Diode, CDU, lamps and other elctrical things!

Discussion in 'FAQs' started by dturnerfish, May 14, 2005.

  1. dturnerfish

    dturnerfish New Member

    I read the messages under “diode matrix”. Went to Radio Shack and was able to make a simple 3 turnout one. It seems to work fine. It does only work with a DC power supply and not an AC power supply. With AC the Diodes do not stop the return power. Is this normal?

    Now the component I am missing is the CDU, and even after reading I am still a bit confused about this. :confused: To my understanding the CDU goes before the switches and after the power supply. Where can I buy a CDU? What Size, and how much do they usually run?

    One other electrical question:

    If I have an 18V AC power supply and I am powering a 12V lamp, would I need to put some type of resistor in between the power supply and the lamp. If so what size. Is there a table to look up such things? Should I be using AC or DC to supply lams

    Is there a more appropriate place for me to post electrical questions? Other resources?

    Thank you,


  2. 60103

    60103 Pooh Bah

    DT: This sounds like a Technical Q to me. Maybe we have an A.
    The diode matrix requires a DC power supply. You can probably start with a single diode or a bridge rectifier -- the direction has to match the diodes in the matrix. AC will creep up the wires in the other direction and confuse the switch machines. (Insert cartoon of confused switch machine)
    A CDU is nice - depends if your power supply will throw all the machines at once. Check our threads on CDUs -- there are various opinions and some diagrams. A good hobby shop should have some in its electrical section. It is attched to the AC output that you use for the switch machines and the other end connects to the switch machine controls -- one wire to common and the other to the buttons or whatever. The CDU counts as the DC power supply.
    You need something to run a 12V bulb from an 18V supply. I favour another bulb of the same specs -- that makes a 24V bulb that will last longer. Or each bulb gets 9V. There may also be a warming of the colour.
  3. ezdays

    ezdays Out AZ way


    To answer your question on the diode, that's the function of a diode. Current in one direction will flow easily with low resistance, but is basically blocked by high resistance in the other direction. This is the first step in turning AC into DC.

    As far as where to get a CDU and how much, I can't answer that, but there is a link in another thread here to a site that has a schematic on how to build one.

    Thirdly, you can not run a 12 volt lamp on 18 volts for very long. How long? Probably less than a second. You do need a resistor, but the value will be determined by the specs on the bulb, like the amount of current it will draw at the rated voltage. If you know that number, I will calculate the resistor size for you. The current rating can be as low as 50 mA and as high as 500 mA (1/2 amp) and anywhere in between, so it does make a difference.
  4. dturnerfish

    dturnerfish New Member

    Maybe I need to rethink this.

    Tell me if I am on the wrong track...

    I am going to have 1/2 a dozen buildings or so. Would it be posible to make a circuit board that is fed by a transformer. On the board the power would be split to 6 different resitors that lead to 6 lamps (some same - some not)?

    I have an old Transformer (16VDC, 18VDC variable). Could I use this for my lighting. With the D.C side I could use the variable - to dim. Is it fine to power lamps with A/C or D/C.

    The Lamps I do have are:

    14 volts, 200ma screw based lamps (Radio Shack #1487) Average life 30 hours.

    I am also a bit concerned about the 30 hour life on the lamps. Are there LED's bright enough to light buildings? Any lamps with a longer life?

    Thank you. I currently only have this forum and another to ask questions. Hopefully I will eventually meet some other railroaders in my home city.

  5. ezdays

    ezdays Out AZ way


    For starters, if you put a bit more info in you user profile, you'll be able to hook up with some local modelers a bit easier. There's a local train swap meet in Phoenix next weekend and there will be at least four of us from this forum meeting there, and there are other areas in the US and Canada where forum members get together frequently, so let others know who you are and where your from and you'll find others will respond. :wave::wave:

    Now you can size each resistor depending on the current and voltage specs of each lamp. There is one basic formula for that:

    voltage = current x resistance.

    If you are looking for resistance, divide the voltage by the current. In your case, you have 18 volts and a 14 volt 200 mA bulb, so take the difference, which is 4 volts, and divide it by the current (.2 amps) and you get 20 ohms. Now if you increase the resistance to say 50 ohms, you'll reduce the current going to the bulb and extend it's life greatly. If you use 12 volts on a 14 volt bulb, you can probably increase it's life at least ten fold. You also have to be concerned about the wattage of the resistor as well. In your case, 4 volts at .2 amps comes to .8 watts, so use at least a one watt resistor. Remember too that the resistor will get hot.

    Now that being said, wouldn't it be easier to just get a 12 volt source and use it on your 14 volt bulbs? Bulbs are rated to run either on AC or DC, so you could use the variable output to run your lamps, but again, you need to be sure it has a high enough current rating to run them all. You will need at least one amp to light five of your bulbs at full brightness.

    I hope this helps you.
  6. dturnerfish

    dturnerfish New Member

    I see the light!

    So in this example I need a 50 ohms, 1 watt resistor. If I can not find this resitor, could I use two 25 ohms one 1 watt resitors? Would anything under 1 watt burn out?

    I think I will find another power supply

    (updated user profile, I am from Dallas)

    Thank you sooooo much,

  7. kf4jqd

    kf4jqd Active Member

    Here'a a very easy solution. Use a 7812 voltage regulator. You can get them at Radio Shack. Just put your 18 to the input. The output will have 12v!


Share This Page