Modding 7.5v AC adapter into PSU

[DarkDraco]

Im rebuilding one of my computer and i wanted to get rid one of the AC adapters hanging out the back of my PC. its a 7.5v adapter that i would like to run straight off the PSU. Can anyone point me in the right direction

[EvilT]

Im rebuilding one of my computer and i wanted to get rid one of the AC adapters hanging out the back of my PC. its a 7.5v adapter that i would like to run straight off the PSU. Can anyone point me in the right direction
what is it powering?

[four4875]

probably a little psone lcd screen?

what i would do is get some diodes with like a 1 volt forward drop, and use 4 of them to drop it to 8 volts, then a diode with a half volt drop, to take it to 7.5v. just run all the diodes in series with the power line from the 12v rail.

[klingens]

How would you get diodes with 1V drop? Zenerdiodes?

You will need a LOT of them in parallel to make it work if you want to power more than a single LED :)

Use a 7875 which is a 7.5V voltage regulator. Add two 0,22uF tantals at input and output and hook it up to a 12V rail. Can deliver up to 1A and bigger rated ones are available

[DarkDraco]

can i get one from radio shack?

[klingens]

probably yes, but I'm no american so I wouldn't know.
7800 series is very common however. If they can't get you one, get a 7809 and add two 1n4001 diodes in series behind behind it. That reduces voltage 1,2V-1,4V approx.

PS: Farnell shoul have them for sure.

[four4875]

any rectifier diodes should work, and will ahndle a nice bit of current, more than a 78xx. and my local radioshack only stocks 7805s and 7812.

if the forward drop of the diode is a little over or less (i seen as low as half volt drop) you can compensate, just work them in series to get the voltage you need.

[cyberey66]

you can also just obtain 7V by hooking up the devices ground to the 5V and the positve to the 12V. Depending on the device, this should work fine.

[klingens]

Depending on the device you want to power. If it's a secondary display for example, it's a great way to let the magic smoke out. Differing ground while transmitting data signals.... *brrrr*
Any device that has more connections than just this power to the rest via data cables has a high chance of either malfunctioning or killing some chip.

[cyberey66]

Depending on the device you want to power. If it's a secondary display for example, it's a great way to let the magic smoke out. Differing ground while transmitting data signals.... *brrrr*
Any device that has more connections than just this power to the rest via data cables has a high chance of either malfunctioning or killing some chip.

Good post, thats true and we don't want to let the magic smoke escape. Thats thats why I said depending on what it is. Idk what he is trying to power, if it is isolated and just requires power then it should work. If not, a voltage reg like you suggested or a 5v zener diode could work. I'm sure radioshack has a decent chance of having either of the above.

The only thing about picking a diode is making sure the power is rated correctly. The power being dissapted by the diode should be about (voltage drop of diode) * (current drawn by device). Should give you a ballpark range.

What are you trying to power? If it has a tolerance to work off ~7 volts, a 5v zener diode could be a cheap solution.

[DarkDraco]

its an LCD screen

[klingens]

Then use the voltage regulator, zener diode will never give you enough current.

[DarkDraco]

ok so when i go to radio shack, what would i ask for?

[klingens]

1 7875
2 Tantal capacitors 0,22uF
1 Molex connector
1 circuit board looking like this: http://www.musikding.de/images/aloch.jpg

Depending how much wattage the panel needs, a heatsink for the 7875 or even a 2nd 7875.

[DarkDraco]

they didnt have any tantalum ones. is polyestr film ok?
and im still trying to find a store that has 7875, they have everything but that >.<

[DarkDraco]

ok i checked some stores abd they only have 7508 and 7512, will any of those work?

[four4875]

well... the 7508 will give you a half volt too high, but i think the lcd would be plenty forgiving enough to take it fine.

radio shack should also have a variable output one with a schematic on the back for the right diagram, could use that with a VR to adjust till you have 7.5V

[klingens]

No clue what a 75xx series is supposed to be. Either a 78xx series or if not possible, a LM317 which is a variable volrage regulator (nneds a 240Ohms resistor and a poti to define the voltage)

[DarkDraco]

im sorry i meant 7805 and 7812

[four4875]

haha, i cant believe i didnt even notice the 75 instead of 78....

my local radioshack is the same way, only 5 and 12V regulators.

[DarkDraco]

what should i do? x_x

[klingens]

Use can use a LM317T setup, variable voltage regulator 1.25V-20V 1.5A max

[DarkDraco]

how would i go about setting that up? o.O

[four4875]

http://casemods.pointofnoreturn.org/vregtut/tutorial-full.html

that should answer any questions you have :-)

edit: on a computers psu i wouldnt worry about the cap even, i doubt theres that much ripple on the power.

[DarkDraco]

so after i make one of these things, i just cut off the AC plug end of the power cable and hookup ground to ground and power line
to the V out?

[DarkDraco]

just looked over that page. is there a way i can set it to 7.5 without the knob? im trying to have this use as little space as possible.

[DarkDraco]

i decided to pop open the original adapter for the screen and i attached a pic of it to this post.
the pic is rather bright but you get an idea of what it is. i was wondering if it would be easier to just solder the blue and brown lines on the left that connect to the plug to the PSU.

[klingens]

The 120V side of your PSU is "hot" 24/7. The ATX Power On "switch" only runs at 12V like the rest of the secondary side of the PSU.So: no.

http://www.national.com/pf/LM/LM317.html to be more exact: http://www.national.com/images/pf/LM317/00906301.pdf
Look at this. it's a circuit for up to 1.5A and due to the pot can output from 1,25 to Inputvoltage-3V,in our case approx 9V.

PS: you'd want the LM317T version in To220 case.

[four4875]

well, if you were to wire it in like that, it would be the same as the wall adapter being plugged in all the time, which is normal, isnt it? if you do go that route, be sure to get the wires right (one is hot and the other is common) and insulate everything properly, and dont shock the **** ot of yourself while inside the psu (200+ V pulsing DC on the one heatsink, gives quite the zap, and you'll be working with straight 110 AC as well.

[DarkDraco]

i ended up trying to make the adapter. here is a diagram of my exact setup.
but for some reason i get 8.5-9.0 voltage readings. how would i lower it 1 more?

[klingens]

You use 2 and 3 resistors, each with 5% variance. Output voltage is almost solely dependant on the resistance ratios of your two resistor "blocks"

[DarkDraco]

so i shouold go back and replace all the resistors or can o add more in?

[klingens]

replace. only use ONE resistor for each of the 2 places. Or use a potentiometer

[four4875]

you could use a pot, get the voltage set, and then read the resistance, and then get a fixed resistor of as close to that value as you can. but i dont see why you couldnt use a little trimmer pot, they dont take much more space than a resistor, a little taller. the little pcb mounted kind that you use a screwdriver to adjust.

[DarkDraco]

bah, looks like i have to find some really small knob. seems the only want to do this =/

[DarkDraco]

temporary diagram for klingens

[DarkDraco]

temp

[Moto7451]

I don't know if anyones thought of this, but why don't you just use a fan controller or build one using the plans in the cooling section? MOSFETs ftw.

[DarkDraco]

..... went through all this trouble and cant get it lower than 8.4v...
can u post a link incase i dont find it? ty

[four4875]

have you played with lowering the resistance to ground? or increasing it im not sure how it works to tell you the truth....

it should stay variable down to like 1.5 vs.

[DarkDraco]

i started off with 1kohm to ground and i was getting 8.6-9.0 readings or so, i lowered it to as much as 200ohm and its like 8.4-8.8 now

[four4875]

is there resistance between that pin and the 12v? if so, increase that resistance as well.

[DarkDraco]

yea theres a 220ohm resistor there. ill up it a bit to see if it helps

[DarkDraco]

damn, i had a temporary setup that went from 4v-9v but as i went to put together the final product it doesnt work and im getting my old problem again... doh

[four4875]

remember what you did to get it to work properly?

if you were to... put one outside leg of the pot on the 12v side, and the other on ground.. and the wiper to the adjusting pin of the lm317.. wouldnt that give you a complete range of resistances and thus voltage present on that pin, giving a full range of adjustment? im tempted to get one to play with, see if i can figure somethin out. actually i tihnk i have one on a breadboard around here somewhere, that i bought back when i got my little lcd, but i never got around to putting it together, caus im good about that sort of thing.. buyin parts and never finishin them.. i just may have to find that. that is if i can find it... sigh...

[DarkDraco]

no, i tried figuring it out but i ended up blowing off the potentiometer. im just gonna wait till i find a place that sells LM350Ts and just follow the casemod side exactly. mixing guides gets confusing and leads nowhere...
anyone know a site i can get one from?

[Moto7451]

http://www.ocforums.com/showthread.php?t=351815

Just in case :D.

[THE_BAIT]

Sorry to link another site, But its handy
http://www.uoguelph.ca/~antoon/circ/circuits.htm

[DarkDraco]

where can i order a LM350 online? i live in US

[four4875]

Sorry to link another site, But its handy
http://www.uoguelph.ca/~antoon/circ/circuits.htm

you dont have to be sorry linking to outside sites, just not in sigs. or if you have a financial interest in the site you're linking, thats a nono. (Advertising)

the link to that site is fine, and thanks, i pass my time readin through schematic sites like that, and consider tryin to build some stuff, but mostly look at what it does and how it does so, so if i ever need to do that in the future ill have an idea how... keeps me busy and educates me, splendid!

[nd4spdbh2]

JUST do what i did... use a 5 volt 7805 regulator from radio shack caculate for the correct voltage and put the resistors in place... i use a 5 volt one modded to be 6 volts on my RC car's servos you can easily adjust the voltage of a "Fixed" voltage regulator by placing the right resistors between the Vout, COM, and gnd