[SOLVED] BC327 transistor as switch

[Veketti]

Dear All,

Please help newbie in simple task why this PNP transistor doesn't work in PWM output as it should? If I supply to base constant +5v that led is on but pwm output from microcontroller doesn't dim it. Is it too slow or what's wrong? I know I should use P channel mosfet instead but I don't have them on hand at the moment..



Your help is greatly appreciated.

 

[BigBoss]

If uC output drops down to 0V, the transistor will conduct the current.But 10 Ohm is too low..
If we assume Bdc=200, this resistor should be approx. 3K Ohm.

 

[Borber]

It doesn't work because you used PNP transistor, small 10ohm resistor and maybe PIC is dead. Use NPN BC337 instead, 1k base resistor and hope PIC has survived. Interchange 12VDC and ground and turn arround your LED.

 

[KlausST]

Hi,

Your circuit doesn´t work. It is allways ON.

The PIC will switch between 0V and 5V, but emitter is at 12V.
Therefore the voltage across BE and R is 12V and 7V respectively.

********
Use an NPN as low side switch. depending on b of NPN your 3k could be sufficient. I´d rather go to 1k

Klaus

 

[Audioguru]

I think the base-emitter diode of the transistor from 12V and the 10 ohm resistor caused a 630mA to 1.1A current surge to the output of the PIC that destroyed the PIC. It also probably destroyed the poor little overloaded transistor.

The maximum allowed output current for a PIC is only 25mA. The transistor is not even needed because the PIC can directly drive a 2V LED through a series resistor of about 120 ohms when the LED is connected to +5V or 0V.

 

[Veketti]

In the first place I thought that I'm not able to put this transistor to negative wire as it's grounded elsewhere. Am I right that if I had to put it between positive wire I need to use PNP transistor instead (transistor between positive terminal and load)?

I had the negative wire on hand too so I did like Borber described and it works. Thank you all.

 

[KlausST]

Hi,

@ Audioguru: The LED is rated 12V/260mA. A transistor is helpful.


@Veketti: Be sure the LED has built in current limiting resistor.
Instead of NPN you could use a N channel logic level MOSFET. (The benefit is: lower voltage drop = lower dissipated power, about no PIC pin current. )


Klaus

 

[Audioguru]

I don't think you have a negative wire. Instead you have 0V (which is ground), +5V and +12V wires.
DO NOT use +12V for the transistor or for the LED if you use a PNP transistor.

Why do you power the LED from +12V?
Did you try using the PIC output to directly drive the LED to 0V or +5V directly through a series 120 ohm resistor?

- - - Updated - - -

Audioguru: The LED is rated 12V/260mA. A transistor is helpful.

Thanks, I missed that. A blinding bright 3W LED.
I was concerned about the PNP transistor, LED and power supply being upside down with grid lines all over the place.

 

[Veketti]

Alright, I'll use n-channel fet IRF510 which I have few extra instead.

Sorry, still bit unclear to me that is there possibility to use n-channel fet or NPN transistor between +12V and load? At least in the first place I wasn't able to make it work with IRF510.

 

[KlausST]

Hi,

the IRF510 is NO logic level MOSFET. Mind the guaranteed only 250uA !! at 4V Vgs.
You need Vgs-th to be lower.



Circuit:

LED anode to 12V
LED cathode to FET-Source
FET-Drain to GND
PIC port to FET-Gate

Klaus

 

[Audioguru]

Use a Mosfet with the prefix of "IRL" (not IRF) where the "L" stands for Logic level.

Does the LED have a circuit or series resistor inside it that limits the current to 260mA?

Klaus describes an odd circuit using a P-channel Mosfet as a source-follower that WILL NOT light the LED.

 

[KlausST]

Hi,

Klaus describes an odd circuit using a P-channel Mosfet as a source-follower that WILL NOT light the LED.

Sorry, i mixed up Source and drain.
Circuit:

LED anode to 12V
LED cathode to FET-drain
FET-source to GND
PIC port to FET-Gate

Klaus

 

[Audioguru]

In Klaus's post #12 he correctly describes how to connect a logic-level N-channel Mosfet to a 12V LED that has current limiting built inside it.

 

[Veketti]

Thank you all and sorry for my ignorance, I wasnt aware of these logic level fet's which turns on on lower voltage. Thought that they're all the same 4V Vgs. Just ordered bunch of IRL's.

 

[D.A.(Tony)Stewart]

• There are over 100 logic families but fewer input threshold standards.
• There are many standard test thresholds for MOSFETs, 10V, 4.5V , 2.5,1.8, 1.5 0.9, etc. while the actual thresholds are lower.
• There are over 25k different active FET p/n's.