ahmedragia21

Hi, im trying to use a uC like 89S52 for energizing a fan using a TIP12 , i connected a base resistor Rb , and supplied the collector with 12 V and put a protection diode , i dont know if a value of 1k base resistor would saturate the transistor at 5V or no ?

Rb= 5- VBe(on) / iB
= 5-2.5 / 12mA = 0.2 kOhm
at VCE(sat) 2V, Ic = 3A, Ib=12ma

is this correct ?

mneary

Your example shows 200 ohms, not 1k. 1k would not saturate the transistor.

Don't forget to check the 89S52 for its voltage drop when driving 12 mA.

ahmedragia21

thanks for your reply, well then my calculations were right ?

also this is from the datasheet of the 89S52
VOH Output High Voltage
(Ports 1,2,3, ALE, PSEN)
IOH = -25 uA 0.75 VCC V

so i think it wouldnt be a problem of drawing 12 mA from it ?

mneary

-25 uA is 10e3 less than -25 mA. It has no hope of working.

crutschow

You'd need a 200 ohm base resistor as you calculated for the TIP120 (I assume it's a TIP120 not a TIP 20), if your uC output were capable of supplying 12mA at 5V. But according to the 89S52 data sheet it can only supply 800uA at 2.4V output.

Thus you need another transistor to add gain. You could us a 2N2905 or similar PNP transistor by connecting the emitter to the +5V, the collector to the TIP12 base through a 200 ohm resistor, and the 2N2905 base to the uC output through a 3.6k resistor. (I used an PNP instead of an NPN since the uC output is capable of sinking more current than it can source and I need 1.2mA of current from the uC to saturate the transistor using a nominal forced beta of 10.)

Note: This added transistor does provide an inversion so the TIP12 transistor is on when the uC output is low.

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Carl

ahmedragia21

thanks carl , but how about using a ULN2003 ?
from the datasheet

VCE(SAT)
Collector-emitter saturation
voltage (Figure 5.)
IC = 100 mA, IB = 250 μA 0.9 1.1
IC = 200 mA, IB= 350 μA 1.1 1.3
IC = 350 mA, IB= 500 μA 1.3 1.6

i think that would work with the 89S52 ?

regarding the 2N2905 i think it wont work also because from the datasheet it says
VCEsat collector-emitter saturation voltage IC = -150 mA; IB = -15 mA; note 1 - -400
mV

so IB is not less than 800uA so that the 89S52 can supply it , or im wrong ?

audioguru

You said that the collector was connected to +12V. Then it won't work because the transistor is an emitter-follower with a max output voltage at the emitter of only about 1V.

The emitter must be connected to 0V and the fan must be connected between the collector and +12V. A reverse-connected diode must be connected across the fan.

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Uncle $crooge

ahmedragia21

Audioguru, yea that's what i did , i were waiting your answer , so what's your opinion about the above posts about using 2N2905 or ULN2003 ?

audioguru

Hi Ahmed,
I am surprised to see that the Amtel micro-p has hardly any output current when compared to a PIC that has 25mA high and low output current.
The Amtel has an output high of only 10uA when its output voltage drops to 4.6V, or 25uA when its output voltage drops to only 3.75V, or only 60uA when its output voltage drops to only 2.4V.
Its output low current is 1.6mA when its output voltage rises to 0.45V.

Do the math to see if it can drive a few transistors and darlington transistors.

Is the max fan current 3A or is that its running current. Its peak starting current might be 10 times more.

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Uncle $crooge

ahmedragia21

Do the math to see if it can drive a few transistors and darlington transistors.

would you guide me to a name of a transistor that can be driven using this uC ?

audioguru

A TIP120 darlington needs an input of 12ma at 3V for a collector output of 2V at 3A.
Add an emitter-follower transistor then its input will be 600uA at 3.7V.

IF you find a very sensitive TIP120 and a very sensitive little transistor then it will work.

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Uncle $crooge

Roff

Why don't you just use a logic level MOSFET like NPD4060L? Connect the source to GND, the drain to the fan, and the gate to your 89S52 through a 47 ohm resistor. Connect a 1N4001 diode across the fan, anode to +12V.
The MOSFET will switch slowly (microseconds, or tens of microseconds), due the the limited current capability of your 89S52, but I can't see how this would be a problem, unless you are PWM'ing the fan. If you are, then this probably won't work.

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Ron

ahmedragia21

thanks but im so surprised with the current caps of my 89S52 , then it cant be interfaced to any thing , also there's a problem that we dont have this mosfet here in egypt , can i use the IRF540 ? or any other common BJTs

Im not PWIng the fan , but how atmel makes a uC like that ?!

audioguru

An IRF540 Mosfet needs 10V on its gate to turn on completely. A few out of 100 will turn on pretty well with only 5V on the gate. Try a few hundred of them and pick the most sensitive one.

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Uncle $crooge

mneary

Should be cathode to +12V.