Dim Bulb Tester Build and How-To

Discussion in 'DIY' started by EngineerNate, Feb 3, 2018.

  1. EngineerNate

    EngineerNate AK Subscriber Subscriber

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    I put this together based on the pics that @Jailtime posted a while back, and figured I'd take some in process shots. There were a number of threads on these with dead links to imageshack or photobucket (the scourges of the internet) so this should be a good post on the issue that won't have dead links for a few more years at least. :)

    Firstly we need the following:
    -1 grounded socket
    -1 socket -> standard light bulb converter
    -1 light switch
    -1 plastic in wall enclosure, I used an "old work" one meant to mount to sheet rock
    -1-2' of 14-2 romex or equivalent. I only ever used single strands out of this, but it's nice to have the color coding without having to buy multiple things and they strip easily.
    -1 switchplate
    -1 sacrificial grounded extension cord.
    -Suggested: bulb protector of some sort

    We'll be following this schematic courtesy of @inkboy1 with the addition of a gonggr not sures in the schematic. The ground is pretty self explanatory: connect all the green stuff together.

    [​IMG]

    Step 1: Prepare the socket. Break off the tabs linking the two outlets on both sides. I purposefully bought a cheaper outlet for this because the tabs were less substantial. Since this will see intermittent use we don't really need the hospital grade outlet.

    [​IMG]
    [​IMG]

    Step-2: Next, cut the end off of your extension cord and insert it into the box through one of the built-in slots. These are nice because they provide some level of strain relief. Strip back the outer sheathing and trim the inner wires so that you have about 4" for the hot wire and 6-7" for the other two. Black is hot, white is neutral, green is ground. You'll want about an inch of exposed wire on each to wrap around the post.

    [​IMG]

    Step 3: Attach the hot wire to the bottom post of the switch:
    [​IMG]

    Step 4: Next, prep the outlet side. We have already electrically isolated the two sockets, we now need to connect them. Run a black wire from the "neutral" side of the bulb socket (in my case, the upper one) to the hot side of the plug you'll use for whatever equipment gets hooked up.

    [​IMG]
    [​IMG]
    [​IMG]

    Step 5: Connect the switch output to the hot side of the bulb socket:
    [​IMG]
    and connect the neutral to the neutral side of the test socket:
    [​IMG]
    Lastly, connect the ground to the socket and if desired, ground your switch as well as shown. This isn't strictly necessary but is good insurance against any unwanted zappage.
    [​IMG]
    Step 8:
    Mount everything in your box. I may swap mine around so that the plug comes out the back.
    [​IMG]
    Step 9: Install switchplate and bulb socket + bulb and you're done!
    [​IMG]

    To make sure that you've gotten all the connections properly, stick a probe from your multimeter into the hot side test socket. With the bulb unplugged, even with the switch turned on the circuit should be open when connected to the hot side of the plug:
    [​IMG]
    And with the bulb installed, it should be closed:
    [​IMG]

    That's it! I chose this style for it's compactness-I live in an apartment with no garage and storage space is limited. This will store away a lost smaller than the switchplate + ceramic socket versions. If you bought a few different bulb sockets you could have different wattage bulbs ready to go, no screwing required!

    Cheers
    Nathan
     
    Last edited: Feb 3, 2018

     

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  2. I LIKE MUSIC

    I LIKE MUSIC Super Member

    Very nice!

    Great instructions for those wanting to built a DBT.

    Thanks for the pictures.
     
  3. spark1

    spark1 Super Member

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    Yes, another in the pantheon of DBT build instructions, but a very good one. The only suggestions I might make (annoying as that might be) are:

    • include a schematic, and
    • include the suggestion of a protective frame/shield for the bulb

    Mine is fairly similar, but mounted on a piece of 2x6 for stability when in use. The weight keeps it from moving around, and you can even clamp it to your bench.

    Thanks for taking the time to put this together.
     
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  4. EngineerNate

    EngineerNate AK Subscriber Subscriber

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    I'll grab the schematic from another thread and include it with credit where it's due.
     
  5. spark1

    spark1 Super Member

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    The thread from which you pulled the schematic has a good suggestion for further simplifying the circuit.
     
  6. EngineerNate

    EngineerNate AK Subscriber Subscriber

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    That's actually a good call, since the bulb polarity is a non-issue. We're already doing something hugely non-standard with the plug so no reason not to do it that way. I have a second socket lying around here, I might wire one up that way and update the how-to.

    Easy to get caught up in doing things the "correct" way (hot on narrow prong) even when the entirety of what you're building is technically incorrect if you're going by those standards.

    Cheers
    Nathan
     

     

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  7. awillia6

    awillia6 Super Member

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    Would you two mind cluing in the rest of us...?
     
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  8. EngineerNate

    EngineerNate AK Subscriber Subscriber

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    If you reverse the hot/neutral on the bulb plug, you can leave the tab connected on the got side of the test plug and not have to use a jumper.
     
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  9. ghazzer

    ghazzer Sansui addict Subscriber

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    Thanks Nate. I think that this is something that needs to be re-hashed from time to time, as a reminder to some and an eye opener to some others.

    Also need to remind folks not to use a DBT with a CFL on their favorite amp.
     
  10. whoaru99

    whoaru99 Epic Member

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    But, there is no downside to doing it the "correct" way because the jumper isn't hard and negates any debate about which plug is bulb or which is DUT re. maintaining polarity.

    As well, while another step, pigtailing the grounding wire rather than two wires under one screw would be another suggestion. That said, I didn't pigtail mine either, but using all stranded wire I used crimp-on flanged spade terminals at all the terminal connections.
     
    Last edited: Feb 4, 2018
  11. mkane

    mkane AK Subscriber Subscriber

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    excellent, thanks
     

     

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  12. saabracer23

    saabracer23 AK Subscriber Subscriber

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    Awesome and informative build! No would a DBT benefit some if I already have a 20A variac? It has the capability of telling me current/amp being drawn.

    I know you can’t use a variac with a FET amp. Just curious.

    Dan
     
  13. Binkman

    Binkman Addicted Member

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    Glamour light bulbs not required. :D Haven't seen one of those in years. reg. 100w bulb maybe sometimes 75w

    I assembled mine without cutting both tabs noted "Speed Racer" Eng. Nate. Don't need the ground wire either as most units are two prong. I note the hot 2 prong e.g. 14 or 16ga. lamp type cord w/plug, the hot is the smaller prong. the test only takes a few seconds or stays bright so no big deal. if ibulb stays bright you know you got to get in there anyway. so keep it simple.
     
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  14. EngineerNate

    EngineerNate AK Subscriber Subscriber

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    That's actually a good point. If you reverse the polarity and happen to hook up the bulb and DUT backwards whatever you're resting will see reversed polarity (using the term polarity loosely here). While this usually isn't a big problem with AC, it could bypass protective circuitry inside the device under test during the inrush phase, as often the protective circuit elements are on the hot side only. It's be momentary but sometimes that's enough to cause a problem.

    With it jumpered as shown, if the two were swapped, the circuit would be reversed and inrush current would hit the device first instead of the bulb but the bulb would still be limiting the overall power through the circuit and the polarity on the DUT would be correct.

    I thought about doing a y-connection for the ground but since grounding the switch isn't strictly necessary in the first place I figured doing it quick and dirty wasn't going to be an issue. The connection seemed solid even with the stranded wire clamped under the solid.

    Cheers,
    Nathan
     
  15. EngineerNate

    EngineerNate AK Subscriber Subscriber

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    I wanted a clear glass bulb so that I could more easily watch the filament. I thought that maybe a standard bulb with the whitish glass would be harder to interpret. Also, these were literally the only standard incandescent bulbs at Walmart. All of the other non-CFL incandescents were halogen types and I wasn't sure if that was a good choice or not.

    Grounding just makes the device itself safer as much as anything and takes almost no additional effort. :)

    Cheers,
    Nathan
     
  16. EngineerNate

    EngineerNate AK Subscriber Subscriber

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    I like it! Part of the reason I built the dbt was that variacs are so expensive. It's added safety at very little cost.
     

     

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  17. awillia6

    awillia6 Super Member

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    Lotta nasty surprises when ramping up a variac and all of a sudden the VRs break over and come on board, feed the short, and KAPOW! Nothing at all to 70 or 80V, then smoke. DBT.
     
  18. awillia6

    awillia6 Super Member

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    More like "For sure smart and careful."
     
  19. Binkman

    Binkman Addicted Member

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    Mine looks very similar but I suppose your extended design is "safer" for testing grounded instruments. Only seen two audio units service manuals, (pre's that had a ground and that was for studio use.) with ground lugs. but any transistor type 120vac device with a power supply can be tested.
    btw.... I have an old pink bulb 100w.. ya know the kind that were very sturdy glass. Pink actually shows up pretty good then dims off like 'okay' in dark pink. :D
     
  20. Ray Gianelli

    Ray Gianelli AK Subscriber Subscriber

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    I used one for years that was just a real quick and dirty "need this now" affair. Took an extension cord, cut one side of it and placed a ceramic lamp socket in series with the cut side. Only within the past week did I finally mount it on a piece of wood.
     

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