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I have a simple 5 volt DC circuit powered by a LiON battery. Running a fan that is rated at 5 volts 100mA-300mA. The battery has a 1000mA output max so the fan draws its full 300mA. I am looking for a way to lower the current available to the fan to 100mA so I can, in theory, triple my battery life. How do I accomplish this?

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    \$\begingroup\$ PWM.. .. .. .. . \$\endgroup\$ – user3528438 May 1 '17 at 20:08
  • \$\begingroup\$ But for you, the simplest way to do it might be to connect the fan to the battery, assuming your battery is around 3.6V, and that should drop the power consumption of the fan by quite a bit. \$\endgroup\$ – user3528438 May 1 '17 at 20:16
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    \$\begingroup\$ If you want to reduce the current to 100 mA, all you have to do is probably to let it spin up. \$\endgroup\$ – Dampmaskin May 1 '17 at 20:24
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    \$\begingroup\$ Put one or more diodes in series. If it is still capable of running (it works on 9V and 12V fans, not sure about 5V ones) it will run slower and will use less power. \$\endgroup\$ – Sredni Vashtar May 1 '17 at 20:39
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    \$\begingroup\$ 1n4001 to 1n4008, but practically any common silicon diode would do. Avoid Germanium and Schottky diodes since its the voltage drop you're after. \$\endgroup\$ – Sredni Vashtar May 2 '17 at 22:23
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The fan will draw less current when it is given less volts. The current draw is roughly proportional to applied volts as is the speed. Consult the fan manufacturers specs and/or check on a variable lab supply to obtain a minimum voltage for reliable operation.

Lower fan voltages will mean that less air is shifted. Now make some PWM to control the fan and you will save battery power. If you run the fan at say 70% speed meaning about 70% voltage and about 70% current then power is halved if your proposed PWM system is perfect.

What you can do is connect your PWM control to a temp sensor so the fan speeds up when hot shifting its full specified amount of air and slows down when cold saving the battery and making less noise and increasing bearing life. Sure this is more complicated than a resistor but these days it is worth it when SMD is used.

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    \$\begingroup\$ Yes, you can make simple PWM circuit using a 556 and a MOSFET, @David. Run it at 2 kHz and put an adjustable resistor preset in to twiddle the PWM mark:space ratio 'til you're happy. \$\endgroup\$ – TonyM May 1 '17 at 20:52
  • \$\begingroup\$ GreatScott has a good video where he uses PWM with a 12V fan, but the concept should be almost the same. Video \$\endgroup\$ – AngryCupcake274 Jun 20 '17 at 19:52
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A 50 ohm resistor would limit the max current to 100 mA. But that might be too much. How about a 50 ohm pot and adjust it till you get the current you want.

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    \$\begingroup\$ This burns up the excess current as heat in the potentiometer. \$\endgroup\$ – Bryan Boettcher May 1 '17 at 20:13
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    \$\begingroup\$ While a resistor will reduce current, it's hardly the best way to increase battery life. \$\endgroup\$ – The Photon May 1 '17 at 20:13
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    \$\begingroup\$ yeah, thats 5 watts down the drain \$\endgroup\$ – Makoto May 1 '17 at 20:30
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    \$\begingroup\$ @Makoto - Would you care to try again? Modelling the fan as a 16.7 ohm resistor, putting a 33 ohm limiting resistor in series drops the current to 100 mA, and wasted power is 0.33 watts. \$\endgroup\$ – WhatRoughBeast May 1 '17 at 21:57

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