Pwm motor makes a buzzing noise

skmdmasud · Oct 27, 2013

audioguru said:
No.
The diode from drain to source is not needed because it would block the drain from going negative which never happens and the Mosfet has a diode there anyway as part of its construction.
The drain does not go negative, instead it goes POSITIVE when the Mosfet turns off then the diode in the Mosfet has avalanche breakdown from the high voltage which causes heating. The diode must be parallel to the motor to prevent the positive voltage spike at the drain of the Mosfet. The cathode of the new diode connects to +VDD and its anode connects to the drain of the Mosfet. If its wires are long (far away from the motor) then it might cause oscillation ringing.

Ok so finally i had some time to open up the dashboard of my car, located the blower and installed a snubber diode in the blower. After testing my mosfet is still heating up like before which means i need to built the driver circuit.

One question do i remove the diode which is across my mosfet or leave it there?

Regards.

audioguru · Oct 28, 2013

Maybe the voltage spike from your motor switching off was not high enough to activate avalanche breakdown of the diode in the Mosfet.
If you use a real snubber diode then you can select one that switches quickly. The diode in a Mosfet switches slowly which causes some heating.

skmdmasud · Oct 28, 2013

audioguru said:
Maybe the voltage spike from your motor switching off was not high enough to activate avalanche breakdown of the diode in the Mosfet.
If you use a real snubber diode then you can select one that switches quickly. The diode in a Mosfet switches slowly which causes some heating.

I used diode uf5408. I will try making driver and make it a good complete job. There is no shortcut to success...no more parts skipping.

audioguru · Oct 28, 2013

The UF5408 is a nice fast diode and will work well parallel with the motor.

skmdmasud · Oct 29, 2013

alec_t said:
You could try this driver circuit for the FET. Q2 and Q3 are the pair of transistors DrPepper mentioned. If your PWM source is low voltage (3.3V, 5V?) the additional Vbe drop imposed by Q2 could prevent the FET turning on fully. Use of a higher voltage for +V, plus Q1, can overcome that. Bear in mind, though, that Q1 inverts the effective PWM duty cycle.
View attachment 81568

Hi I finally made the driver, as you said my output was inverted so had to change some code and things started to work. I connected the driver +ve to the car 12V. Even after the driver circuit my mosfet is getting pretty hot dont understand why.

One mysterious thing happened, with the car turned off every thing worked except for the mosfet heating (tested over 15 min), the moment i turned on the car i heard a soft exploding noise and then smoke, I turned off the car and opened the blower compartment and found out that the diode that i connected with the blower exploded it got tosted. This can only happen if the polarity in the blower change.

alec_t · Oct 29, 2013

This can only happen if the polarity in the blower change.

It would certainly happen if the diode was connected the wrong way round or was incorrectly rated.
If the FET still gets hot it may be oscillating. It is important that the wire from driver to gate is as short as possible.

skmdmasud · Oct 29, 2013

alec_t said:
It would certainly happen if the diode was connected the wrong way round or was incorrectly rated.
If the FET still gets hot it may be oscillating. It is important that the wire from driver to gate is as short as possible.

Hi..
diode was not connected the wrong way nor its rating was low, i tested it the day i connected it.

Anyway yesterday i drove for over 25 min with ac on and did not notice any smoke , am i correct to say that if my mosfet does not burns up in 25min then it will not burn at all.

alec_t · Oct 31, 2013

am i correct to say that if my mosfet does not burns up in 25min then it will not burn at all.

No. There could be a gradual heat increase. Did you check how hot the FET was after 25 min? Could you keep your finger on it then?

skmdmasud · Oct 31, 2013

alec_t said:
No. There could be a gradual heat increase. Did you check how hot the FET was after 25 min? Could you keep your finger on it then?

I did not check after 25 min but did testing for over 15 min. I can keep my figure for around 3 or 4 sec on the heatsink. I can keep longer if some one challenges me lol .

alec_t · Nov 1, 2013

I wouldn't be happy with it running that hot. What size heatsink are you using?
What voltage is +V for the driver circuit?

skmdmasud · Nov 1, 2013

alec_t said:
I wouldn't be happy with it running that hot. What size heatsink are you using?
What voltage is +V for the driver circuit?

hi..
heatsink is very tiny about 1 inch long and 1cm width, you can only put it in one fet or vregulator t12 package. driver circuit +V is direct car battery 12.xxV

alec_t · Nov 2, 2013

Try a bigger heatsink.

skmdmasud · Nov 2, 2013

alec_t said:
Try a bigger heatsink.

hi, can't put a bigger heat sink lack of space. i think it will last if it doesn't and burns out then next time i will put 4 cheap fets in parallel to handle what ever is happening.

thanks for helping me(armature hobbyist) out.

alec_t · Nov 2, 2013

if it doesn't and burns out then next time i will put 4 cheap fets in parallel to handle what ever is happening.

There may not be a next time. If it burns out, your car may too .

skmdmasud · Nov 4, 2013

alec_t said:
There may not be a next time. If it burns out, your car may too .

lol it may happen. what else can i do??

alec_t · Nov 4, 2013

Can you mount the circuit elsewhere so that there's space for a bigger heatsink?
What is the length of the wire/trace linking the driver to the gate of the FET?

skmdmasud · Nov 4, 2013

alec_t said:
Can you mount the circuit elsewhere so that there's space for a bigger heatsink?
What is the length of the wire/trace linking the driver to the gate of the FET?

hello. the space in the front panel is very right, after few day i will go to the market and will try to find a bit bigger heatsink.

driver and mosfet is next to each other with a 2 inch wire connecting them.

alec_t · Nov 4, 2013

2" of wire hopefully isn't too much and causing high frequency oscillation of the FET.
Did you replace the burnt diode? One with a higher current rating preferably? Without the diode your FET may fail from back-emf spikes. Those spikes may be what is causing the FET heating.

skmdmasud · Nov 4, 2013

alec_t said:
2" of wire hopefully isn't too much and causing high frequency oscillation of the FET.
Did you replace the burnt diode? One with a higher current rating preferably? Without the diode your FET may fail from back-emf spikes. Those spikes may be what is causing the FET heating.

Hi..
no i did not replace the diode, the diode worked fine while i was testing and suddenly in just moments it exploded, which makes me wonder that the blower output polarity must be getting reversed with some settings. I need to further look into the wiring.

alec_t · Nov 5, 2013

It's a very strange arrangement if some 'settings' (?) change the polarity

Pwm motor makes a buzzing noise

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