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SCR driven lights get dimmer as more lights come on

Discussion in 'Electronic Basics' started by Wes, Sep 16, 2003.

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  1. Wes

    Wes Guest

    Hi - I'm hoping someone can help me here. The basic problem is
    that I have a circuit consisting of a bunch of #47 mini-lamps
    that get turned on and off via a processor, and everything works
    great EXCEPT that the lights get noticeably dimmer as more total
    lights are on at the same time. The lights are controlled in
    groups of 4, and each group of 4 that turns on has a relatively
    dramatic effect on the brightness of all the bulbs that are
    currently on. I would like the bulbs to stay close to their
    designed brightness (relatively constant brightness regardless
    of how many lights are on).

    There are 16 lights total (4 groups of 4). Each group of 4 is
    controlled by a shift register (595) output. The 595 output
    drives a pair of MCR-106 SCR gates, and each SCR drives 2
    bulbs. I have a 1K resistor connected between each 595 output
    and the pair of SCR gates it drives.

    For lamp power, I'm using a 12.6VCT 4A xformer. I've fed the
    center tap and an outer lead to the AC inputs of a bridge
    rectifier, so I get the 6.3VDC I need to drive the bulbs -
    this works fine. The lamps are 6.3V / .15A, so with 4 amps
    available I would think I should be able to drive 16 lamps OK.

    I connected a sort of "status bulb" directly across the 6.3V
    lamp power supply. I thought I would see it dim along with
    the SCR-driven lights, but this one bulb stays relatively
    nice and bright.

    So, if just 1 group of 4 lights is on, they look nice and
    bright. Calling that 100% brightness, when the next group
    of 4 comes on, then all 8 lights drop to, oh, 85% bright.
    Then when the 3rd group of 4 comes on, then all 12 lights
    are about 70% bright. Finally, 16 lights on are all at
    about 55% or so.

    Anyone have any ideas as to how I can keep all of the lights
    near 100% brightness? Thanks!! -Wes
     
  2. Dana Raymond

    Dana Raymond Guest

    Hi Wes. Thats an interesting design you have there... Why are you using
    SCRs?

    Anyway, use a scope or meter to see where the voltage drop is occuring.
    Also, make sure that your wiring and/or PCB traces are heavy enough to
    handle the combined current.

    Since the bulb power is 120Hz half wave, the SCR's trigger point may be
    affected by small IR voltage drops between the SCR cathode and TTL ground.
    Higher SCR cathode voltage will probably result in later turn on of the SCR
    (if the trigger level is marginal to start with) and result in lower
    brightness.

    Using FETs instead of SCRs would lower parts count and eliminate problems
    like this.

    Dana Frank Raymond
     
  3. Wes

    Wes Guest

    Hi Dana - thanks for the reply! I decided on SCRs simply because I've
    become pretty familiar with older Bally pinball machines, and it uses
    SCRs to control all of the feature lights.


    Well, I bypassed the electronics completely and just hooked up all the
    lights directly to the transformer in parallel. Same problem - the
    more I hook up, the dimmer the bulbs are. I'm using AWG22 solid, and
    I'm thinking maybe I should try about AWG16 or so. Any thoughts on
    that?
    Well now I am using a full wave bridge rectifier, and I'm not
    attempting any phase control with ZCD or anthing like that. Just
    straight DC from the + and - outs of the BR.
    Worth looking into - thanks again!
     
  4. What is the shift register for? Are the lights getting turned on in
    sequence? This sounds like a problem with 'duty cycle' rather than voltage
    to me. The more groups, the less total time they are on.
     
  5. Dana Raymond

    Dana Raymond Guest

    OK, so you hooked up all of the bulbs directly to the transformer and they
    get dimmer the more you add. Exactly what does that look like? Just bulbs,
    sockets, wires, and a transformer? You need to be much more specific so that
    we can help you, OK?

    Did you make any measurements? Did you know that a transformer rated X Volts
    @ Y Amps does not have constant voltage as the load increased from 0 to y
    Amps. You need to measure the voltage at the point where all of the lamps
    are connected together in common (at the transformer or bridge or whatever)
    to see if that is your problem.

    You also said that you connected a status bulb across the power supply and
    it did not dim. But you say above that you connected all of the bulbs to the
    power supply and they did dim. Thats contradictory to me.

    Get your meter out and make some measurements, OK?

    Also, try the AWG16 instead. The current of 16 bulbs may have a significant
    voltage drop if its carried by a single AWG22 wire or wires.

    BTW I used to fix pinball and video games for a small arcade chain 20 years
    ago.

    Good luck, and tell us what you found.
     
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