Resisting current to fan

Thread Starter

GRNDPNDR

Joined Mar 1, 2012
545
I removed the labels because it was for a different transistor, but had the footprint I needed (I think, I'm struggling with eagle)

They are both D1761's, but I'll start with some lower values.

what's the reason for starting with a lower value?
 

AnalogKid

Joined Aug 1, 2013
10,987
To make sure the base has enough current to make 50 mA flow through the collector. With a gain of 100, you need 0.5 mA (500 uA) of base current to get the most out of your fan. At lower input voltages you need a smaller resistor to guarantee the minimum base current.

ak
 

THE_RB

Joined Feb 11, 2008
5,438
I'd originally thought of doing that and almost didn't post this thread because of it, but using a resistor would leave the fan underpowered if/when the driver dropped to 12V.

A 300 Ohm resistor will give me 50mA @ 15V but at 12V it's only 40mA.

Is this significant? no, not likely, but ideally I don't want the fan slowing down at all, so I may just stick with AK's circuit for the regulation.
Given the forward Vf curve of the LED when the LED is at 12v it will only be using a fraction of the current compared to the 1A at 15v. At 12v the LED might be at 200mA. So having the fan running at 80% speed then should be fine.

But anyway, a voltage regulator will perform better if you want to do it that way. :)
 

Thread Starter

GRNDPNDR

Joined Mar 1, 2012
545
Given the forward Vf curve of the LED when the LED is at 12v it will only be using a fraction of the current compared to the 1A at 15v. At 12v the LED might be at 200mA. So having the fan running at 80% speed then should be fine.

But anyway, a voltage regulator will perform better if you want to do it that way. :)

at 12V it pulls 950mA. at 15V it's 1.05A
 
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