555 LED Dimmer -- question about oscillating load above 200mA

Discussion in 'General Electronics Chat' started by beginnersluke, Nov 9, 2016.

  1. beginnersluke

    Thread Starter Member

    Aug 26, 2015
    35
    2
    Hi!

    I have a question which I've been unable to find an answer to thus far.

    Backstory: I have some LED lights from IKEA, but they're crazy bright. I've build various 555 circuits before and have seen schematics and videos online for using the 555 as a PWM generator to dim LEDs. So far all is easy and good.

    Here is my problem: I have 4 lights, 2 power supplies (transformers). The power supplies are rated at 350mA on the output. Looking at the datasheet of the 555, I see that it can sink or source up to 200mA. That's not enough!

    I was thinking I could perhaps use a solid state relay. I had trouble finding data on how "fast" these could be (maybe I need help knowing the right specification).

    It that a good solution? Is there a better way?

    (My other thought was to build four 555 circuits -- one per light, but wasn't sure how having the 555s share a potentiometer (would that work? I'd be very thankful if someone could explain why, either way). I also figured they would not be perfectly in sync, which seemed a problem, since if only one was on, it seems it would get more current that it should.)

    I apologize for my lack of knowledge and if this is simple to the people here. I am working hard to learn, but didn't get anywhere with google and forums trying to figure this out.

    Thanks you!
     
  2. AnalogKid

    AAC Fanatic!

    Aug 1, 2013
    8,029
    2,279
    First, standard solid state relays are not fast enough.

    Second, the "power supplies". Just transformers, or something with rectification, current limiting, and maybe some filtering? Photos will spare many electrons.

    Multiple 555's are tempting, but the complications you've brought up are significant enough to make a single 555 circuit with an added higher-power output stage less work, fewer parts, etc. The standard 555 circuit cannot adjust below 50% duty cycle, but adding two small diodes and one pot will get you 5% - 95% adjustability. After that, a 1-transistor output driver *probably* will work, but we can't tell without more data.

    First, what do you have? Part/model numbers, link to IKEA website product page, photos, anything... There are WAY too many possibilities to guess.

    ak
     
  3. beginnersluke

    Thread Starter Member

    Aug 26, 2015
    35
    2
    Here is a photo of the power supply. It outputs DC so it does have rectification. Sorry for the lack of detail on that.

    In case it's not super easy to read, here is what it says:

    LED Driver
    Class 2 Power Supply

    Input: 100-120VAC
    50/60Hz 0.06A

    Output: Constant Current:
    DC350mA 0.5V...7.2V
    Constant Voltage:
    DC8.5V ≤ 300mA

    (This brings up another newbie question, there's no switch, so how does one "select" the mode between constant current or voltage?)

    I don't see these lights anymore, but if you look up DIODER, you'll find some that are quite similar.

    Yes, the circuit with the diodes is what I've found for a dimmer. The transistor sounds good (I don't know why my mind jumped to a relay and didn't think of a transistor -- I'm just learning!).

    Hopefully that information on the power supply is helpful, and I'd appreciate detail on how to properly connect the transistor.

    Thank you!
     
  4. mcgyvr

    AAC Fanatic!

    Oct 15, 2009
    5,394
    1,194
    Buy some spray on tailight tint and give them a quick coat.. Done :)
    Or sandpaper the plastic lens
     
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  5. AnalogKid

    AAC Fanatic!

    Aug 1, 2013
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    2,279
    The regulating circuit does it automatically. The circuit makes a constant current into external loads that are low enough in equivalent resistance that the voltage developed across then by that current is within the range of 0.5 V to 7.2 V. As the external resistance increases, the output voltage must increase to keep the output current constant at 350 mA (basic Ohm's Law). But the power supply has a maximum output voltage set by its power transformer turns ratio, in this case 8.5 V, at which it no longer can keep increasing to supply the full output current. This tells us that the supply is limited to 3 watts, the limits of the envelope defined by 8.5 V and 0.35 A.

    ak
     
  6. AnalogKid

    AAC Fanatic!

    Aug 1, 2013
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    https://images.search.yahoo.com/sea...2/4/52433239757b7fb7798b4567.png&action=click

    https://learn.sparkfun.com/tutorials/led-light-bar-hookup/example-circuits

    Here is the basic plan I grabbed off the innergoogle, although there is a variation I prefer that drives the timing resistor from the output pin 3. If you disconnect pin 7 from everything, disconnect the top of R2 from 12 V, and connect R2 to pin 3, then R2 sets the dimming limits, R1 sets the percentage between those limits, and pin 7 isn't sucking down unnecessary power. Also, R3 is too large; reduce to 100 ohms.

    ak
    555-Dimmer-h.gif
     
  7. beginnersluke

    Thread Starter Member

    Aug 26, 2015
    35
    2
    This is great. Thank you. I think with your first post, it's helped my googling and I've come upon something similar. I was trying to make it overly complicated and that hurt my searches.

    I hope to work on this soon, so I will update. This is for my daughter's room and she loves learning about electronics, so I'm just trying to learn to help her and encourage her. Thanks for the answers and the patience.

    Luke
     
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