Dimmer not too bright

Discussion in 'General Electronics Chat' started by bidwin, Feb 14, 2015.

  1. bidwin

    Thread Starter New Member

    Jan 22, 2015
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    I tested my foam cutter (120VAC -> fuse -> Lutron Dimmer -> stepdown transformer 25.2V -> cutting bow) and found the dimmer reduced the transformer output by about 35%, which is not satisfactory. I guess that's caused by the time-slicing power reduction method of the dimmer. I may need all the 28V+ the transformer actually puts out so I want to lose as little as possible. I looked at a rheostat, but since I'll be using very low amps, <1 on input, <2 on output, it looks like that will waste quite a bit of power. I've been unsuccessfully googling replacements for the dimmer, so I'd like to know if any of you can recommend a relatively inexpensive (<$20), relatively efficient item or kit I can use to regulate/lower/adjust AC power going into the transformer.
    Thanks.
     
  2. #12

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  3. MaxHeadRoom

    Expert

    Jul 18, 2013
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    I did not like the idea of controlling the input side to the Transformer, I wired it on the output side.
    You also need to have a transformer of the correct Va otherwise the voltage will collapse or decrease under load.
    The circuit I built is a little more than the average Triac/SCR controller but even the 120v dimmer type should work on the output side.
    Max.
     
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  4. #12

    Expert

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    Good point. Most people underestimate the power needed to heat things.
     
  5. bidwin

    Thread Starter New Member

    Jan 22, 2015
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    Thx for the response 12. I tried two identical dimmers and they both yielded yhe same results. Both bad
    .!?!

    Regarding the links in your thread link below:

    The low end of the output range of the first is too high for my needs. FYI, I looked at all the similar options on ebay and found none would work;

    The Lutron dimmer probably won't work - I spoke to Lutron tech support who gave me two possibilities that I'll investigate;

    Third option breaks budget.

    Skimming the thread below I found nothing to answer my question. However I do use the Jacobs site and bought my wire from them. Not nichrome, but rene 41, which is a better choice for me.

    Thanks for your response.

     
  6. bidwin

    Thread Starter New Member

    Jan 22, 2015
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    DOH! WHY DIDN'T I THINK OF THAT?! Must STILL be in the box.
    Thanks for yor response, Max. I'll try your suggestion.

     
  7. bidwin

    Thread Starter New Member

    Jan 22, 2015
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    Forgot to respond to an important part of you msg. My transformer is rated 25.2V 2A. I might get close to 2A but not reach or exceed it, so I hope I'll be OK.

    I did try to get an understanding of the difference between watts and VA, Max, but my non-tech understanding is that for resistive loads there's no difference. If that's not correct please enlighten me.
     
  8. MaxHeadRoom

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    That is only (25x2) 50va so quite a small transformer, so a 100Va min. would give you some margin.
    Max.
     
  9. bidwin

    Thread Starter New Member

    Jan 22, 2015
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    Putting the dimmer on the output side is a mute point, Max, as some would say. Both dimmers passed full voltage when turned on, regardless of the dial setting. So the search continues.
     
  10. ian field

    Distinguished Member

    Oct 27, 2012
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    Not sure - but I think you need to use either a snubber or PFC capacitor to drive a transformer with a dimmer.

    The transformer primary is predominantly inductive, so current lags voltage - the end result apparently; the triac doesn't conduct for the full phase angle that was intended.

    If you use just a PFC capacitor - watch you don't choose the resonant capacitance, otherwise it can generate some wicked voltages!
     
  11. joeyd999

    AAC Fanatic!

    Jun 6, 2011
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    Hmmmm...interesting.

    Since a predominantly inductive load stores energy (as opposed to dissipating it), you are suggesting I can power a resistive load on the secondary for, essentially, free!
     
  12. ronv

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  13. bidwin

    Thread Starter New Member

    Jan 22, 2015
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  14. ronv

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  15. #12

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    I still suspect the transformer isn't big enough. Can you plug it all together with no dimmer and measure the output then?
     
  16. bidwin

    Thread Starter New Member

    Jan 22, 2015
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    ronv, my dimmers have triacs, and, according to Lutron tech support, those won't work with transformers well. When dimmers were put on the output side it seemed they weren't there at all. Max voltage passed regardless of dial setting.

    12, with only the transformer connected to AC its output was about 28.7V.

    ++++

    I think it's time to bag this one and use my DC solution. I've spent enough time searching for a seemingly nonexistent cost-effective solution. Maybe I should just get out my old Lionel train transformer and see how much power that thing puts out!

    Thanks to everyone who responded to this thread.
     
  17. ian field

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    I never suggested anything of the sort - but you can cheat the electricity meter with way out of phase VI.
     
  18. MaxHeadRoom

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  19. jamesd168

    New Member

    Nov 8, 2014
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    what I have found is that AC based foam cutters do not cut as cleanly as variable DC power supplies, so you should look into getting a variable DC power supply for your project. you can get a really inexpensive one like this (0-30V and 0-3A): http://www.volteq.com/volteq-regulated-variable-dc-power-supply-gps-3030d-30v-3a.html

    If you wish to stick to AC based foam cutter, then try to get a variable transformer, or variac, which allows you to lower the voltage if you need less output, and you don't waste any electricity. Volteq has small ones starting at $44.50: http://www.volteq.com/variac-auto-transformer.html

    If you still want to continue your own way, I will do the following:

    1) make sure without any dimmer, your power is enough to heat up the foam cutter
    2) once you are sure of that, you can put in a variable resistor to reduce the voltage on the foam cutter. you need to make sure that the resistor can handle the current you are putting through without getting hot.
     
  20. MaxHeadRoom

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    Jul 18, 2013
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    The one I showed actually uses variable DC due to the SCR instead of Triac, I was unsure about the half wave effect but it works fine.
    Another reason I used it on the secondary.
    Max.
     
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