Connect with us

Help with finding a resistor to drop 220v to 110v

Discussion in 'General Electronics Discussion' started by betacrash, Dec 3, 2014.

Scroll to continue with content
  1. betacrash

    betacrash

    3
    0
    Dec 3, 2014
    I run a manufacturing facility that has no documentation whatsoever. We do have a guy that has been building our machines since day 1 and just does what he has learned but has no information beyond this. Here's the dilemma.....

    We build a machine that uses a small AC lamp (find below)

    http://www.alliedelec.com/search/productdetail.aspx?SKU=70081530

    When we build a 220v version of our machines we solder a resistor to one leg of the lamp to allow it to run without burning out prematurely. The resistors are old (I know, I know) so the indicating marks on them are unreadable. I just need to know if anyone would know what size resistor we would need to use for this lamp.

    Specs on the lamp:

    Base Style : Cartridge
    Bulb : Cylindrical
    Color : Red
    Color, Emitted : Red
    Color, Lens : Red
    Current Rating : 1.2 mA
    Current, Rating : 1200 mA
    Dimensions : 1.45 in. H x 0.28 in. Dia.
    Lens Color : Red
    Life : 25000 hrs.
    Life Hours : 25000
    Resistance, Series : 4.7 Kiloohms
    Type : NEON
    Voltage Rating : 118 VAC/VDC
    Voltage, Breakdown, AC : 110 to 125 VAC
    Voltage, Breakdown, DC : 110 to 125 VDC
     
  2. Bluejets

    Bluejets

    5,251
    1,101
    Oct 5, 2014
    Arouse1973 likes this.
  3. KMoffett

    KMoffett

    723
    75
    Jan 21, 2009
    Looking at the actual DIALITE spec sheet:
    http://www.dialight.com/Assets/Brochures_And_Catalogs/Indication/507-xxxx-xxxx-640.pdf
    The internal resistor is 47KΩ, not the 4.7KΩ as shown in the Allied link. That had seemed awfully low. I have no idea where the "Current, Rating : 1200 mA" came from. So the voltage drop across the internal resistor 1.2mA*47000Ω is 56V. Neons trigger at about 90V and then drop to ~70V. So 56V+70V=126V...the approximate supply voltage. So, to go from 220V to 126V a resistance of (220V-126V)/1.2mA=94V/1.2mA=~78000Ω. The old resistors were probably under rated for power. The power rating should be at least twice the calculated value of 95V*1.2mA=0.11W, so 1/4W, as Bluejets said. Also, 78K would be easily replaced with a more common 82KΩ 10% 1/4W metal film resistors:
    http://www.radioshack.com/nte-qw382...lameproof-resistor/55049453.html#.VH_Hv8nYy2k

    Ken
     
    Last edited: Dec 4, 2014
    hevans1944 likes this.
  4. KJ6EAD

    KJ6EAD

    1,114
    159
    Aug 13, 2011
    Ken, you have a little unit typo (0.0012A or 1.2mA).
     
  5. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,501
    2,841
    Jan 21, 2010
    1/2W metal film resistors are about the same size as 1/4W carbon resistors.

    I (being one to err on the side of caution) would go for the extra insurance of a 1/2W resistor over the 1/4W especially given the voltage. My limited experience suggests voltage ratings of between 200V and 500V for name brand 0.25W metal film resistors. This one will have a peak of around 140V across it which is probably well within the bounds of reasonable for a 1/4W metal film resistor. In any case, 1/2W resistors may be more easily available and are also suitable.

    My guess is that 1200mA was a typo from 1200μA in the specs of the device.
     
    Arouse1973 and hevans1944 like this.
  6. betacrash

    betacrash

    3
    0
    Dec 3, 2014
    Thank you thank you for your help with this........

    But please forgive my ignorance. Now I need to know what Ohm I would need. When I looked up 1/2W I saw a selection of 10 10K and 100K
     
  7. KMoffett

    KMoffett

    723
    75
    Jan 21, 2009
  8. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,501
    2,841
    Jan 21, 2010
    As much as Radio Shack is a supplier of last resort, and even considering that a single resistor should cost you single digit cents, if this is all that you're ordering them that's ok given it seems to include postage.
     
  9. KMoffett

    KMoffett

    723
    75
    Jan 21, 2009
    Some times you just spend a one-off price for a prototype part at a down the street Radio Shack, before you buy in bulk for production. Especially if you have little or no experience. ;)

    Ken
     
  10. betacrash

    betacrash

    3
    0
    Dec 3, 2014
    You guys are great! Thank you very much!
     
Ask a Question
Want to reply to this thread or ask your own question?
You'll need to choose a username for the site, which only take a couple of moments (here). After that, you can post your question and our members will help you out.
Electronics Point Logo
Continue to site
Quote of the day

-