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Author Topic: Dimensioning a darlington switch  (Read 2548 times)

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speed89

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Dimensioning a darlington switch
« on: July 02, 2011, 12:12:44 PM »
[img]Hi guys,
I followed this tutorial http://pcbheaven.com/wikipages/Transistor_Circuits/ and I'm trying to design something like this but using a dc vibration motor(like those used by the mobile phone).My idea is using a pic10f200 for delay the playing of the dc motor.
The pic waits 10 seconds and then keep on turning on and off the gpio0 every 2000 cycles(internal clock 8Mhz).
Anyway the code of the pic is not the problem.
When I try to connect the motor it works for just a second and then the pic reset itself waiting again 10 second and powering the dc motor for that second.Actually it schould keep it on until I get out the power supply.
If I change the vibrator motor with another dc motor as big(i think Iload almost the same) it works properly as well as with a led.
Changing the power supply until 5 Volts doesn't change anything.
With the resistor Rb(6.2k) or without it works in the same way.In fact the current from the pic is IOH=-3mA indipendently by the resistor.
The transistor is a BDW236 (darlington npn) hfe=1000(min) so Ib=0.1A/1000=1mA(needed to turn it on).

Whats wrong?

kam

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Re: Dimensioning a darlington switch
« Reply #1 on: July 02, 2011, 12:24:50 PM »
There are 2 reasons why this happens. Most unlikely is a brown-out reset. This is caused if the motor draws instantly a lot of current, causing the supply of the PIC to fall bellow a certain level and reset (brown-out).
Most likely though is that the motor sends spikes when is turned off. A motor is an inductive load. Inductors store energy when current flows, and when current stops flowing they release this energy. This energy causes the PIC to reset.

To find out which of the 2 happens, try this: Let the PIC turn on the motor for 2 seconds and then turn it off. Watch when the PIC resets. If it resets upon turn-on, it is a brown out, the only solution of which is to increase the supply current capacity.

If the PIC resets upon turn-off, it is bad news. The best solution is to supply the motor from a completely different power source (a different power supply for example). Otherwise, you need to install filters. First of all NO MATTER WHAT, you need a flyback resistor across the motor. That is totally necessary. Also, you need a big capacitor across the supply of the motor, and if necessary, you need to make an LC filter instead to "separate" the PIC supply from the motor supply. Make sure also that you have 0.1uF and a 10uF capacitors across the supply of the PIC. These measures will work for you.

speed89

  • Guest
Re: Dimensioning a darlington switch
« Reply #2 on: July 03, 2011, 19:51:22 PM »
Thank you or you answer  ;)
I just tryed changind the pic.using a pic16f690 it works as it should!
I tested it in the same condictions and writing the same code(adapted with f16 registers).It supply the darlington npn transistor and is able to supply even load much more heavy.
If you read the electrical specifications they have the same electrical levels at all!
I need to use the 10f200 for this aim and not the 16f690.
What could be wrong now?Any suggestion?

speed89

  • Guest
Re: Dimensioning a darlington switch
« Reply #3 on: July 03, 2011, 21:00:39 PM »
SOLVED!
I added a decoupling capacitor of 16uF in paralel with the motor (and the flyback diode actually added from the begin).
Thank you a lot! :) :) :) :)