2N2222 getting too hot

Discussion in 'The Projects Forum' started by mattstamp, Sep 28, 2011.

  1. mattstamp

    Thread Starter New Member

    Sep 28, 2011
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    Hello,

    I am making a back light for a picture frame for my son. It's going to be lit by LED's. I found this site and followed the instructions and put it together on a breadboard. I tested with 3 LED's at 12 volts and my 2N2222 is burning up almost instantly. It doesn't fail, just gets too hot to touch. Is there anything I can do to this circuit to get a cooler transistor in there?

    http://pcbheaven.com/circuitpages/LED_PWM_Dimmer/

    Thank you for your time,
    Matt
     
  2. t_n_k

    AAC Fanatic!

    Mar 6, 2009
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    I suspect a wiring error. The schematic looks incorrect - IC pins 7 & 3 should probably be swapped with respect to their function. Pin 7 to potentiometer terminal 2 and pin3 to R1/T1 base node.
     
  3. mattstamp

    Thread Starter New Member

    Sep 28, 2011
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    To update the high temperatures only occur when the variable resistor is turned down completely, at it's lowest brightness.
     
  4. mattstamp

    Thread Starter New Member

    Sep 28, 2011
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    Swapping those pins and I loose all dimming functionality.
     
  5. Hi-Z

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    Jul 31, 2011
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    The suggested led configurations look pretty dodgy to me. I assume you're using 3 leds in series with a 47 ohm resistor. The author seems to be relying on the fact that the leds are exactly 3.6V, and the power supply no more than 12.3V (never mind 13.5V!) , if the led current spec is to be adhered to.

    It seems to me that your leds have a lower forward voltage than 3.6V, and this would lead to (potentially) very large currents. You could quickly check the voltage across the resistor. Ohm's law will tell you what the current is (I = V/R).

    Lower voltage leds are actually an advantage, because once you've got the right value resistor, you'll not need to have such an accurately defined power supply voltage. So, do check the voltage across the leds: you can then subtract this from 12V and use this value (V) to calculate the resistor value you'll need for the desired current. So, R = (12-V)/I.

    Assuming you're not aiming for very large currents, and/or large numbers of leds, you shouldn't need a heatsink for the 2N2222.
     
  6. mattstamp

    Thread Starter New Member

    Sep 28, 2011
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    Forward voltage per the datasheet says 3-3.2v. Yes I am using 3 in series with a 47 ohm resistor.
     
  7. Hi-Z

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    Jul 31, 2011
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    Actually the figure is 13.0V, not 12.3V (I forgot about the transistor Vbe). But the principle remains: the led circuit is sailing far too close to the wind, and don't forget that your leds probably have a very different Vf anyway.
     
  8. mattstamp

    Thread Starter New Member

    Sep 28, 2011
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    My understanding of this circuit is that it is flashing the LED's to make them appear dimmer. Why is it that at full brightness the transistor is cool to the touch but at it's lowest brightness I burn my finger?
     
  9. Hi-Z

    Member

    Jul 31, 2011
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    OK, so you could easily have 3V or more across the 47 ohm resistor, so you'd be getting 60mA+ led current (which is probably more than you'd want). The 2N2222 would have to dissipate rather more than 40mW, which shouldn't be a problem. However, it's still worth checking the voltage across the 47 ohms - your leds may be less than 3V each.
     
  10. Hi-Z

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    Jul 31, 2011
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    That's interesting. It sort of suggests that the drive to the transistor isn't behaving properly: it should be a "clean" PWM, i.e. it should exist as either 0V or +12V at any given time. But even so, I wouldn't expect all that much difference in power terms, even if there were "bad" voltages being applied to the transistor's base.

    Have you double-checked the wiring? (Not that I can imagine any misconnection that would result in correct operation AND the symptoms you're describing.)
     
  11. mattstamp

    Thread Starter New Member

    Sep 28, 2011
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    I have double checked the wiring. I'm at a loss for why this is happening and am too inexperienced to make any determination. Should I go find another PWM dimmer schematic?
     
  12. Hi-Z

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    Jul 31, 2011
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    Just a possibility: R1 seems rather high at 4k7. I would imagine you'd get a more satisfactory drive if you were to reduce this to 1k2, say. Also, do check that you're getting the right led current (when the leds are fully lit).
     
  13. t_n_k

    AAC Fanatic!

    Mar 6, 2009
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    I've not seen that circuit layout before - but I'm not a regular 555 timer user.

    I expected to see something more like the attached which I've just tested OK on the proto-board.
     
  14. Hi-Z

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    Jul 31, 2011
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    I think you should try a different connection for the 2N2222 - at the moment it's an emitter-follower, but I think a saturated switch would be a far better bet:

    Connect Tr1's emitter to 0V and its collector to the bottom of the led chain, and the top of the led chain (not forgetting the resistor - which probably could do with increasing from 47 ohms) to +12V. Leave R1 at 4k7.

    The 2N2222 will now be doing what it's best at doing - switching sharply between on and off.
     
  15. Tealc

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    Jun 30, 2011
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    I tried that PCB Heaven circuit a while back and I couldn't get it to work properly. I just assumed a wiring error on my breadboard but now I'm not so sure.

    This is the one I went with in the end, although I may have loaded the LED and resistors onto the emitter rather than the collector, but it seemed to work just fine.

    [​IMG]
     
  16. t_n_k

    AAC Fanatic!

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    The PCB Heaven circuit is an unusual one. It seems to be a departure from the typical 555 Timer astable configuration - albeit with the steering diodes to control the duty cycle. Tealc's case which worked, is what one would normally expect to see. As I indicated in my earlier post, I have a similar configuration working on the prototyping board.

    It would be interesting to get Bill Marsden's comments on the PCB Heaven circuit.
     
  17. SgtWookie

    Expert

    Jul 17, 2007
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    I've seen similar to the PCBHeaven schematic, but I don't care for them.

    The 555 OUT pin 3 can source/sink up to 200mA. Why use pin 7, where the max sink current is not documented?

    Why did they use the 2N2222 as a voltage follower rather than a saturated switch?

    The pot R2 really needs some resistance between the wiper and pin 3. Otherwise, it'll really get blasted with current when set to the extremes.

    The circuit by Tony van Roon that Tealc posted in reply 15 is far better.

    I see that Tealc found the schematic here:
    http://www.reuk.co.uk/LED-Dimmer-Circuit.htm

    but the original source for the schematic was Tony van Roon's site:
    http://www.sentex.ca/~mec1995/circ/pwm555.html
    who re-drew it from an Elector magazine article.
     
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  18. mattstamp

    Thread Starter New Member

    Sep 28, 2011
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    Well I didn't quite understand where the base was in all of this. Moving the transistor down as you stated and leaving the base the same I blew up the transistor with a quick pop. Opps hahaha.

    I think I'm going to give that schematic a try. I've seen it a lot around as a adaptation of a motor speed control. Thank you all for your help and insight. I'll let you know how it goes.

    Thanks,
    Matt
     
  19. Hi-Z

    Member

    Jul 31, 2011
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    Sorry to have made you blow up one of your transistors - I hope you've got a spare! However, I don't understand how that could happen - you've still got the 47 ohms in series with the leds, I assume? And the base is connected to pin 7 (and not pin 3)?

    I think the circuit as I suggested is probably the simplest way of achieving a high led current - but do note that there is still a big question mark regarding the led forward voltages, and therefore the correct value of series resistor.

    There is another slight disadvantge in using pin 7: it's a saturated open-collector output, and it may require perhaps a larger pullup current than that provided by 4k7 for fastest/cleanest switching. I don't think you'd go far wrong in taking R1 down to 1k2, if you've got something suitable to hand.

    I would try to find out the led voltages first; then you can choose an appropriate series resistor (I'll bet it's not 47 ohms!). If you're brave enough, you could then try as I've suggested - though I do think we need to explain why the transistor blew up...
     
  20. Hi-Z

    Member

    Jul 31, 2011
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    Just trawling around, looking for "555 pwm", I found this:

    http://www.dprg.org/tutorials/2005-11a/index.html

    It's very similar to the circuit you've been playing around with, in that it's using the pin 7 output. In this case, it happens to be driving an n-channel fet to control a motor, but there's nothing to stop you using the 2N2222 in a similar connection, as I've suggested above.

    But, I'll repeat, you need to get to bottom of two things: why the tansistor blew up, and exactly what voltage leds you have. The two may be interrelated, I suppose.
     
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