does a ballast resistor drop voltage to 6V?

Discussion in 'Mech Tech' started by grub, Apr 24, 2014.

  1. Hi all,

    I have bought a 6V searchlight to fit to my bay.

    I seem to remember that years ago when I had my Triumph Stag, it was fitted with a 6V coil which was wired via a ballast resistor.

    Just had a quick look on ebay and all it says for ballast resistor is 1.6 ohms - not sure what that means but will a ballast resistor drop the voltage down from 12v to 6v for my lamp?

    Thanks,
     
  2. matty

    matty Supporter

    the easy way is just to wire another bulb in series
     
  3. As @matty said, you could wire another 6V bulb in series, or you could crudely drop 6V using a wire wound resistor (which is going to get hot). What wattage is the bulb?

    <edit> You could always try to find a 12V bulb that fits ;)
     
    Last edited by a moderator: Apr 25, 2014
  4. The best solution would be to replace with a 12V lamp, the next best solution is to wire two identical 6V lamps in series. The worst solution is a ballast resistor, as it must dissipate the same power as the actual lamp in heat (ie if the lamp is 50Watts, then the ballast resistor will produce 50W of heat), and the whole lot will require 100W of power from the battery/alternator (50W for the lamp and 50W wasted by the resistor).

    To put that heat into context, imagine a pre-energy saving 40Watt or 60Watt domestic bulb. It will get that hot.

    If you must use a ballast resistor, let us know the wattage of the 6V lamp, and I will try to find something appropriate. For the 50W example above, R should be 0.72Ohms. I would choose this one below:
    http://uk.rs-online.com/web/p/panel-mount-fixed-resistors/6150549/

    (0.68 Ohms, which is close enough), and up to 100W power. A 50W resistor is also available, but would be operating at the top end of it's spec and may not last long.
    These resistors are designed to be mounted on a metal panel, which helps with heat dissipation. You would have to carefully chose where to fit it to allow adequate cooling.
     
  5. ^this, exactly. Maplin do nice wirewounds, if you go down that route, but as @MarcT512 says, they're going to get hot as they're effectively dissipating the same power as the bulb.
     
  6. Bhubesi

    Bhubesi Supporter


    Good thinking Batman!!
    Tony
     
  7. Simplest solutions are always the best!
     
  8. Bhubesi

    Bhubesi Supporter

    KISS.................keep it simple, stupid!
    Tony
     
  9. thanks for the replies guys

    Sealed beam unit, so can't just replace the bulb.

    Sounds like the best plan is to wire another bulb in series - presumably that would have to be a 6v bulb as well?

    I assume you can still get 6v bulbs ok - maybe I could fit a proper bulb holder in the dash then that would light up when my searchlight is on - coolio!

    Sounds like the ballast resistor is not a good idea as if it gets too hot and touched a wire, it'll probably melt it!
     
  10. If you wire another 6V lamp in series it also has to be the same wattage as the sealed beam unit.

    Post up a pic of the bulb & fittings. I'm sure between us we can find a 12V option.
    As as side line I do lighting for live events (theatre, musicals, bands, indoors and out) - so have had to deal with finding replacement/equivalent lamps for all sorts of equipment over the years.
     
  11. As marc says, you'll need a bulb of the same wattage - going to be v bright on your dash!
     
    fritt likes this.
  12. A quick google confirms that spot light is 5 inches diameter. It looks like a sealed beam PAR (Parabolic Aluminized Reflector) lamp. These are measured in eights of an inch, so that fitting probably takes a PAR36 (4.5 inches) or PAR38 (4.75 inches) lamp.
    Can you measure the diameter of the bulb? You might need to open it up to get the exact diameter, and if you do look for any manufacturer codes on the reverse of the lamp.

    At a guess, I think one of these (below) might be a suitable 12V replacement - but with a bit more info we can be sure:
    Type #4405 12V 30W PAR36 - http://www.lamps-on-line.com/4405-12-8v-30w-par36.html
     
    Last edited: Apr 30, 2014
    paradox likes this.
  13. Im looking forward to seeing this lamp fitted as ive thought about getting one for my van
     
    MarcT512 likes this.
  14. Can I get to the bulb if its a sealed beam unit?

    I've done the scary bit & drilled through my windscreen pillar - trial fitted it and it looked good.

    Tested the buld with a motorcycle battery & it worked fine, so just need to sort the voltage issue & fit it/wire it after a bit of fettling.
     
    MarcT512 likes this.
  15. Looking at the picture on eBay from the side, I reckon the front ring unscrews. You must be able to get in there somehow - the bulb will need replacing at some point.
    If it puts up a fight, wrap a large rubber band around it - you'll get loads more grip.
     
  16. 'tis true - looks like it unscrews.
     
  17. Just taken it apart - it is a sealed beam unit and the bulb is sort of resin-moulded into the lamp, so no way of replacing it - it just says 'made in usa' on the back of the lamp unit.

    It is a westinghouse allstate series 2030 make - made in america in the 40s or the 50s I think and the lens just says usa 4515 30w 6-8v spotlight..
     
    Last edited: May 1, 2014
  18. snotty likes this.
  19. You can also use a stepdown transformer. 12V in -> 6V out.
     

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