# I need just a moment of higher amps to get a fan spinning. How?

Hello.
I am using the PWM out put to control a 12V fan motor.
The fan is being powered by a 5v battery run through a 12v boost converter. The battery is rated at 1.5amps output and that matches the fan's rating ... except to get the fan running I need just a split second of higher amperage. It seems to want 1.8amps.
It is REALLY close to working, it just stutters. A few times it has actually managed to get the fan running.

My question, Can I do something to get just a brief, little jolt more of amps to spin-up this fan ... or do I just need to go shopping for a new power source?

Thanks,
Gary

Gary_Arduino:
The fan is being powered by a 5v battery run through a 12v boost converter. The battery is rated at 1.5amps output and that matches the fan's rating ... except to get the fan running I need just a split second of higher amperage. It seems to want 1.8amps.

What is the rating for the battery and for the fan, and what is the current capacity of the boost converter? Using a boost converter, you will need a lot more current from the 5v battery than is needed by the fan at 12v. If the fan is rated at 1.5amps at 12v, that is 18 watts. With a 100% efficient boost converter, that same 18 watts requires 3.6amps from a 5v battery, so expect something between 4 and 5 amps as a realistic amount.

If your fan needs 12v at 1.5A, you need to be able to supply 12/5 = 2.4 times that current at 5v, or 3.6A assuming the converter is 100% efficient (conservation of energy).

So, we must conclude that even when it is able to spin the fan, considerably less power is being delivered to the fan than you think (most fans will run at well under their rated voltage, just not as fast). or you're able to pull far more than 1.5A from the battery.

In conclusion, the power source is not suitable for powering that fan. Get a bigger power source or a smaller fan.

A start capacitor is a common means to boost a motor out of stall.

Gary_Arduino:
Hello.
I am using the PWM out put to control a 12V fan motor.
The fan is being powered by a 5v battery run through a 12v boost converter. The battery is rated at 1.5amps output and that matches the fan's rating ... except to get the fan running I need just a split second of higher amperage. It seems to want 1.8amps.
It is REALLY close to working, it just stutters. A few times it has actually managed to get the fan running.

If your fan is 12V at 1.5A then it is a 12 * 1.5 = 18W load on the converter. (P = V * I)

If you have 90% efficiency, then you will need;
18 / 0.9 = 20W input to the converter.

So the 5V supply will need to supply;
20 / 5 = 4Amps (I = P / V)

What do you mean it is rated for 1.5A?
Batteries are rated in Amp- Hours.

Thanks.. Tom..

Thanks all.

The fan is listed as (printed on fan's label) 12v 1.5a
My battery is listed as (printed on the side) Output - 5v 1.5a (max)

The boost converter is an ebay purchase described as "USB 5V to 12V Fixed Output Step Up Boost Power Supply Module"

Using a bench power supply, I was able to see that the fan was actually pulling 1.8amps when it first starts spinning. It does this for just a brief moment (maybe 1 second) then settles back down to a 1.3 amps draw.

I guess I am just running at the ragged edge of compatibility here. Time to look for a higher amp battery I suppose.

Hi,
If you replace the 5V battery with your bench power supply and see how much current is required on the 5V input side.

Tom..

TomGeorge:
Tom..

Hi Tom. I didn't state that very clearly, sorry about that. Yes, where I measured was between the bench supply and the power booster. That is where I read the 1.8amp draw.
Just for fun, next time I am at the bench I will read the current between the fan and the output of the booster.
I am always up for gathering more data!

UPDATE: Fun note. I measured the current draw between the DC power booster and the fan ... only .46A at peak draw! So it would appear that it is the booster that is drawing th bulk of the current.

Another fun update: I had another USB, rechargeable battery laying about. This one has "Output: 5v/2.1A" printed on the side. The fan works perfectly off of this one!