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Update 1 :

After some help and advices from nice people here i'm going to use this circuit : http://everycircuit.com/circuit/5857347480322048

enter image description here

the 25 ohm resistance being the fan.

I'm requesting your help for a little electronic circuit, a simple transistor switch. I need help because i don't know how to choose the right transistor or the right resistance for my circuit. I need advice for adding a protective diode.

I explain myself :

it's a circuit that switch a 5v 0.2A dc fan to cool my raspberry pi because i use it intensively.

here is the circuit i designed :

updated (look up)

the online link to it : Updated to new link (look up) (the 25ohm resistance is the fan's resistance, i didn't know how to modelise it).

I use the 5v pin to power the fan and a gpio pin (3.3V 16mA) to turn the transistor on/off by a python code i will do myself later.

is the resistance choosen ok ? how can i choose the right transistor after it's specs ?

if i get to make the circuit work fine, i will then use the PWM pin of the RPI to make it's speed variable with the temperature of the CPU, but that's after.

i hope my question is complete, What do you guys think ?

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    \$\begingroup\$ The 18 k ohm resistor is a bit high, that could prevent the transistor from fully closing and making it hot. I would Use a 1 kohm resistor instead of that 18 kohm. Include a link to the datasheet of the fan, not all fans like a PWM signal on their supply. \$\endgroup\$
    – Bimpelrekkie
    Commented Aug 30, 2017 at 15:08
  • \$\begingroup\$ Link to the transistor datasheet would be good too. \$\endgroup\$
    – pericynthion
    Commented Aug 30, 2017 at 15:18
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    \$\begingroup\$ Assuming a 0.7V drop from base to emitter, the base resistor will see 3.3V - 0.7V = 2.5V. For 16 mA to flow the base resistor would have to be 2.5V / 0.016A = 156 Ohm. So in order to keep the current below the limit of the GPIO, 220 Ohm should be plenty. \$\endgroup\$
    – Dampmaskin
    Commented Aug 30, 2017 at 15:25
  • \$\begingroup\$ @Bimpelrekkie i got the fan with the pi's case, all i got is voltage/current. \$\endgroup\$
    – Corn
    Commented Aug 30, 2017 at 16:36
  • \$\begingroup\$ @pericynthion i do not own a transistor yet, i though i'd choose one with the circuit's demand. Isn't that right ? \$\endgroup\$
    – Corn
    Commented Aug 30, 2017 at 16:38

1 Answer 1

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Try using a common switching transistor such as 2N4401. You should have Ib in the range of the fan current divided by 20 to have the transistor fairly well saturated. 10:1 (Ic/Ib) would conform to the guaranteed specs, but 20:1 is good enough for this, at 200mA or less. If you look at Fig 17 in the linked datasheet, things go pear shaped at higher collector currents:

enter image description here

If you had a much higher current fan, it would likely be better to use a MOSFET rather than a BJT.

@Dampaskin's suggestion of 220 ohms for the base resistor will be fine for this. And you're done.

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  • \$\begingroup\$ I better understand now, one more thing that i don't get is, Hfe here is 196mA/11mA ~= 17, is that normal ? \$\endgroup\$
    – Corn
    Commented Aug 31, 2017 at 6:35
  • \$\begingroup\$ Yes, it is normal. The datasheet saturation voltage is rated at 10 "forced beta". You must keep the forced beta << hFE (as measured with large Vce) to have the transistor Vce as low as practical. So you pick the Ic/Ib to be sufficiently low that the transistor is well "on" and it thus runs cool. \$\endgroup\$
    – Spehro 'speff' Pefhany
    Commented Aug 31, 2017 at 10:21
  • \$\begingroup\$ It is not simple to understand this but i got it, thank you very much. \$\endgroup\$
    – Corn
    Commented Aug 31, 2017 at 14:39
  • \$\begingroup\$ @Corn. Good.. it's a bit hard to describe in words. \$\endgroup\$
    – Spehro 'speff' Pefhany
    Commented Aug 31, 2017 at 14:43

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