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how to reduce 12v 7Ah to 6V?

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Oh, I see hero999. Yes of course - the heat power generated from a resistor would be half if 50% duty cycle, not the voltage.

However, the design of output stage to the motor will affect this voltage or effect behaviour. Is there a half bridge or is there a single transistor where a protection diode is placed in parallell to the motor?

Assuming the fan motor is inductive and the output stage is a half bridge. Then the dc voltage (as seen from the motor) should be around half as much. If that isn't enough (motor isn't pure inductive), the half bridge can be followed by an inductor to make a (almost) steady dv voltage and there could be a cap parallell to the motor. That should give output ≈ 6V.

Use the LM7806
Lot of heat and presumabely the chip can't provide enough power itself. It's a fair chance it will blow.
 
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However, the design of output stage to the motor will affect this voltage or effect behaviour. Is there a half bridge or is there a single transistor where a protection diode is placed in parallell to the motor?

Assuming the fan motor is inductive and the output stage is a half bridge. Then the dc voltage (as seen from the motor) should be around half as much. If that isn't enough (motor isn't pure inductive), the half bridge can be followed by an inductor to make a (almost) steady dv voltage and there could be a cap parallell to the motor. That should give output ≈ 6V.

I understand that the motor is a non-Ohmic load and won't behave like a normal resistor.

I think that connecting a capacitor (other than a tiny 100nnF RF suppression capacitor) will be counter productive because it will draw large surges when the transistor turns on.

I think the frequency is also too low for the motor's inductance to be significant.

What will make a difference is the fact that when the motor is spinning, it generates a voltage. Think of the motor as a DC generator, in series with a resistor which represents the losses in the windings and core. If the voltage stays the same, the faster the motor spins the higher the generated voltage inside the motor, the lower the current drawn.

I haven't experimented with PWM vs DC on the speed of a motor. I don't know if it's perfectly linear i.e. 50% duty means half the average voltage and half the full speed or whether it's a square law.

Perhaps someone with more experiance of this will pipeup? :D
 
**broken link removed**

i try to reduce the 12v7ah sla battery to run 8" fan which required 6V, but only 1-15 ohms 2watt registor in series is work but the R is may burn within a minit. what should i do to down 12 v to 6 volt

This will teach you for asking such a simple question ! Oh , while we're at it . I have a brick but it is too big . What I really need is a half brick. How do I go about turning a full brick into half a brick. How many different ways are there of doing it ?
 
ok all process involve new Money but i ve not so much money,

OK thanks to all, i use the battery for operating CARFAN [radiator maybe].
 
That's a good idea, you can probably pick up an old radiator fan for next to nothing, just make sure it's the type which runs off s motor, not a belt connected to the engine. :D
 
Unfortunately a typical stock electric automotive radiator fan motor draws about 50 times what that 1 - 2 watt 6 volt fan draws.

Or at least all the ones I have played with where in that 50 - 100+ watt range.
 
What so what?

:p

Does it make a difference if two fans rotate at different speeds as long as they are not blowing into the same system?

Temperatures here climb to 46deg/C daily and I have three fans running all day long. They are rated for different power and don't care what the others do.

They just do what they are supposed to do: move air!
 
you could get hold of a car ventilation system fan, when I banged my first car up I took it out, it was a wopping 85 watts but I built a simple PWM speed drive and was able to run it off a 36 W power pack up to a certain speed, my boss who hated anything i did that did not directly benefit him was too impressed to tell me to take it home
 
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