# amperage/voltage question



## Fitseries3 (Jan 7, 2009)

i simply need to know how to properly figure out how to figure the amperage this will draw so i can get a psu that will handle it.

(8x) 24v 1.53amp but i'll be running them between 7-12v FFB1424SGH- info

i assume that you just do 1.53a x 8 fans and get 12.24amps @ 24v but how would i figure it at 12v or 7v?

doesnt the amperage go up when its undervolted?

help!


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## department76 (Jan 7, 2009)

try out this power supply calculator, you have to click a couple times to get to the app, the lite version seems to work well.


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## Fitseries3 (Jan 7, 2009)

well the problem is its not computer related at all. it doesnt even run on a computer psu. 

i do thank you for the link though.


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## ZenZimZaliben (Jan 7, 2009)

Depends on if the fans are hooked up in series or parallel. If in parallel each device receives the same voltage..the amount of power is dependant on the resistance of each device.


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## Fitseries3 (Jan 7, 2009)

most likely parallel.


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## ZenZimZaliben (Jan 7, 2009)

OK..so to find current draw you need to treat the fans as resistors. Find out what thier resistance is with multimeter. then apply ohms law. this will give you total current draw at specified voltages with known resistance.


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## Silverel (Jan 7, 2009)

fitseries3 said:


> i simply need to know how to properly figure out how to figure the amperage this will draw so i can get a psu that will handle it.
> 
> (8x) 24v 1.53amp but i'll be running them between 7-12v FFB1424SGH- info
> 
> ...



Amperage generally doesn't change. Undervolting will give you a power decrease, causing loss of RPM's, and depending on the fan, they might not even spin up though the motor will attempt to turn and eventually burn out, unless it has sensors to remain off.

The operating voltage on these is 14-28, unless it's a recommended range, it shouldn't even power on at less than 14v.

What you _might_ be able to do, is splice the connectors to use 2 12v leads, and add equal resistance to each line to get down to whatever range between the 14-28v.

If they spin up fine on 12v, there isn't anything to worry about imo.


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## Fitseries3 (Jan 7, 2009)

the thing is i only have one for now. im debating on returning it or getting 7 more.

it spins all the way down to 3v no problem so thats not an issue.


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## Silverel (Jan 7, 2009)

Ah, good times. I think what you're looking for is wattage, not amperage.

You can assume each of those fans is going to demand the same amps safely, I think I'm understanding what you're getting at now.

So start with 8 x 1.53a
You'll need a line that can handle 12.24a, recommend about 10% on top of that so 14a would be fine.
Next take voltage @ 12v x 12.24a
146.88W of continuous draw once they all spin up.

As long as your power can handle 14a and 160W, you should be good to go. Otherwise, you'll need to split off onto different lines.


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## Fitseries3 (Jan 7, 2009)

this is becoming a PITA. i cant find a meanwell(or similar) that has enough to keep the fans going. i may have to split it into 2 psu's but thats more money.

what if i ran them in series or a combo of series and parallel?

i can find higher voltage psu's easier than lower ones. if i have to run a 24v psu but they still run about 30-50% speed i'll be happy.


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## Silverel (Jan 7, 2009)

fitseries3 said:


> this is becoming a PITA. i cant find a meanwell(or similar) that has enough to keep the fans going. i may have to split it into 2 psu's but thats more money.
> 
> what if i ran them in series or a combo of series and parallel?
> 
> i can find higher voltage psu's easier than lower ones. if i have to run a 24v psu but they still run about 30-50% speed i'll be happy.



Series vs. Parellel isn't going to matter much when it comes to the draw against the PSU. It's still going to ask for 12v to push 1.53a at each fan.

Series would cause the last fan to fail first with lack of power.
Parallel would cause them all to slow down, until they all fail.

Most PSU's should have ample amounts of juice to power this small array of fans. What else is going to be loaded on it?


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## Fitseries3 (Jan 7, 2009)

fans only.

http://cgi.ebay.com/3x-Mean-Well-RS...5|66:2|65:12|39:1|240:1318|301:0|293:1|294:50


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## Fitseries3 (Jan 7, 2009)

see what i mean?


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## Silverel (Jan 7, 2009)

Ah, sorry. lol... had to leave work.

If it's just for fans you can get a dirt cheapo PSU and do the paperclip trick so it stays on.

14a with 160W seems pretty easy to find. o.o


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## Fitseries3 (Jan 7, 2009)

nah... gotta be a meanwell or similar so i can regulate the voltage. a 12v meanwell can run 7v-16v giving me a good range of fan speed.


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## Silverel (Jan 7, 2009)

Something like this one?

http://www.computronics.com.au/meanwell/s-201/

Ah, voltage range on that is too low... I'll look with ya


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## Arctucas (Jan 7, 2009)

@fitseries,

I believe you would want to run your fans in parallel. Voltage across parallel resistive loads is equal across all loads. 

Current flow (Amperage draw) across parallel resistive loads is the sum of the Amperage draws of all loads in the circuit.

Given the specifications you provided; each fan draws 1.53 Amperes at 24 Volts for full speed.

Eight fans in parallel would then draw 8 x 1.53 Amperes = 12.24 Amperes at 24 Volts.

If the fans can run at lower voltages, I would expect the Amperage draw to remain the same, only the fans would spin at a lower RPM.


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## Dia01 (Jan 7, 2009)

You can use the V=IR or P=VI equations mainly for resistive loads though.

So, if you use V=IR you can transpose the formula once the resistance of a load is known for example 24V/15.68 ohms = 1.53amps.

Therefore at say 12V:  12V/15.68 ohms = 0.76amps

Generally for motor windings though the resistance will change to suit which voltage it will be used for so in other words, for a 24v fan to do the same amount of work as a 12v fan it will draw less current which is actually the opposite of the above formula.

In summary:

Voltage = Amps x Ohms
Amps = Volts / Ohms
Ohms = Volts / Amps

Power = Amps x Voltage
Amps = Power / Volts
Volts = Power / Amps


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## Silverel (Jan 7, 2009)

You could always do the cheapo PSU and a fan controller


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## Fitseries3 (Jan 8, 2009)

cant use a fan controller. the fans kill them. i've killed all 3 of my controllers already.

a cheapo psu doesnt go over 12v either.


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## Dia01 (Jan 8, 2009)

My guess is that when undervolted it will actually draw a little less current as the speed will decrease and inadvertably reducing the back EMF generated hence less magnetic flux cutting the windings.  I can actually simulate and log it when I get home if you want to know for sure.


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## Fitseries3 (Jan 8, 2009)

so good or bad? sounds great


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## Dia01 (Jan 8, 2009)

fitseries3 said:


> so good or bad? sounds great




Less current is always good assuming I'm right, I am wrong alot though!


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## Fitseries3 (Jan 8, 2009)

...inadvertably reducing the back EMF generated hence less magnetic flux cutting the windings.


that sounded like a bad thing. idk.


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## Dia01 (Jan 8, 2009)

fitseries3 said:


> ...inadvertably reducing the back EMF generated hence less magnetic flux cutting the windings.
> 
> 
> that sounded like a bad thing. idk.




Actually the more back EMF the more current draw as back EMF opposes the flow of current if my memeroy serves me right.


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## Arctucas (Jan 8, 2009)

Dia01 said:


> Actually the more back EMF the more current draw as back EMF opposes the flow of current if my memeroy serves me right.



What is back EMF? I get the Electro-Motive Force part.


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## Dia01 (Jan 8, 2009)

Arctucas said:


> What is back EMF? I get the Electro-Motive Force part.



Electromagnetic Force - It is always present.


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## Arctucas (Jan 8, 2009)

Dia01 said:


> Electromagnetic Force - It is always present.



I was doing some reading on back EMF (Google is your friend) and the references I found were saying Electromotive Force, i.e. Voltage?

As I understand it, back EMF is the induced voltage produced by the rotating electrical field inside the permanent magnets of the DC motor. This back EMF tends to counteract the applied voltage, thereby acting as sort of regulator by increasing in strength proportional to motor RPM, until the point where it is equal to the applied voltage and prevents any further increase in rotational speed. 

I am still unclear as to how this affects the current draw of the motor, as I would tend to believe that the resistance would remain the same (since DC current sees an inductor as basically a short circuit or zero resistance).


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## JrRacinFan (Jan 8, 2009)

Hiya fit, just my .02. .....

You have both +12v and -12v on a psu, if you take +12v to hot/red and put -12v to ground/black that would give you 24v. Same can be said for -5v, but at 17v. Remember ground is 0v. Even though you probably already know this, figured I would bring it up in case you forgot. So you can obtain 24v from a PC power supply.

EDIT: Just trying to help you out in your quest.


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## Fitseries3 (Jan 8, 2009)

im trying to get 7v really. pc power supply wont work. for some reason at 7v the fans wont spin but they will at 3.3v and 5v and 12v. i think its the combining of the 2 voltages thats preventing it from working.


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## Dia01 (Jan 8, 2009)

Arctucas said:


> I was doing some reading on back EMF (Google is your friend) and the references I found were saying Electromotive Force, i.e. Voltage?
> 
> As I understand it, back EMF is the induced voltage produced by the rotating electrical field inside the permanent magnets of the DC motor. This back EMF tends to counteract the applied voltage, thereby acting as sort of regulator by increasing in strength proportional to motor RPM, until the point where it is equal to the applied voltage and prevents any further increase in rotational speed.
> 
> I am still unclear as to how this affects the current draw of the motor, as I would tend to believe that the resistance would remain the same (since DC current sees an inductor as basically a short circuit or zero resistance).



Pretty much, that is why a ballast in a fluroescent light fitting is sometimes known as a choke.  Back EMF or Electro Motive Force does however oppose the applied current, that's why you do not get a dead short when voltage is applied to a coil.


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## Arctucas (Jan 8, 2009)

Dia01 said:


> Pretty much, that is why a ballast in a fluroescent light fitting is sometimes known as a choke.  Back EMF or Electro Motive Force does however oppose the applied current, that's why you do not get a dead short when voltage is applied to a coil.



But that is Alternating Current, and Inductive Reactance and Capacitive Reactance come into play along with Resistance to give you the circuit Impedance.

I thought we were discussing DC motors?


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## Dia01 (Jan 8, 2009)

Arctucas said:


> But that is Alternating Current, and Inductive Reactance and Capacitive Reactance come into play along with Resistance to give you the circuit Impedance.
> 
> I thought we were discussing DC motors?



Yes, granted, effectively though the circuits still sees the same type of load whether AC or DC being that EMF is still present.  I can not really recall DC and AC machines theory exactly, like I stated, I believe I am correct though, a practical test will prove, I'll do a test this weekend and monitor the current draw of a 120mm fan at 12v compared to 7v and see the results.


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## Fitseries3 (Jan 8, 2009)

well i think i decided its cheaper to get 2 meanwell S-100-12's and use one for each bank of 4 fans.


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## Dia01 (Jan 9, 2009)

Alright, probably not intersting but here's the results.

Arctic Cooling Fan
92mm x 92mm x 38.5mm
2000RPM
34.8CFM / 59m3/h
12V, 200mA

Tests: (Using Fluke Multi-Meter)
@12VDC - 137mA
@7VDC -   90mA
@5VDC -   62mA

The results a quite proportional i.e. less voltage = less current, you could roughly work out a fan motor say with a FLA of 300mA @ 12V could roughly draw 180mA @ 7V for example.  Not exactly but roughly enough.  In a parrallell circuit consisting of 8ea x fans just multiply for 12VDC 300mA x 8ea.


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