# can anybody tell me the gain of this transistor for 0.15 amp collector current?

http://pdf.datasheetcatalog.com/datasheet/stmicroelectronics/4128.pdf

the transistor is tip 122

the gain is required for collector current = 0.15 amp and collector-emitter voltage of 12v.

Gain is approx 800 → base current <20uA
This isnt exactly easy to obtain…
For 15 mA: waht about a single transistor, saturated and a resistor to adjust to 15mA ?

thanks bro. can you refer me some transistor for so low base current which have a gain of 100 or any other gain but low.

What are you trying to do? Be specific.

very common is the 2n2222 gain min 75 at 150 mA (typical gain 300) It can handle 500mA max. What do u want to build..?

zacmackra: the gain is required for collector current = 0.15 amp and collector-emitter voltage of 12v.

You really want to dissipate 1.8W in your transistor? That does seem rather wasteful.

What are you trying to make?

i want to control a small dc fan of rating 12v, 0.15amp with arduino. i am not getting a higher rating small dc fan in my area. since arduino can deliver low current so i am using a transistor to amplify. is there any better way to do it?

hi guys , i have this transistor bd139 which i will be using for my project http://pdf.datasheetarchive.com/indexerfiles/Datasheet-01/DSA00650.pdf

my requirement is to supply collector current of 0.15 amp. however i am unable to calculate the base current as the transistor shows gain minimum: 40 and maximum: 250 for 0.15 amp collector current. so what should be my maximum base current be.

Note- i want my transistor to fully supply 0.15 amp at my maximum base current .

So another way to put what your asking (to rephrase your question) is: What should I use for base current to guarantee saturation ? In this case I (col) should be 0.150A To guarantee saturation, assume the gain is minimum (40) so 0.150/40=0.00375A (3.75mA) If you assume a base-emitter voltage of 0.9 V when saturated: 5V-0.9V=4.1V 4.1V/0.004A (4mA) =1025 ohm You can use a 1 k resistor for the base resistor and the arduino output will drive it to saturation.

This was more precise. You can use your TIP120-transistor. Connect 12V to fan+. fan- to collector. emitter to GND. (common for Arduino and fan power) Connect Arduino output via 330..1k resistor to base. Measure voltage over transitor when fan is running. It should be well below 2V. add a diode parallel to fan (kathode at 12V, anode at collector) ..... The fan itself will draw its 150mA while running. (misread - altered current)

thank you for giving such good explanation. Just one thing i put 1 k but transistor is heating up to a high temperature. is it normal for transistor to heat up or am i doing something wrong? What about heat sinks?

Why have you started a second Thread on this subject? Previous Thread

Why have you started the second Thread in the wrong section after your first Thread was moved (IIRC)?

DON'T double post.

...R

It heats because it is not saturated. (you can measure a Vce = ??... to high.. should be below 1V) Increace base current to achieve saturation. => reduce resistor value, but never below 100 Ohm ............. from datasheet Vbe=1,3V (5V-1,3V)/R = max 40mA => R=100

thanks for all your replies. one question tip 120 has a high gain of 800-1000 and i want just collector current of 0.15 amp. so we need to reduce the base current to 0.15/800=1.875 x 10^-4. so a resistance of atleast 30 k will be sufficient. why do you advice i should use 330......1k? please explain me and pardon me if i am wrong somewhere.

For one thing, you are making the "design" beta dependent.

It isnt a sound amplifier !? Its a switching circuit? (so I belive)

If U’d collector to 12V and emitter to GND - then it would be very important to control the current, to avoid short circuit!
In your case, you have a motor involved on the collector side. This shall be switched on or off?
OK. If u put a low voltage (<.5V) on base - the transistor breaks any connection C-E → no current flow
When u saturate the transistor by driving enough current into its base, Vce drops to belov 1V and the transistor if fully open for current to flow. The motor (resistor) in the collectors path decides how much current that will flow. You said that your fan is marked 150mA
A chart in the datasheet indicates that you need more than microamps base current to achieve saturation.

Try increasing the base resitor and observe that the Vce rises → and the transitor gets hot.
In a switching circuit you should reduse base resistance until transitor saturates.
You will even hear that the fan/motor slows down if base resistor is to big.

because i need to control a dc fan whose maximum rating is 0.15 amp. so when i give 5v from arduino i hope that my transistor must be fully saturated and when i lower voltage (depending upon the duty cycle of pwm output) i want my transistor not to be saturated and draw less collector current.

Your fan takes the current it needs to run. Please forget the 150mA for a while. (startup current is probably 500mA+) Go for a low resitor value to avoid transistor heat_up.

## Run PWM as you've planned. dont forget the diode parallel to the fan. (kathode to 12V)

edited current...

Since when is .15Amp = 15mA????

150mA to the fan. 12V.

The transistor Vbe is about 0.7V. The datasheet rating of 1.3V is a maximum at maximum base current.

To make sure a transistor is saturated, you should put in a base current between 1/20 and 1/10 of the collector current. So about 8 to 15mA. This is because gain of a transistor goes down when in saturation, so the gain at 15mA unsaturated is not an accurate assessment of what it is when saturated. In addition, you want the transistor overdriven so a different batch, temperature, or aging doesn't prevent saturation.

(5V - 0.7V)/10mA = 430 ohms (5V - 0.7V)/15mA = 287 ohms

The output of an Arduino will not go all the way to 5V when you are drawing current from it, so I'd go with 330 ohms.

Then the Vce saturated should be around 0.1 to 0.2V.

BTW, 15mA is 0.015A. 0.15A is 150mA. The prefix "m" lower case or "milli" means one thousandths.