a lot of power. Mind you, most devices are less than one amp, but, I know some people with insanely high powered audio systems surely use more on the same electrical lines.
 
Nazo said:
As you can probably gather from the message before me, you can tell all you need to know with the wattage rating since it includes current.

I would tend to say that if your PSU can handle it, probably the car adaptor will work. Cars are supposed to be able to work with a lot of power. Mind you, most devices are less than one amp, but, I know some people with insanely high powered audio systems surely use more on the same electrical lines.
Click to expand...

Yeah I guess I didn't think about that :rolleyes: oh well
 
mikeblas said:
10 watts on 7.5 volts is 1.3 amps. That means you're not going to get very far with any three-terminal linear regulator -- particularly not the LM317, as the input voltage is too close to the output voltage. (Check the data sheet for the dropout voltage vs. output current graph.
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There are three-terminal regulators that will work. Try this one (a 5A low-dropout regulator). You can probably go with something even smaller like this 3A one (It'll save you a whole $.09! :) ).

 
Mohonri said:
There are three-terminal regulators that will work. Try this one (a 5A low-dropout regulator). You can probably go with something even smaller like this 3A one (It'll save you a whole $.09! :) ).
Click to expand...

Good find! I didn't think low-dropout linear regulators were available above, say, 500 mA.
 
mikeblas said:
Good find! I didn't think low-dropout linear regulators were available above, say, 500 mA.
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I think there was even a 7A one at mouser somewhere, too.

 
Ok, so I was reading through a worklog here on the [H] and someone else wanted to do the exact same thing as me...woot! Someone posted this:

12vto7.gif


Now I just need to figure out how to read these diagrams... :p

 
fenton06 said:
Now I just need to figure out how to read these diagrams... :p
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Before you learn to read schematics, learn to read the thread. I've already explained why the LM317 won't work. If you want to take the frustrating path towards learning why, go ahead: build that circuit.

Otherwise, you should get a low-dropout regulator like Mohonri suggested.
 
Going from memory, the PSOne screen only takes 250mA (maybe less), so he should be fine with a 317. Mohonri was just making a WAG when he said 5Watts.
 
Well, I guess we're hacking and not designing, then.
 
Ok, so I need to build the circuit that Mohonri already posted:

317.gif


Is there some huge glarign difference difference that I am missing? I believe they both use the LM 317.
 
fenton06 said:
Is there some huge glarign difference difference that I am missing? I believe they both use the LM 317.
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Mohonri's circuit gets its power from the mains. As such, it includes a very large input filter capacitory, and a back-flow protection diode as recommended by the National Semiconductor data sheets.

The mains-powered circuit shows the regulator an input of about 17 VDC (if I'm not having a bad math day), while you were intending to give the other circuit 12 VDC directly, without the full-wave rectifier.
 
You know, it just struck me that I had an old universal car adapter in a box of junk. I dug it out and looked, and it's definitely able to manage 5W. It says 800mA and goes up to 9V (with 7.5 as one of the choices,) so might actually manage a bit more at 7.5V. It uses a LM317T (and, btw, has a very small little heatsink, so you might need to do that too.) Actually, the board in the thing is pretty simplistic, so if you think it'd help you out, I can probably provide a picture of it for you to look at.
 
Nazo said:
It uses a LM317T (and, btw, has a very small little heatsink, so you might need to do that too.)
Click to expand...

Yep; note that a car supplies about 13.6 VDC nomninally, not the 12.0 VDC of a drive rail. That's more than ten percent more headroom for the dropout voltage.
 
Why? Cars are officially supposed to do 12V, not 13.6. Anyway, that was just when I checked it.

BTW, ImageShack is having issues on the main server (though the image server is ok) so, since it finally managed to work I thought I'd go ahead and do an image just in case. If it's not of any use, that's fine, just let me know and I'll remove it sometime when ImageShack is working normally. Here's the image:


EDIT: Or maybe the image server is having troubles too. Well, it can't be helped. It will work later.
 
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