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Step up from 5v Powerbank

BGT

Oct 22, 2020
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I have a fan that is rated as 0.9a at 12v, and it would be very convienient if I could connect this to a 10,000mah USB powerbank

I have found a step up transformer thing
which is rated at being able to work at 3a input.

from my very basic understanding. the fan draws 10.8 watts (0.9*12). so therefor on the 5v side will it be running at 2.16a ?

So would a fully charged powerbank, safely and reliably work this fan ? (all seems a bit too good to be true)

Alternatively - I have a number of 18v li-ion batteries (1.5 & 3.0 ah) would I be better aiming for a step down transformer ? but would I then be faced with incorporating a low voltage cut off ?
 

Harald Kapp

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The fan will draw more from 5 V due to conversion losses in the step-up regulator. The XL6009 claims an efficiency up tp 94 %, let's realistically assume 90 %. Then teh current from 5 V is I = 0.9 A × 12 V / 5 V /0.9 = 2.4 A.
If your powerbank can deliver 2.4 A or more on the 5 V output, this setup will work.

When using LiIon batteries, a step-down controller will be required as well as an undervoltage lock-out to prevent damage to the battery (the 5 V powerbank should have that included).
 

Alec_t

Jul 7, 2015
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Would you have enough air movement if the fan could work at 5V?
 

BGT

Oct 22, 2020
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Thing is; I am trying to make use of what I have.
6 - 12v 0.9a 10.8w centrifugal fan
a 10,000mah usb 5v powerbank (max output 2.1a)
couple of 1.5ah cordless drill batteries and charger

I will probably be wanting the device I am making to run for about 3 hours! so the 10ah would be far far better than the 1.5ah.

As it happens the fan is a little too powerful, and I am wanting to try it at 9v (or somewhere between 6&12v)
so how does the maths work out for that
12v 0.9a 10.8w

so would 9v reduce the power used to 8.1w ? (or is it more complicated than that)
 
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Alec_t

Jul 7, 2015
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(or is it more complicated than that)
Yes it is. The power to drive a fan varies approximately as the cube of the rotational speed. Reducing fan speed also reduces the fan motor back-emf, so the current increases.
 

BGT

Oct 22, 2020
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So if I run the fan at 6v instead of 12. it would still flow the same amount of air ? and still in essence be a 10.8w fan.

I was thinking lower voltage lower power output
 

Alec_t

Jul 7, 2015
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So if I run the fan at 6v instead of 12. it would still flow the same amount of air ?
No. The flow rate would be less, but I doubt it would be halved. The wattage would be less too.
 

Harald Kapp

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There's no other way (at least I can think of) other than
  1. either study the datasheet of the fan or
  2. simply test it on your power bank without the step-up controller
 

BGT

Oct 22, 2020
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Sorry if this is all a bit basic on an Electronics forum - but i will ask regardless

The fan is clearly labelled as 12v 0.9a & 10.8w

done a quick check on it with it wired into a 12v supply, and I checked that it was 12v.
the fan runs perfectly and is easily as powerful as it need be,
However it is only showing 0.45 amps, so I wired it into a second multi meter and same result.
as this is quite pivotal on how I proceed with this little project, am I missing something obvious or not understanding something obvious?

xAmpM.jpg


wonderful if it is only 0.45 amps as this would make it only a 5.4watts fan, and should give 4 hours on the 18v battery that is rated at 24wh (3 hours run time would do)

And this would also suggest the use of the usb powerbank (rated at 5v ... 1A/2.1A (max)) as it would only be drawing 1.2A (?)

Although mu gut feeling is the 18v Li-ion would be the better choice, even if that did mean step down and power cut off things to be added (but that would all be very intereting to do and far from unwelcome)
 

Alec_t

Jul 7, 2015
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Perhaps the 0.9A rating is for the peak (start-up) current, rather than the normal full-speed running current?
 

Bluejets

Oct 5, 2014
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Without seeing any fan specs, impossible to say.
Could be wrong label on the fan, different fan, or the fan may have an internal pwm regulator so the Op is seeing just the average current.
 

BGT

Oct 22, 2020
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I have decided to go with the 18v li-ion battery and have ordered an adjustable step down transformer. But as mentioned I will need a circuit for a low voltage cut off. Any suggeztions on what I could buy or if possible to make something ?
 
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