Topic 'Re: hot npn transistor when transistor base is fed with pwm signal' in forum 'Electronics' - dslreports.com

Re: hot npn transistor when transistor base is fed with pwm signal

Tue, 14 Nov 2017 21:01:21 EDT

shdesigns posted : You must be doing something wrong. I've switched motors that take several amps with transistors. Always switch the low side as it is easier.
Never a problem with current feeding back. I do usually use a MOSFET as it is easier to drive.

Re: hot npn transistor when transistor base is fed with pwm signal

Tue, 14 Nov 2017 20:44:07 EDT

teletone posted : some advice for anyone who finds this thread and is thinking of hooking up a 4pin pwm fan to an IOT board like a arduino or esp8266 based device:

you'll probably use 3 wires of the 4, positive, negative and pwm. don't implement a npn transistor to switch the fan on and off if your pwm line is connected from the fan to your board. you'll be opening the negative/ground and the current from the fan will back feed from the fan through the pwm line to your board. if you want to control speed with pwm and and have a transistor to turn the fan completely off, i'm guessing you should use a pnp transistor on the high side (positive line) or you could use a simple switch. don't switch the low side with a npn!

Re: hot npn transistor when transistor base is fed with pwm signal

Sat, 11 Nov 2017 17:07:28 EDT

teletone posted : i guess the "acceptable operational range 21 kHz to 28 kHz" is set so people don't hear the pwm frequency when they adjust their fan speeds on their computers.

i've been playing around with different pwm frequencies. if you could somehow control the pwm frequency with a midi keyboard, you could use the fan as a speaker. obviously no polyphony, single note only.

i hooked up a speaker to where the fan was hooked up and if you set the pwm anywhere in the frequency range of human hearing, you can hear the frequency come through on the speaker.

Re: hot npn transistor when transistor base is fed with pwm signal

Sat, 11 Nov 2017 16:37:21 EDT

SparkChaser posted : Great! I didn't understand where the PWM was coming from (MY BAD)

There are two things that can happen (maybe more) You get it out of the range of hearing or what is acting as a transducer can't operate at that frequency.

My hearing cuts off at ~8KHz

Re: hot npn transistor when transistor base is fed with pwm signal

Sat, 11 Nov 2017 16:23:35 EDT

teletone posted : i tried that and it didn't work. i also tried vintagewino's diode method with no results.

i did try something else however. i was looking at the specs for the pwm pin of 4-pin pwm fans and saw:

PWM Frequency: Target frequency 25 kHz,
acceptable operational range 21 kHz to 28 kHz
Maximum voltage for logic low:
VIL = 0.8 V
Absolute maximum current sourced:
Imax = 5 mA (short circuit current)
Absolute maximum voltage level:
VMax = 5.25 V (open circuit voltage)

then i said to myself, if i ever want to hook up a 4 pin pwm fan to by device, i need to set the pwm frequency to somewhere in the range of 21 kHz to 28 kHz . the pwm from my device is clearly not in that range because i can hear the frequency at around 1000hz

so then i added this code to my arduino sketch in the void setup:

analogWrite(D5, PWMRANGE);
analogWriteFreq(25000);

and it set my pwm frequency to 25000hz. i also changed the cap across the positive and negative of the fan to 0.22uf. the noise is either gone or above the threshold of hearing. so far, the transistor seems to be cool.

Re: hot npn transistor when transistor base is fed with pwm signal

Sat, 11 Nov 2017 11:54:17 EDT

SparkChaser posted :

said by teletone:

put a 10uf capacitor from positive to negative of fan, put a 6.8 ohm resistor in series with the positive of the fan. i didn't have 4.7 ohm resistor.

Just for fun - Have you tried a capacitor 22UF from the pos of the fan to the emitter of the transistor. Leave in place the 6.8 resistor.

Re: hot npn transistor when transistor base is fed with pwm signal

Sat, 11 Nov 2017 11:10:33 EDT

vintagewino posted : If Sparkchaser's schematic is what you're dealing with:

You start charging the cap when the transistor switches on. Capacitor appears as if it's a short circuit for the first instant as it starts charging. If you're using 1000Hz, as you mentioned, reactance of 10uF capacitor is ~16 ohms @ 1kHz. That's why the transistor gets hot.

When the transistor switches off, the cap now discharges through the fan. That's why the fan now runs quiet.

You might want to try a diode across the fan (cathode to +12V) instead of the electrolytic cap. Might work better, as it quenches the spikes.

MG's comment would make life much easier.

Re: hot npn transistor when transistor base is fed with pwm signal

Sat, 11 Nov 2017 06:44:39 EDT

SparkChaser posted :

said by teletone:

i wonder why using a larger capacitor brings the heating back? is it the current draw the capacitor causes every time it charges because of the pwm?

Yes, that's why way back in the beginning I mentioned the cap. There is the current charging the capacitor plus the capacitor keeps the transistor from saturating. The longer it is out of saturation the more heat is generated.

Re: hot npn transistor when transistor base is fed with pwm signal

Sat, 11 Nov 2017 01:46:04 EDT

teletone posted : could you use a pwm signal from a 3.3v gpio on a 12v 4-wire fan?

p.s, i've been playing around with esp8266 based boards and had a few 3-wire pc fans kicking around and i thought it would be fun to control them with a smartphone. the whole experience taught me quite a bit on transistors. obviously there is quite a bit more to know.

Re: hot npn transistor when transistor base is fed with pwm signal

Fri, 10 Nov 2017 22:46:42 EDT

TheMG posted : Any reason you can't just use a PWM-ready (4-wire) fan instead? You simply feed the PWM signal into the 4th wire and that's it, circuitry built in to the fan motor handles the rest. No fuss, no PWM noise.

Re: hot npn transistor when transistor base is fed with pwm signal

Fri, 10 Nov 2017 19:53:17 EDT

teletone posted : so with the pwm on the base of the transistor, the transistor is turning on and off many times per second and each time it is causing a current spike which heats the transistor?

i wonder why using a larger capacitor brings the heating back? is it the current draw the capacitor causes every time it charges because of the pwm? would i have to use a larger value resistor if i use a larger value capacitor? is there a calculation for this or is it just a trial and error thing?

Re: hot npn transistor when transistor base is fed with pwm signal

Fri, 10 Nov 2017 19:41:28 EDT

shdesigns posted : The resistor add a low pass filter. It reduces the big current spike when the transistor turns on.

You out the cap across the fan and the resistor between the fan and the transistor?

Re: hot npn transistor when transistor base is fed with pwm signal

Fri, 10 Nov 2017 19:18:08 EDT

teletone posted : put a 10uf capacitor from positive to negative of fan, put a 6.8 ohm resistor in series with the positive of the fan. i didn't have 4.7 ohm resistor.

the noise is quieter, the transistor is cool, the resistor is warm. what's the resistor's function?

edit

i changed the 10uf to 47uf and the noise is gone... but the transistor is getting hot again.

Re: hot npn transistor when transistor base is fed with pwm signal

Fri, 10 Nov 2017 18:38:00 EDT

shdesigns posted : Add a resistor in series with the fan. Leave the cap on the fan. 4.7ohms should make a difference.
An inductor would work even better but would require proper clamping.

Re: hot npn transistor when transistor base is fed with pwm signal

Fri, 10 Nov 2017 18:00:58 EDT

teletone posted : it's electrical. sounds about 1000hz. 0.22 uF in series with 100 ohm resistor didn't help.

if it helps, it's a 120mm brushless dc fan. standard 3pin case fan. only using positive and negative on the fan. not using the tach signal. definitely doesn't like being controlled by a NPN transistor whose base is being fed by a pwm signal. it works but it whines at anything other than 0 or 100%.

Re: hot npn transistor when transistor base is fed with pwm signal

Fri, 10 Nov 2017 17:51:18 EDT

SparkChaser posted : Ahhh.. Is the noise coming from the fan, in which case it's mechanical? Or, it is electrical and being transduced to mechanical to hear it. Does that make sense?

Re: hot npn transistor when transistor base is fed with pwm signal

Fri, 10 Nov 2017 17:41:52 EDT

teletone posted : the fan. it's not installed in a computer. i'm controlling it with an iot device, nodemcu.

Re: hot npn transistor when transistor base is fed with pwm signal

Fri, 10 Nov 2017 17:21:21 EDT

SparkChaser posted : Didn't think to ask this. Where is the noise coming from the computer speaker?

Re: hot npn transistor when transistor base is fed with pwm signal

Fri, 10 Nov 2017 17:15:02 EDT

SparkChaser posted : Sorry, in place of that 22UF you had. Across the motor.

Re: hot npn transistor when transistor base is fed with pwm signal

Fri, 10 Nov 2017 17:10:57 EDT

teletone posted : where do i place them?

Re: hot npn transistor when transistor base is fed with pwm signal

Fri, 10 Nov 2017 17:04:29 EDT

SparkChaser posted : Try something like a 0,22 uF in series with 100 ohm resistor. You can play around with the values to see what works.

Re: hot npn transistor when transistor base is fed with pwm signal

Fri, 10 Nov 2017 16:32:49 EDT

teletone posted : yup. with a 3904 and now a diode in reverse bias between the positive and negative of the fan. i got rid of the cap and the heat is gone. only issue i seem to be having is pwm of anything other than 0 or 100 duty on the input at the base of the transistor causing the fan to make a tinnitus inducing noise. that noise must be the frequency of the pwm. how to i stop it? the cap was doing it but it was causing the transistor to get hot.

should i put a capacitor between the base and collector of the transistor? to smooth out the pwm?

edit, i did that and the noise is gone. i used a 4.7uf ceramic capacitor. between collector and base. gonna run the fan at low speed for a while and check the temp of the transistor.

edit 2, transistor is hot again with cap between collector and base. :(

Re: hot npn transistor when transistor base is fed with pwm signal

Fri, 10 Nov 2017 16:26:47 EDT

teletone posted : thanks. i notice in spec sheets, transistors don't list a maximum current for base. i guess that transistor takes what it needs and the only thing i have to watch out for is not pulling too much mA from the digital pwm pin of the MCU pin that feeds the base of the transistor. i think the specs of the MCU say to keep it under 12mA. so 6mA is fine and dandy.

Re: hot npn transistor when transistor base is fed with pwm signal

Fri, 10 Nov 2017 16:21:08 EDT

SparkChaser posted : Just wondering, is this what you have? (humor me I'm old :p )

[att=1]

Re: hot npn transistor when transistor base is fed with pwm signal

Fri, 10 Nov 2017 16:06:45 EDT

mackey posted :

said by teletone:

i measured the hfe of the transistor with one of those lcr-t4 component testers.

The hFE varies depending on the CE current, I find it extremely unlikely your hFE is anywhere near 217.

said by teletone:

i purchased them on aliexpress. is that the problem maybe?

There's nothing wrong with that per se, however keep in mind generic Chinese parts treat specs as nothing but suggestions that don't need to be followed. Mislabeled or outright counterfeit parts are common as well.

said by teletone:

i put a 330Ω resistor like you said and the base of the transistor is drawing 6.3ma when the pwm signal to the base is 100% forgive me, i'm new at this, but i thought the base was only supposed to draw only 1ma for the transistor to be fully on?

oh wait, 1ma at the base would only let the value of the hfe pass from C to E in mA. so if the hfe is 100, 1 mA at the base would allow 100mA to flow between C & E. so for a 130mA fan, it would need at least 1.3mA for the transistor to be fully on? so 5 times more current is okay?

Like I said, your hFE is almost certainly nowhere near 217. 6.3 mA is nothing when you have 130 mA flowing CE and is perfectly fine.

said by teletone:

also, i notice in spec sheets, there is a minimum hfe and maximum hfe. which value is used when calculating the resistor value for the base?

Depends on what you're trying to do. In this case you want to make sure the transistor is fully on and spends as little time in the transition region as possible, so you'd use the minimum value and round down to a smaller value resistor.

said by teletone:

for the suppression diode, a schottky between the positive and negative of the fan in reverse bias? anything else needed?

Nope, that's it.

Re: hot npn transistor when transistor base is fed with pwm signal

Fri, 10 Nov 2017 15:55:15 EDT

teletone posted : for the suppression diode, a schottky between the positive and negative of the fan in reverse bias? anything else needed?

Re: hot npn transistor when transistor base is fed with pwm signal

Fri, 10 Nov 2017 15:31:39 EDT

teletone posted : i put a 330Ω resistor like you said and the base of the transistor is drawing 6.3ma when the pwm signal to the base is 100% forgive me, i'm new at this, but i thought the base was only supposed to draw only 1ma for the transistor to be fully on?

oh wait, 1ma at the base would only let the value of the hfe pass from C to E in mA. so if the hfe is 100, 1 mA at the base would allow 100mA to flow between C & E. so for a 130mA fan, it would need at least 1.3mA for the transistor to be fully on? so 5 times more current is okay?

also, i notice in spec sheets, there is a minimum hfe and maximum hfe. which value is used when calculating the resistor value for the base?

Re: hot npn transistor when transistor base is fed with pwm signal

Fri, 10 Nov 2017 15:02:20 EDT

teletone posted : i measured the hfe of the transistor with one of those lcr-t4 component testers.

where should i put the cap?

p.s, these are the transistors i have, some pnp and npn:

S9012
S9013
S9014
S9015
S9018
A1015
C1815

A42
A92
2N5401
2N5551
A733
C945
S8050
S8550
2N3906
2N3904

i purchased them on aliexpress. is that the problem maybe?

Re: hot npn transistor when transistor base is fed with pwm signal

Fri, 10 Nov 2017 14:59:54 EDT

SparkChaser posted : If you have it right across the fan, you are trying to charge the cap and drive the motor.

Re: hot npn transistor when transistor base is fed with pwm signal

Fri, 10 Nov 2017 14:58:17 EDT

mackey posted :

said by teletone:

I got the value of the resistor by using a "fixed base bias" calculator. i measured with a multimeter and the 4k3 keeps the base current under 1ma. since the transistor has a hfe of 217, it only needs 1ma to be "fully on". wouldn't 330Ω allow too much current at the base?

Are you sure about that h_FE? Every 2n3904 datasheet I can find maxes out at 100 @ 1mA and is only about 30 @ 100mA. At any rate, such a large resistor will make it take a really long time to turn on. A 330Ω resistor will still be well under 10mA and should be fine.

Re: hot npn transistor when transistor base is fed with pwm signal

Fri, 10 Nov 2017 14:51:22 EDT

SparkChaser posted :

said by teletone:

i got the value of the resistor by using a "fixed base bias" calculator. i measured with a multimeter and the 4k3 keeps the base current under 1ma. since the transistor has a hfe of 217, it only needs 1ma to be "fully on". wouldn't 330Î© allow too much current at the base?

Don't know where you're getting 217 but it's know where near that at 130 mA. (based on minimum)

If you have a 2n2222 around, try it

Re: hot npn transistor when transistor base is fed with pwm signal

Fri, 10 Nov 2017 14:49:25 EDT

teletone posted : the cap is between the positive and negative of the DC computer chassis fan's cable.

Re: hot npn transistor when transistor base is fed with pwm signal

Fri, 10 Nov 2017 14:46:05 EDT

SparkChaser posted : Do you have that cap right across the motor?

Re: hot npn transistor when transistor base is fed with pwm signal

Fri, 10 Nov 2017 14:40:33 EDT

teletone posted : i got the value of the resistor by using a "fixed base bias" calculator. i measured with a multimeter and the 4k3 keeps the base current under 1ma. since the transistor has a hfe of 217, it only needs 1ma to be "fully on". wouldn't 330Ω allow too much current at the base?

Re: hot npn transistor when transistor base is fed with pwm signal

Fri, 10 Nov 2017 14:34:21 EDT

teletone posted : the current draw of the motor is 130ma at 12v.

p.s, without the 22uf cap, i can hold on to the transistor. it's just warm to the touch. the problem is, how do i get rid of the noise the fan makes from the pwm input on the transistor base?

Re: hot npn transistor when transistor base is fed with pwm signal

Fri, 10 Nov 2017 07:12:42 EDT

SparkChaser posted :

He's saying he has a 22uF in parallel with the fan. If he is trying to drive that cap, that transistor is operating in the linear region most of the time, unless it's completely on.

Re: hot npn transistor when transistor base is fed with pwm signal

Fri, 10 Nov 2017 06:41:27 EDT

mackey posted : That limiting resistor is way too high, especially for 3v3. I'm surprised it doesn't get hot even when completely on. The base has a fair amount of parasitic capacitance, and with such a large resistor I doubt it's making it out of the active region before having to switch again. I'd try a resistor somewhere around 330Ω.

I'm assuming you have a suppression diode in there somewhere as well?

Re: hot npn transistor when transistor base is fed with pwm signal

Fri, 10 Nov 2017 04:22:05 EDT

SparkChaser posted : 3904 does not sound like the right part. Get rid of the cap and see now it is. What's the current draw of the motor

EDIT: sent the above at around 4 laying in bed :D
You really have to know the motors current. The 4300 resistor is not much drive for the application, even if the 3904 could handle the current. .

hot npn transistor when transistor base is fed with pwm signal

Fri, 10 Nov 2017 03:56:42 EDT

teletone posted : greetings,

i have a 12vdc powered, 130ma computer case fan being controlled with a 2n3904 (217hfe) npn transistor connected to 3.3v pwm signal (from an esp8266 based device) at the base of the transistor. also, between the base of the transistor and the 3.3v pwm signal from the esp8266, there is a 4300Ω resistor to limit current to the base. everything works fine when the fan is completely on or completely off. the problem is, lets say, if the pwm is set around 50%, touching the transistor for more than a few seconds is painfully hot. also between the positive and negative of the fan, i have a 22uf capacitor to get rid of the 1000hz whine the fan makes.

what's with the heat? is it normal?