7v / off / 12v mod troubles

[Nforcer]

not sure what im doing wrong here:

red molex goes to the first pin on the switch

black wire from fan goes to the middle pin on switch

black wire from molex goes to last pin on switch

yellow wire goes straight through to red wire of fan


i get full power (12v) and nothing else??

what am i missing??

[Diggrr]

That sounds correct to me...have you tried a different fan? Some may not work on 7 volts, and some may be confused by the double positive so that the timing circuit doesn't fire the coils as it should.
Does the fan motor get warm when on 7 volts?
Have you tried the fan plugged directly into the molex to make sure it will work? It might be the switch is bad too.
It's an SPDT switch? If it's a DPDT, are you using the contacts all on the same side? (single/double pole, double throw)

One more thing about the switch, is it marked on-off-on or off-on-on? That would mix up the needed wiring diagram.

Let me know.

[Nforcer]

ok

the fan is fine - its running straight at 12v right now

it doesnt get warm at 7v to my knowledge

the switch is marked on / off / on

the switch is SPDT

when switched to 7v the psu bogs down to about half (the fan inside slows down i mean)

and other people have had success with this exact fan

sunon 108CFM 120mm fan

??

[Diggrr]

Well, if the mod is going to fight your power supply so much, you may have to use either a resistor or a rheostat to slow the fan down. Some power supplies just don't like the 7 volt mod. My Sparkle psu's see it as a short and shut down instantly.

To figure out the needed resistor use [12v-7v=5]/[fan amperage]= resistor in ohms. And make sure the resistor can handle the fan's amperage. You can also wire the resistor into the same switch, so your panel stays looking the same.

[Nforcer]

see im using this fan in a water station and the power supply is just a cheapy AT 250w psu - total cheapness

all the powersupply has to do is power this one fan

anyways the fan is a 6.8w and its rated at 0.57A

this is all enclosed in a wood box so i wont see any of it - easiest way to wire it up?

if this doesnt work im gonna pick up a rheostat and see how that goes - i have a feeling that the fan will stall out real easily though if i turn it to far down !

[Diggrr]

Oh, I see.
Well in the case of an AT psu just running one fan, you need a constant load on the 5 volt line to keep it running normally. I had to do this on a pair of AT's that were joined to be a TEC power supply (until I got my big one) so that it would put out full voltage on all lines.
On one of the other molex connectors, put a small light bulb on it and see if that helps it with the bogging down problem. Something like a 12 volt automotive lightbulb should be enough. You can just tape some wires to the bulb's base for testing it out.

I won't swear it'll work, but it's worth shot, and cheap too. Use the light as a fan/psu indicator if it works.

[Nforcer]

but the psu will turn on without anything hooked up to it

so it should be able to do the fan ??

i dunno

i think that i will get a rheostat before i get a bulb to hook up to the 5v line

anything else use 5v that i might have laying around??

[Nforcer]

thanks for all of your replies

and the switch is a SPDT switch from radio shack and has 3 "prongs"

on / off / on

rated up to 6a

275-654a is the part number i think

im starting to think this crappy new psu is just that - crappy

[Diggrr]

Got an old, small speaker (junk drawer quality)? A flashlight bulb (3+ batteries)? even an 80mm fan that would run on 5 volts? A small DC motor?

Just some ideas.

[Nforcer]

i guess i can find a fan or something kicking around

why do the at's need constant 5v load??

[Diggrr]

I'm not really sure, I just picked up the tip from an article that I read about using psu's without a computer. If I don't have a load on mine, the 12 volt only reads like 9.5 volts using a multimeter, and the 5 volt line is low too. It does work for whatever reason.

I'm no engineer, but I'm handier than Red Green.:D

[Crazy Jayhawk]

Yeah, you need to have the 5V line loaded with an AT PSU to keep the voltage stable. You could get some multi-watt resistors from Radio shack and hook them to the 5V line. Just be sure you put some sort of heatsink on them.

[Nforcer]

thanks for the reply but still no luck

i wired up a small 80mm fan to run on 5v and no probs

then i plugged in my 120mm sunon to run on 7v and it tries to spin but just wont turn over

i can feel the motor of the fan "buzzing" - wanting to spin but not enough juice i guess

weird that the exact same fan but in 80mm runs fine on 5v ?

i know my wiring is correct for the 12 > 7v adapter

im gonna try and find a rheostat that will work i guess

[Mysphyt]

Before you go for the rheostat, try starting the fan on 12v and switching down to 7. For some fans, in some cases, the voltage needed to start a fan spinning is higher than the voltage needed to keep a fan spinning. Also, do you have a multimeter handy? If so, check your voltages--is your 12v actually running at 12v? Is your 5v actually running at 5v? If your 5v is high and your 12v is low, then your 7v mod might actually try to get your fans to run at 6.5 or so--which might be too little.

[Nforcer]

well no worries

i picked up a lm317 set at 8v and now i have it wired to a switch

so low is 8v / off / 12v

works good - no heat......and fan starts every time!!

thanks for the help though !