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Author Topic: PMW dimmer issue  (Read 5319 times)

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mantella

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PMW dimmer issue
« on: July 27, 2011, 16:51:01 PM »
I built the pmw dimmer, but it won't dim. I read all the posts about it to try and troubleshoot, but nothing has worked. I've tried two different 555 chips. I've tried two different 2N2222 transistors. I've tried two different pots. And I've tried different LED's. I've gone over the connections and it all looks right to me. Can anyone see what I'm doing wrong? Thanks.

cheerio

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Re: PMW dimmer issue
« Reply #1 on: July 27, 2011, 21:07:41 PM »
plase make 2 pictures
1 close-up like you did and 1 where we can see all cables connected

mantella

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Re: PMW dimmer issue
« Reply #2 on: July 27, 2011, 22:36:11 PM »
okay, it's kinda blurry. But you should be able to see the connections...


kam

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Re: PMW dimmer issue
« Reply #3 on: July 27, 2011, 23:34:37 PM »
What happens if you...
1] ...remove R1 (4K7) from the circuit?
2] ...disconnect pin 7 of 555?
3] ...remove R1 and disconnect pin 7?
4] ...replace R1 with a 10K resistor?
5] ...replace R1 with a 22K resistor?
6] ...replace R1 with a 22K resistor?
7] ...replace C1 with a 1uF electrolytic cap?

Are you sure that...
a] ...this is a 2N2222 transistor? Is it a KSA?
b] ...the battery is not dead? Did you test it with a power supply?
c] ...pin 6 and 7 are correctly connected? (i cannot see from the picture very clear)

Finally, do this. Remove all the LEDs and the transistor. Connect only 1 LED between R1 and V+, with anode facing V and cathode facing R1. Then, replace R1 with a 1K resistor. Tell me what happens when you play with the potentiometer.

In general, from what i can see, the connections are correct.

mantella

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Re: PMW dimmer issue
« Reply #4 on: July 28, 2011, 00:38:20 AM »
What happens if you...  LEDs don't light
1] ...remove R1 (4K7) from the circuit?  all of them light up as before, but they don't dim with the potentiometer
2] ...disconnect pin 7 of 555?  nothing lights up
3] ...remove R1 and disconnect pin 7?  all of them light up as before, but they don't dim with the potentiometer
4] ...replace R1 with a 10K resistor?  all of them light up as before, but they don't dim with the potentiometer
5] ...replace R1 with a 22K resistor?  all of them light up as before, but they don't dim with the potentiometer
6] ...replace R1 with a 22K resistor?  all of them light up as before, but they don't dim with the potentiometer
7] ...replace C1 with a 1uF electrolytic cap?  all of them light up as before, but they don't dim with the potentiometer

Are you sure that...
a] ...this is a 2N2222 transistor? Is it a KSA?  it says 2N2222 A338 on it
b] ...the battery is not dead? Did you test it with a power supply? battery reads 8.6Volts. I haven't tried a power supply.
c] ...pin 6 and 7 are correctly connected? (i cannot see from the picture very clear)  I double-checked and as far as I can tell they are correct.

Finally, do this. Remove all the LEDs and the transistor. Connect only 1 LED between R1 and V+, with anode facing V and cathode facing R1. Then, replace R1 with a 1K resistor. Tell me what happens when you play with the potentiometer.  the LED doesn't light up or do anything

thanks for your help. Does that tell you anything? I'm not really sure what's going on. It seems like such a simple circuit.

kam

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Re: PMW dimmer issue
« Reply #5 on: July 28, 2011, 18:01:01 PM »
thanks for your help. Does that tell you anything? I'm not really sure what's going on. It seems like such a simple circuit.

you have some very funny results  ::)

first of all, do you have a datasheet for 2n2222 for this specific package that you have? I think that the pinout is not CBE but it is EBC. If you connect an EBC  transistor as a CBE, then it should work as you mention. So you can make a test (with the danger to fry the transistor) and connect it as if it was EBC. This is the best i can think for now.

Also, regarding the last test, i think you did it wrong. Here is what i said:
Quote
Finally, do this. Remove all the LEDs and the transistor. Connect only 1 LED between R1 and V+, with anode facing V and cathode facing R1. Then, replace R1 with a 1K resistor. Tell me what happens when you play with the potentiometer

So, remove the transistor, R1 and all the LEDs. Then connect the anode of one LED to the positive supply. Then connect one end of a 1K resistor to the cathode of this led. The other end of the resistor should be hooked up to the 7th pin of the 555.

mantella

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Re: PMW dimmer issue
« Reply #6 on: July 28, 2011, 21:01:36 PM »
okay, I re-did the tests as you said, and still nothing.
I even went to rat-shack and got some more 555s and 2N2222s to try.

The 2N2222 is rated at V=40,  I=600,  power dissipation 625,  h=200

tried hooking up the transistor both ways, but both directions gives the same result (LED are on, but won't dim)

the 555CN says 18V max, 600mW, 200mA

mantella

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Re: PMW dimmer issue
« Reply #7 on: July 28, 2011, 22:50:42 PM »
okay! I have (some) success.
I took it apart and completely rebuilt it using a different breadboard, and also use the 12Volt configuration along with a power source this time.
Now they light up (really bright) AND they dim when I turn the potentiometer.
But, they don't turn all the way off - unless I unplug the power completely.
Is that a function of the pot? can I get it to turn all the way off (and maybe not so bright) if I use a different value of potentiometer?

thanks for all your help.

kam

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Re: PMW dimmer issue
« Reply #8 on: July 30, 2011, 08:42:20 AM »
hmmm that is strange. you can try using a larger capacitor for C1 or indeed try a larger pot value.

i think that the LEDs you have have small forward current, or they light at only a few mWatts. if this is the case, the you should replace the diodes with shottky diodes which have very small forward voltage drop and can achieve duty cycle closer to the edges (0% and 100%). OR, here is an idea that just came to me. Increase R1. What is the hfe of the transistor you have? Can you send a datasheet link for this?

mantella

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Re: PMW dimmer issue
« Reply #9 on: August 23, 2011, 06:45:26 AM »
sorry I haven't found a datasheet for that transistor. I kind of got distracted with other projects. It's just a 2N2222 I found at Radio Shack.

I do have another question however. I'm trying to build the 555 PWM circuit with 32 LEDs. They are 3.0-3.4V, 24mA and my power source is a rechargeable lithium battery. The battery output will typically be between 3.7V and 4.2V.  I don't have all the parts yet, so I'm just trying to plan it out at this point. But would you suggest using your 5V setup, or should I try a different resistor since I'm not giving it 5V? Or will the PWM circuit even work with less than 5V?