You can probably PWM 12V @ 50% into the fan with no problems

 

Oh...
hmm. I don't think I have any that small lying about.
I do, however, have some Kanthal Wire. I could coil that and make a sub-ohm *resistor* (inductor with resistance)...
The output resistor's won't have to dissipate much heat, will they?

Since power equals I²R, it would be milliwatts.
But why do you want to use two regulators?
You still have to dissipate that same amount of heat.

 

Well, just using what I have on hand for testing and learning. I want to see if I can drop the LM's heat generation into a range where they will be able to sustain the load instead of overpowering a single one and having it blow.

What my thought process is, because the total heat generated for one is above what it can handle, if I use two of them, that will bring their individual heat load to ... say ... 75% of capacity. That would be manageable as they would both have their individual heat sink. So yes, the total heat dissipation would be the same, but spread across two different pieces of equipment, they can handle it.

I'm going to play around with it this evening and see what comes of it.

 

PWM.
You send pulses of 12V to the motor, on half the time, off half the time, thousands of pulses per second. Effective voltage across the motor is 6V. It's like 99% efficient. All you need is a 555, a transistor, and a few resistors.

 

@strantor - I like the suggestion but where I fail to get what you are talking about is the current draw.
The LM555 can't pass that much current, 1.7A. Can you be a bit more elaborate with your suggestion?

I have a bunch of 555 chips so that would be ok, but I don't know which transistor I would need to pass the current.

Much appreciated

 

@strantor - I like the suggestion but where I fail to get what you are talking about is the current draw.
The LM555 can't pass that much current, 1.7A. Can you be a bit more elaborate with your suggestion?

I have a bunch of 555 chips so that would be ok, but I don't know which transistor I would need to pass the current.

Much appreciated

555 doesn't pass the motor current. It switches a transistor on/off. Transistor carries the motor current. There are lots and lots of transistors that would work. See many examples here:

https://www.google.com/search?q=pwm...hgvHTAhWB1CYKHenOAV8Q_AUICSgB&biw=360&bih=512

 

If you want to consider PWM control as strantor suggested, which eliminates the need for a heat-sink, here's a 555 circuit:

Pot P1 allows adjustment of the duty cycle, which varies the motor speed, but since all you want is a fixed 50% duty cycle, you can substitute two 25kΩ resistors for the pot.
(Eliminate pot P1 and put the two resistors in series connected to D1 and D2 with the junction of the two resistors connected to C1 and pins 2 & 6.)
This will actually generate a duty-cycle of about 52%, but that will compensate for some of the inefficiency in the circuit, giving an average voltage to the motor of near 6V.

 

I see now.
I will have to check to see what parts I can scrounge up.
Thanks very much for the suggestions and explanations again.

 

I see now.
I will have to check to see what parts I can scrounge up.
Thanks very much for the suggestions and explanations again.

One important thing to know about PWM is why it has such a big advantage. It's because the switching transistor is almost always in only one of two states, fully on or fully off. It is NOT acting like a resistor and burning off excess power while it is somewhere between on and off. Almost all of the power consumed by the load passes right through the transistor while it is fully conducting. It's a bit like dimming a lamp by flipping the light switch on and off really fast.