FWR upstream of Triacs ok?

Discussion in 'Lights - Store Bought and Home Made' started by zeph, Jan 7, 2012.

  1. zeph

    zeph New Elf

    Joined:
    Dec 27, 2011
    Messages:
    30
    Likes Received:
    0
    I have a good number of 120VAC LED lights which are half wave rectified (HWR), meaning they only conduct half the AC half-sine-waves. I would like to control them (along with some incans) using a dimming controller with ACSSRs.

    0) Any problem so far (in light of the considerations below)?

    However I'd like to convert these to Full Wave Rectified (FWR), conducting for both half-sine-waves - as it looks better and is brighter. And I'm wondering if I can insert a FWR upstream of the controller (between it and the mains), so I don't have to create an adaptor for every channel. My thoughts follow.

    1) Triacs
    I think the Triacs will be OK with conducting in only one direction; and I think they will be able to turn off OK between half cycles. Right?

    2) Zero Crossing
    However, I'm not sure if the zero crossing detector on the controller will be OK with this. The voltage fed into the controller will not actually cross zero between half cycles (unless there's some inductive overshoot). In fact, I don't know what it will do after the LEDs stop conducting as the voltage approaches zero - and I no longer have a working scope.

    So - should this work OK or not? Do different controllers have different zero crossing detection circuits which would affect the answer?
     
  2. David_AVD

    David_AVD Good news, everyone!

    Joined:
    Jun 12, 2010
    Messages:
    3,345
    Likes Received:
    22
    Location:
    Victoria Point (Brisbane)
    All of the low voltage multi-function controllers I've seen employ a full wave rectifier on the power input and SCRs on the channel output. Off the top of my head I can't why TRIACS (instead of SCRs) would be unhappy with that.

    The one problem you could have is the power feed for the controller's electronics. You absolutely must not feed the controller's power transformer with the FWR output. It requires true sinusoidal AC.
     
  3. wjohn

    wjohn Apprentice Elf

    Joined:
    May 22, 2010
    Messages:
    73
    Likes Received:
    0
    Zeph,


    HWR 120v AC led strings have no additional rectification other than that offered by the LEDs themselves.

    FWR 120v AC Led strings have a additional (4 diode) module in line with the LED string.

    For the sake of simplicity, FWR 120v AC 60 hz will be around 200 v DC with two positive going cycles every 1/60 th of a second rather the the one for the HWR strings. The voltage will return to zero every time, it just won't go negative.

    Placing a four diode bridge rectifier in line and up stream from the AC SSR should be fine. The 'DC' voltage from the rectifier will still return to Zero, so that should unlatch the Triac.

    You should be able to buy a 400v 2 amp bridge for a few dollars from Radio Shack. Cut a regular extension cord and wire the male side to the AC pins of the Bridge, and the female side to the Pos and Neg pins of the Bridge.


    Run a test with this setup, and see what the effect is. NOTE: be cautious around 120VAC!!!
     
  4. OP
    OP
    zeph

    zeph New Elf

    Joined:
    Dec 27, 2011
    Messages:
    30
    Likes Received:
    0
    Summarizing so far: The triacs should work fine, but any transformer on the controller would not enjoy receiving FWR input. I'm imagining that a transformer would be used for generating DC for the electronics, and maybe also for generating the zero crossing signal, right?

    I'm still choosing a controller, so I don't have anything to test (or to fry) yet.

    Hmm. Perhaps this approach would work better with a remote-SSR style controller - run the controller per se on AC, but feed FWR'd power to the remote ACSSR boards (eg: 4 channel SSR boards connected via RJ45 cables).

    Thanks for the tip re wiring a FWR - I already did that part for one string, using a 400v 4a bridge. It was brighter and less flickery as expected (this was actually for a kitchen counter light, not xmas).

    I just didn't want to have to do that for every channel in order to use my existing HWR LED strings.
     
  5. David_AVD

    David_AVD Good news, everyone!

    Joined:
    Jun 12, 2010
    Messages:
    3,345
    Likes Received:
    22
    Location:
    Victoria Point (Brisbane)
    Correct, you can't feed a transformer with rectified AC.
     
  6. wjohn

    wjohn Apprentice Elf

    Joined:
    May 22, 2010
    Messages:
    73
    Likes Received:
    0
    No Problems at all,


    Experimentation is good. sounds like you already have seen what FWR will do to improve the operation of a LED fixture.


    IMHO, all the LED MR16s downlights I have taken apart have had a FWR upstream from the LED board so that they can be operated from AC or DC.


    Upstream of 4 Channel SSR would make sense. e.g. the SSRez has no on board transformer, only the TRIACs on the AC side of the board, so it would be unlikely to be impacted by the placement of a FWR upstream.


    a number of people I know still use their AC RENARD16s and place a downstream FWR to power their LEDS with DC from a AC controller.
     

Share This Page