lighting up 24vdc small car bulb

Discussion in 'General Electronics Chat' started by lloydi12345, Dec 6, 2012.

  1. lloydi12345

    Thread Starter Member

    Aug 15, 2010
    103
    0
    Hi, I'm controlling 24v rated light bulb with a microcontroller but it seems like sometimes it doesn't light. I tried resetting the circuit then it worked again. Then after a while the bulb again dies. Everything works again after random times of resetting the circuit and the process goes on. I am using PIC18F4620.

    This is my schematic:

    [​IMG]

    Do i need protection diode for this one? I am using TIP120.

    I am flashing 20 bulbs with 20 transistors. I am using different patterns to create sequences.

    Edit:
    I am trying two bulbs here only. They are rated 24v 300mA. My power supply is 24Vdc 2A. When I am replacing the bulbs with leds, the circuit works properly.

    thanks,
     
    Last edited: Dec 6, 2012
  2. panic mode

    Senior Member

    Oct 10, 2011
    1,321
    304
    20 bulbs times 0.3A = 6A. that is 300% of your power supply rating so I hope they are not all on at the same time. you can check if the lamp works when you short transistor (C-E terminals). if that still gives you problem, don't look at the transistor and PIC, problem is elsewhere. you don't need a diode for this circuit because load is resistive and not inductive. if you can turn on the bulb sometimes, it should work all the times. maybe take a closer look at that power supply and see if it is overloaded. 24VDC supplies are common in industry. most of them tolerate overload conditions rather well. not all but many do not shut down on overload, or similar and reset automatically. it could be that your unit has thermal shutdown that resets automatically after while.
     
    Last edited: Dec 6, 2012
  3. IC-Man

    New Member

    Jan 3, 2012
    26
    4
    Beside the power supply current limitation, the inrush current on cold light bulbs might be the problem. Using a smart current limiting driver, like the iC-DXC (http://www.ichaus.biz/product/iC-DXC ) or similar could help. Another problem might be the fact that the microcontroller I/O ports are in the input mode after reset. This mean that the base of Q2 is floating. Here you find some useful hits to interface to the 24V environment http://www.ichaus.biz/wp1_mcu_interface .
     
  4. n1ist

    Active Member

    Mar 8, 2009
    171
    16
    Do you have a common ground between the 24v supply and the logic supply?
    /mike
     
  5. Audioguru

    New Member

    Dec 20, 2007
    9,411
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    When a 300mA incandescent light bulb is cold then it draws 3A or more until its filament reaches 2000 degrees C.
    An LED doesn't DOO DAT.
     
  6. lloydi12345

    Thread Starter Member

    Aug 15, 2010
    103
    0
    Thanks everyone for your replies. Yes I already considered a 6A supply but I'm trying for now 1A since I'm only using 2 bulbs for a while. I am using now a dc power supply with regulated 24Vdc 1A output from one channel and another channel for 5Vdc 1A output.

    n1st: yes I got both ground shorted.

    IC-man/audioguru: 3A? do really bulbs like that suck up that much current when they are fully off?

    I am still trying to check whether it's the dc power supply problem. Later on a few hours, I will be powering up everything through 12Vac 10A transformer rectified to roughly 17Vdc (the light looks fine i checked it on dc power supply). I will see what I could get when I power them through transformer. I will be using rectifier diodes P600b. It is a diode rated at 6A.

    What if I turn everything on? Will it be fine with the diodes?
     
  7. JohnInTX

    Moderator

    Jun 26, 2012
    2,348
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    Not when they are off but when you apply power to a cold one you can indeed get many times more than the rated current while the lamp heats up. This happens quickly but it may be in 'overcurrent' long enough for your power supply to enter its foldback current limiting mode (assuming it has one but sounds like what's happening to me.) I have a benchtop 20A 12V foldback-limited supply that does this when powering one 12V lamp rated at 1.5 amps. The inrush causes the supply to drop its output voltage (foldback limiting) until the load is removed. I can eventually get the lamp to light by 'pumping' it, connecting intermittently until the filament heats enough to slow the inrush to something that won't trip the current limit.

    Besides the special drivers you can mitigate the problem by adding a keep alive resistor across the darlington driver to provide a current path for the lamp even when the driver is off. Size the resistor to keep the filament just below incandescence i.e. a dull red temperature. Since the filament is already warm, it will pull less current when the driver turns on.

    You might also try a soft-start scheme in firmware. Drive the lamp with a PWM starting at small enough duty cycles to get the lamp warm without tripping the supply then increase the duty cycle as it warms up. The timing of the initial warm up pulses has to be shorter than the response time of the current limit on the power supply for this to work.

    Try a battery to power the circuit for a test. That would help identify a foldback current limit issue on the supply.

    It should be clear that you are not going to be able to hit 20 lamps at the same time without solving some of the inrush issues. Firmware can help here. Stagger the turn-ons a bit, do the soft start etc. Even if you solve the supply issue, you still don't want to slam on a lot of incandescent loads at once.

    BTW. Some switches and relays have specified ratings for incandescent / tungsten loads. When they do, that rating is always way less than the resistive ratings for the unit. Inrush current is why, especially with bigger loads.
     
    Last edited: Dec 6, 2012
  8. lloydi12345

    Thread Starter Member

    Aug 15, 2010
    103
    0
    What do you mean with connecting keep alive resistors across the transistor? Sorry I can't picture it out.

    Hi I think I now have an idea how to light them up safely. I will be first trying a sequence that will light each bulb half a second till everyone lights up.
     
  9. JohnInTX

    Moderator

    Jun 26, 2012
    2,348
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    In your schematic, connect a resistor from C of the transistor to ground. Now, even with the transistor off (or even removed) current will flow through the bulb. How much current depends on the resistor.

    The trick here is to select a value (and wattage) that will just begin to light the bulb - think rheostat, dialing it up and down until the bulb is just barely lit, that's your R value. The R value will be greater than you'd normally use to maybe dim the light by dropping some of the applied voltage. In this case, you want to drop most of the voltage, leaving just enough for the lamp to run it at a dull red glow or even less.

    You won't need that long. While bigger lamps take longer to fully heat up, its still pretty fast. A small flashlight lights up essentially as fast as you can see it. A big outdoor spot light has a noticeable lag (but still much quicker than 1/2 sec). The bulbs that caused the 20A supply to current limit were aircraft nav lights with big, rugged filaments with a turn on long enough to be noticeable to the eye (and the power supply!)

    The best way would be to add a small-as-you-can resistor in series with the lamp as a current shunt and observe the voltage across it with a scope as the lamp is turned on. By small-as.. select the R value so that at the lamp's rated current, it will drop just enough volts so that your scope can measure it on its most sensitive scale. Even that will cut the inrush a bit but you'll be able to get an idea of what it is. Modulate the PWM output accordingly for the soft start. Once the inrush on one lamp has died down, you can soft start the next. Just guessing but even higher current lamps should be pretty much done within 100ms or so.

    Finally, don't try to cut the times too fine. They will vary from bulb to bulb and in each bulb as it ages (due to filament erosion).
     
    Last edited: Dec 6, 2012
  10. Audioguru

    New Member

    Dec 20, 2007
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    Yes.
    Simply measure the resistance of a light bulb when it is disconnected from power. Then use Ohm's Law to calculate its current and power when it is cold.
     
  11. THE_RB

    AAC Fanatic!

    Feb 11, 2008
    5,435
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    I recently did some tests with filament bulbs, the amount of current needed to get the filament to the thermal runaway point (usually seen as the point where it can JUST be seen starting to glow red in a dark room) is around 25-30% of the full running current. Under that current the filament is "cold" and will have a very low resistance and very low power dissipation (ie; very low self-heat).

    In other words it takes quite a bit of current to get a filament "warm" for a warm start.

    I'm worried with your suggestion of resistors that running 25% filament current by using a resistor would be extremely power hungry. :)
     
  12. JohnInTX

    Moderator

    Jun 26, 2012
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    Quite right, of course. Keep-alive is an old-school approach whose use is probably best limited to illustrate the inrush problem. Its certainly not optimal in these energy conscious times.

    Of course, neither is using big banks of incandescent lamps in the first place :eek:
     
  13. Audioguru

    New Member

    Dec 20, 2007
    9,411
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    Previously, the highest electrical power demand was at Christmas time due to the millions of incandescent Christmas tree lights heating the planet.

    Now my electrical utility company gives away for free a new set of LED Christmas tree lights for each set of old power-hungry incandescent ones that are traded in.
    Now electrical power consumption at Christmas time has dropped a lot. So the electrical utility does not need to build additional expensive generating stations.
     
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