Current draw at 5V

DanceMan · Post by **DanceMan** » Tue Aug 31, 2004 7:29 pm

From a thread at Hardware Central:

"I've heard that a 5 volt mod will more likely do some damage"

"I've never heard this, and can't see how it could be true."

"But again, you really have no understanding of how the electronics inside a fan works.
The controller will push more current to compensate for excessively low voltage which can overheat the coil."
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If the load were constant, and the voltage drops, amperage will increase. But since the rpm drops, is the load constant?
If you look at L,M, and H fans, the amperage increases with the speed. So the fan's wiring is likely capable of the current of the H model in any case, and should handle an M fan drawing higher current.
My electrical expertise is insufficient, so I came to the forums with a better educated membership for answers.

sthayashi · Post by **sthayashi** » Tue Aug 31, 2004 8:32 pm

I'll have to check tomorrow when I have access to a power supply, but that reasoning seems largely bullshit.

Most of the fans that I've seen actually REDUCE current rather than increase current for a reduced voltage. I know that a physically jammed fan will draw more power than a nonjammed fan.

But don't take my word for it until I can reconfirm tomorrow with a power supply and an ammeter.

Qwertyiopisme · Post by **Qwertyiopisme** » Tue Aug 31, 2004 8:35 pm

The amperage is reduced when the fan gets a lower voltage, so it souldn't damage the fan's components. One thing that is not good for it though is to give it just enough power so it almost spins but doesn't quite spin, as then the coils dont get any airflow around them (normally there is a minicule amount of airflow (caused by the magnet spinning around, kind of like water in a sallad spinner).

Straker · Post by **Straker** » Tue Aug 31, 2004 11:24 pm

if the fan were just barely capable of spinning at 5V, i could imagine slightly higher amperage, but i doubt it'd be comparable to the amperage when actually starting from stopped, at any voltage. funny thing is this is a basic DC motor thing and has nothing to do with an "understanding of how the electronics inside a fan works", so it's still bullshit.

common sense works too, if any fan controllers actually did this, we shouldn't see nearly linear rpm/voltage.

silvervarg · Post by **silvervarg** » Tue Aug 31, 2004 11:29 pm

If setting fans to 5V caused any problems for the fans you should have seen hundreds of notes in this forum about burned fans. Since you don't you can safely say that there is no problem at all to 5V fans.
All this is assuming the fan starts with 5V. I have not seen reports about fan problems with fans that don't start reliably at 5V either, so the problem is probably not big in this scenario either.

Michael_qrt · Post by **Michael_qrt** » Wed Sep 01, 2004 4:16 am

That guy obviously never paid attention in physics class (or any other basic electronics course). The strength of the electromagnets that drive the fan is proportional to the current put through them not the voltage. The voltage just drives the current. So if running the fan at 5V somehow increased the current then the fan would actually spin faster. When you decrease the fan voltage you will get less current and less voltage in the coils and so they will dissipate less heat, there is no threat to the fan at 5V.

The situation where lowering the voltage causes a greater current draw is in some types of power supply/power regulation circuitry. For example the power circuitry in a laptop needs to deliver constant voltage and current to the processor under load. The battery voltage will sag as the battery discharges. So to deliver the same power the power circuitry needs to draw more current out of the battery at its reduced voltage. Anyway this has nothing at all to do with fans which don't work this way.

sthayashi · Post by **sthayashi** » Wed Sep 01, 2004 10:18 am

According to my MultiMeter, an 80mm Panaflo FBL fan draws 74mA @ 12V and 33mA @ 5V.

Which confirms what everyone else here has been saying.

DanceMan · Post by **DanceMan** » Wed Sep 01, 2004 1:12 pm

Thanks, eveyone. You've confirmed my gut instincts.

burcakb · Post by **burcakb** » Wed Sep 01, 2004 2:54 pm

How about answering with physics? Our famous formula = V = IR where V is Voltage, R is resistance in ohms and I is current in Amps.

Now, if you lower voltage, since the internal resistance of the fan is constant, your current should drop proportionally.

However, this is a bit misleading because what we're interested in is "work done", ie Watts. That means W = IR*I. The only case where the initial comment could become true is when you're expecting the same amount of work to be done by the fan. That is, you want it to turn 2500 rpm @ 5V. In which case, as per last formula, current would increase exponentially. However since lowering voltage will also lower work done, so current WILL go down.

The relationship is not linear (i think) because there's the case of inertia, dynamics, etc, a whole lot of other force interactions that come into play.

cpemma · Post by **cpemma** » Thu Sep 02, 2004 2:08 pm

I found a damn-near-perfect linear relationship down to 7.5V for this fan.

Note the curve doesn't pass through zero as a pure ohmic resistance would. AFAICS the transistor switches in the fan electronics should make the offset constant in the line equation negative, not positive, so more investigation needed.

One explanation is that the fan is having to work proportionally harder to overcome friction as voltage drops, so uses proportionally more current than Ohm forecasts.