DC-DC Vairable 0-12v 6A power supply

[mo0K]

Hi,

i have a circuit i am trying to design and although i fear you are asked the same questions about power supplys over and over i feel my problem has differences that i wouldn't just find, so hope it's ok to start another thread about power supplys.

the brief, to run two 12v 3A 120mm fans from a PC 12v/5v ATX molex connector, the circuit would have two thermistors, one reads the room temp (T1) and the other reads coolant temperature (T2), there needs to be a POT to set the output when the thermistors match resistance, but if T2's temp goes up so will the voltage output (and another POT would set the amount of V per deg C),

[mo0K]

or maybe i could use a

[mo0K]

???,

,er.. i guess as i'm not getting any relpys, no one can help..??

if so and some one could help with just getting a POT to control a transistor that can handle 12v 3A (or a pair), or 12v 6A,

if some one could tell me if the circuit below would do fine for a simple speed controler, and if so which components would do the job..?

thanks..

although pulse modulation with a 555 could make it easier to set the V to degC, ?

[Guest Alun]

What are you trying to do?

Do you want to turn a fan on/off with different temperatures?

Both of your circuits are emitter followers that amplify the current output of a potintial divider.

Do you want to alter the fan speed with temperature.

[mo0K]

hi'ya Alun..


i have two Delta 120mm 4000RPM 190CFM 3A fans for a 240mmx120mm rad in my watercooling system on my computer, they sound like a jet engine..!!, LOL,

now i want the be able to set the base level the fans run at (with a POT) also i want two temperature sensors, i want one to monitor the room temp and the other to monitor the water temperature, now if they are the same temperature i want the fans to run at the set base level, but if the water temp goes up when compared the room temperature i want the fans to go up by say a volt a DegC (but would like adjustment on this really too),

do you see..?

i didn't want to pulse modulated circuit as i find brushless fans are modulated and if you run them on a modulated source the double modulation makes them noisier, but i now don't know if i want to waste power and make heat using transistors and it might be easier to get the control i want from a modulated circuit...


what do you think....

cheers

mo0K  ;D

[Guest Alun]

I haven't tested this but I know the individual circuits work so you should be fine.

This circuit won't do ecactly as you said but should achive the same thing. Normally the fan speed is set by the POT, but when Th1 is hotter than Th2 the fans will come on at full speed.

How it works

The circuit is split into three parts:

PWM circuit:
Even this has two sections.
A is a traingle wave oscilator.
B is a comparator.
When the input voltage is higher than the output of the oscilator the output goes high and turns on the motor, when it's less than the output of the oscilator then the motor is off. Thus adjusting the input volgage will set the level it comes on for and thus the lengh of the cycle the motor is on for.

Temperature Comparator:
When Th1 is hotter its resistance will be low and the voltage on the + pin will be higher and the voltage on the - pin turning C on and thus the motor. When Th1 is cooler the resistance will be higher the comparator will be turn off. The diode on the output ensures it only affects the PWM circuit when the comparator's on.

The third circuit is a simple potential divider to set the default speed of the fans.

A B & C are cheap and cheerfull uA741 op-amps.

Put these circuits togeather and job's a gooden. ;D

[Guest Alun]

Oh sorry here's the circuit:

[mo0K]

hi

cheers for your input,


i do realy want a progressive change of speed proportional to the difference of the two thermisters though, could the difference in resistance adjust the pulse width modulation..?


???