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rheostat + switch

J

jaxx

n00b
Joined
Jul 1, 2004
Messages
26
I am trying to power a pump and a fan on a 1A 12Vdc wall transformer, i want the circuit to consist of: SPST switch, to turn it on, rheostat to control the 6W 12V pump, and a rheostat to control the 2W 12V fan.

any problems?

what rheostats should I use? or even rheostats? what rating of ohms, watts, etc?

thanks,
-jaxx
 
Rheostats to control the pump? I dunno how far youd get with that, a pwm circuit might work better. but in principle
(Make sure you get pots rated for enough current)
Code: 
             |---------[Pot1]-----[Pump]------|
12V --------|                                     |--------
             |---------[Pot2]-----[Fan]---------|
Should work fine. Repeat should, dont come complaining to me if something breaks and your pc melts cos its pumps not working ;)

 
It all depends on how low you want to make the pump and fan go, but as the pump has an effective resistance of 24 ohms(at full speed, at lower speeds it's resistance will increase) you should be able to get half the current with a 24 ohm rheostat. I'd make it 50 ohms, just so you have some leeway, and if your rheostat goes lower than you need it to, then simply don't turn it down all the way, but with a slightly higher value you know you can turn it down a bit more if you need to.

The fan has an effecitve resistance of 73 ohms, so you might try for a 100 ohm rheostat.
You also need to make sure they're rated for enough heat dissipation, now without doing the math you can assume that each rheostat will never dissipate more heat than the power of the device it's connected to.

If you're interested in PWM or other solutions, check out http://casemods.pointofnoreturn.org/pwm/
 
exactly what i was thinking about the ohms, just making sure i wouldnt go crazy.

when you say PWM(pwn for me, since im a n00b at this...), what exactly is a PWM circuit?

Herulach's way does seem to be different from mine... isnt a pot the same as a rheostat?
 
Yes a pot is the same as a rheostat, as far as being a different way, how do you mean? I assumed you wanted seperate control over the pump and fan, and obviusly i left out the switchces, but aprat from that. Anyway, as far as values go, you want linear ones. and some back of the envelope calculations would seem to suggest:

Power consumption of pump, 6w--> 0.5A so assuming it has an operating range of 12 to 5V you want a max value of:
from V=IR, R=V/I
you want a voltage drop of 7V, so that equates to a 14ohm Resistor, assuming that it goes linear, basically youre gonna have to fudge it , prob go for a 20 ohm.

But serioulsy, just using a rheostat like that is asking for trouble, look into either pulse width modulation, linky. Or a proper voltage regulator to do what youre looking into, theyre both a bit more work, but also both a hella lot more reliable.

Edit: Ignore the spelling, i cant be bothered to correct it all.
AS far as what PWM is, essentially it uses a chip to switch the supply voltage on or off really really fast, the link i posted explains it all.
WIth regards to the voltage regulator, you should google it, most places stock cheap adjustable voltage regulators that will do the range youre after with little external components, just make sure its rated at at least an amp, even though you only need half, and make sure its heatsinked if it needs it.


 
ok, waht do i need for a linar voltage regulator?

yeah, it meant to say doesN'T... but i digress.

i think what i am going to do is have the fan on 100% all the time, it is a silenX fan anyways.

i think i will have the pump on 7V... all the time... unfortunately i ordered my transformer and my resistor for ANOTHER project just monday night... i guess i'll place another order... we'll see.

remember, i only have a 1A 12Vdc, and i dont want to mess it up... i really feel stupid saying this stuff, but im pretty clueless sometimes, but somethings with ohms i can get, mainly just finding what resistance i want if i want to regulate voltage / wattage...

so: i'll get a rheostat if i have to, because thats what i'm used to, or i can run it at 7V... the pump i have is: the C-systems CSP-750... http://www.dtekcustoms.com/index.asp?PageAction=VIEWPROD&ProdID=140&HS=1 soon to be replaced, but meh... lol

thanks for all your help... i will make sure i post pix here.
 
Looking at the pump, i dotn how much mileage youd get out of dropping the voltage. Certainly watch your temps, as youll find the pressure drops of a lot faster than the voltage. Thats an advantage of the PWM design, you dont lose any torque. Any reason why youre running it off a wall adaptor rather than your psu? Youll prob find you have ample power on the 12V rail if you have a decent quality supply, unless its going into a shuttle or something.
 
I already water cooled my shuttle, and now i am trying to put the pump, res, rad and fan away from the computer... thats why i cant really run a wire out of my computer alllllll the way down to the pump etc etc, or else i would... but that seems to be more of a hassle than i want it to be.

the transformer i ordered is rated at 1A... if it needs to be more, i can get another.

i just want a way to slow down the pump. thats all i want.... so my circuit will go: wall transformer->spst switch->fan->way to slow down pump->pump->wall transformer.

how do u suggest i go about doing this? just list the ways, how i can buy/get them.

and what do u mean by "i dont know how much mileage youd get out of dropping the voltage"? when the pump was in my shuttle i ran it at i think 7V most of the time...(it was on that zalman splitter thing, unless that was 5V, it came with the slim fan, OP-1)

2600+XP-M @ 2400 @ 1.600V if that is the mileage you are refering to, or to the pump? i really dont care too much about the pump, if it dies ill get the newer 12V pump from dangerden.

thanks
-jaxx
 
12 volts - 7 volts = 5 volts
you want to drop 5 volts
5 volts / .5 amps = 10 ohms
5 volts x .5 amps = 2.5 watts

figure thats what i'll get.
 
I meant that the way brushless motors work, they lose torque if you drop the voltage, meaning of course that the water comes out at a lower pressure. Which will affect your temps, and depending how restrictive the cooling loop is may just make the pump not work, if youve had it working fine at 7V already then no problem.
So, ways to go about it in order of simplicity:
Get a three way switch and go for, on, low voltage, off. Put a resistor on the low voltage leg, not just a 1/4W one, one of the big ones. If youre looking at getting ~7V then you want a 10 ohm resistor as you calculated, I'd go bigger on the wattage, get a 10 amp one, its not like they are particularly expensive, just one of the simple white coffin ones would do.

Rheostat, again, same thing, but dont know how well youd go on dissipating probably around 3-4W through one.

Voltage regulator. link Go for a bigger heatsink obviously, but thats a pretty good regulator circuit for what you need.

Pulsewidth Modulation, again, very good, but lots of effort.

Personally id go for the Voltage regulator idea, you can probably find all the bits for under 10$ and it will do you very nicely.
 
hehe, 10 amps... little excessive eh? think ya mean watts...

mouser i think sells 10ohm 3 or 5 watt, i ordered somestuff from them earlier this week, guess i can setup everything then get that resistor, since i think im gonna go that way.

thanks for all the help,
-jaxx

e: 3w http://www.mouser.com/index.cfm?han...uctid=209551&e_categoryid=362&e_pcodeid=58800

5w http://www.mouser.com/index.cfm?han...uctid=253741&e_categoryid=362&e_pcodeid=58800

10w http://www.mouser.com/index.cfm?han...uctid=217595&e_categoryid=362&e_pcodeid=58800
 
Yeah, i did, 5W would be fine, i just figured that certainly over here its around 5% more to get eh 10W one and then youre set. The only thing is youre prob right on the limit of the 1A draw from your supply, so keep an eye on it, it should be ok, but it might get a little hot
 
i got my stuff today, i've setup my rad, pump and res, just need to drill holes out of the case for power, water and air.

i will also be doing a iPod battery charger this weekend.
 
You can make a fairly simple PWM controller with a FET (or two or three) and a 7555 timer IC. The first timer switches the FET on and off at a set frequency for a duty cycle determent by the second timer. This signal then drives the FET on and off.

If you have 30 bucks you can buy a PWM fan controller that will have 4 or more of these circuits in it.

Great explination of PWM here (some poor newb got pwnd) http://xoxideforums.com/showthread.php?p=661922#post661922

I strongly advise that you don’t use a rheostat on a water pump. Lowering the voltage will pull more current through it then it was built for, and you will severely shorten the life of the motor. If you need less flow or quieter operation, get a smaller or quieter pump, and a second pump for a “boost mode”.

EDIT: ipod now charges off 5v or 12v, so any female USB or FIREWIRE port + computer PSU = ipod battery charger.
 
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