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Thread: How do you determine Positive and Negative

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    Say Rem, I just finished reading all the posts and I bet your head is really spinning. I'm sure you were looking for a simple answer. The two posts are AC and you want to make them DC so you can run some accessories, right? Go to Radio Shack and buy a 4 amp bridge rectifier. 4 amp is not critical, but something close. This will have 4 wires. One marked + one marked - and two with no marks. Connect the two with no marks to your AC posts and you are ready to go, all for about $2.00. Connect the accessories to the + and - leads.

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    Michael, I appreciate that! Yes, I think that would work. My only question is will it convert it from 16v ac to 16v dc? If so, is there a way I can knock that down some? Ideally I would like about 12v.

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    Quote Originally Posted by REM37411 View Post
    Michael, I appreciate that! Yes, I think that would work. My only question is will it convert it from 16v ac to 16v dc? If so, is there a way I can knock that down some? Ideally I would like about 12v.
    In addition to the bridge rectifier, You need a voltage regulator and a capacitor or two for filtering to get smooth 12vdc. This is the circuit I was talking about earlier.

    Paul

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    Quote Originally Posted by REM37411 View Post
    Michael, I appreciate that! Yes, I think that would work. My only question is will it convert it from 16v ac to 16v dc? If so, is there a way I can knock that down some? Ideally I would like about 12v.
    (Edit: Aah. I see Paul beat me to the punch on this. We're talking about the same circuit.)

    The rectifier will knock about 1.5V off the 16. which will get you somewhere in the 14.5V range, depending on the exact rectifier you buy.

    If you want 12V for sure, do this... get a fairly big cap... maybe 10uF, like this one. You really only need 0.33uF, but too much capacitance won't hurt. Hook the + and - leads of the capacitor to the + and - leads on the rectifier.

    Then get one more component... a LM7812 linear regulator, like this one. Now, if you hold it with the metal plate with the screw hole away from you, pin 1 is on the left. Hook Pin 1 to the + terminal on the rectifier (and the cap), and hook pin 2 to the - lead of the rectifier and cap. Pin 3 will be +12V w/r/t Pin 2, so Pin 3 & Pin 2 are your 12V supply output for your accessories. If you really want to make it nice, add a 0.1uF capacitor across pins 2 and 3. Put it all in a little project box and you're good to go.

    The 7812 can handle up to 1A of current, so it should be happy with pretty much whatever your wall wart will supply.

    Also note there's a whole family of 78XX regulators - the last two digits indicate the output voltage... so you can get one that does 5, 6, 8,9,10,12,15,18, or 24V
    Last edited by TwinDad; 21st Feb 2011 at 10:26 AM. Reason: Acknowledging pbender...
    Never mistake a guy who talks a lot for a guy who has something to say...

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    I love that you guys get it. I don't. To me it is easier spending 12-15 bucks on a power supply. I know that when you sparkies explain something, in YOUR head you're thinking "This is easy peasy.....a monkey can do this". But the thing I don't think you guys take into consideration is, most of us don't get it. I don't mean this in a negative way, I really don't. I just don't get it. I need something I can plug n play with. And not that it isn't intriguing! IT IS! I bought an electronics kit to try and GET the basic understanding but haven't had time to really play with it yet. I would LOVE to be able to build the circuits I need for a flasher crossing or something one day. But alas, for now, I am relegated to plug n play.

    Be the kind of person your dog thinks you are.



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    Quote Originally Posted by REM37411 View Post


    I love that you guys get it. I don't. To me it is easier spending 12-15 bucks on a power supply. I know that when you sparkies explain something, in YOUR head you're thinking "This is easy peasy.....a monkey can do this". But the thing I don't think you guys take into consideration is, most of us don't get it. I don't mean this in a negative way, I really don't. I just don't get it. I need something I can plug n play with. And not that it isn't intriguing! IT IS! I bought an electronics kit to try and GET the basic understanding but haven't had time to really play with it yet. I would LOVE to be able to build the circuits I need for a flasher crossing or something one day. But alas, for now, I am relegated to plug n play.
    Yeah, we're also often guilty of answering the question you ask ("how do I power DC equipment from this AC supply?") instead of answering the question you really want the answer to ("What's the easiest way to power my DC equipment?")

    Go buy a 12V wall wart and be happy.
    Never mistake a guy who talks a lot for a guy who has something to say...

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    Note that the terminals labled "to track" can be of either polarity, depending on the setting of the reversing switch on the controller. Track power is thus variable DC, with polarity dependent on direction setting.

    AC "accessory" terminals are generally 16 VAC.

    "Ground" does not enter into it in either case.
    Doug Stuard,

    In the wilds of Northern Virginia

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    Quote Originally Posted by REM37411 View Post


    I know that when you sparkies explain something, in YOUR head you're thinking "This is easy peasy.....a monkey can do this".
    Actually, a monkey COULD do it! It's just that sometimes we are too smart and think that something is hard just because we are not familiar with the terminology. I would wager to guess half of us "sparkies" got started sparkin' with our first Lionel train set (or in my case, Marx). Two wires from transformer ("What's a transformer, daddy?" asks young Doug) to the track and the trains go around. "Can I get shocked touching the track daddy? There's 'lectricity there!"

    It's all a learning experience, and I ain't givin' up just yet!
    Doug Stuard,

    In the wilds of Northern Virginia

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    To be perfectly honest, I didn't even see that it was ac until someone pointed it out. I think I understand that most accessories like incandescent lights work on that output but led lights need a DC power source. So, while I can use one of these to power my CDU for turnouts, I will need a DC power pack for powering lighting. I know dang well I'm going to blow something in the process I just need to make sure it isn't one of these high dollar processors I got for my crossing or some other cool toy.

    As far as it being so easy a monkey can do it, maybe I need to hire that monkey.

    Be the kind of person your dog thinks you are.



    Ron

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    Quote Originally Posted by REM37411 View Post
    To be perfectly honest, I didn't even see that it was ac until someone pointed it out. I think I understand that most accessories like incandescent lights work on that output but led lights need a DC power source. So, while I can use one of these to power my CDU for turnouts, I will need a DC power pack for powering lighting. I know dang well I'm going to blow something in the process I just need to make sure it isn't one of these high dollar processors I got for my crossing or some other cool toy.

    As far as it being so easy a monkey can do it, maybe I need to hire that monkey.
    Ron a cheap way to eliminate the AC DC issue is just getting an inexpensive multimeter for about $20 bucks. Radio shack has them they have auto-range built in with fuse protection so it's an easy cheap way of verifying what power source you are dealing with at the time. I've got about 10 meters up to a $1000 dollar fluke with a built in oscilloscope however I use my little pocket sized $20 dollar meter the most. Plus you have all the sparkies here for backup electricity scares a lot of people yet if you just respect it and verify what your doing with a simple pocket sized meter you can solve most problems yourself. I've also added a Wiki link to show you how to easily use any meter you might choose.

    http://www.radioshack.com/family/ind...goryId=2032305
    http://www.wikihow.com/Use-a-Multimeter
    http://www.techrepublic.com/article/...imeter/1056686

    Pete
    Eighty 1 Fourever

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    How do you use a meter to determine ac or dc? I understand how to check voltage but how do I use it to decide "This is 12v" dc?

    Be the kind of person your dog thinks you are.



    Ron

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    Quote Originally Posted by REM37411 View Post
    How do you use a meter to determine ac or dc? I understand how to check voltage but how do I use it to decide "This is 12v" dc?
    Wallwarts have a 99.9% probability of having a DC output there should also be a standard label on the transformer rectifier that you plug into the wall. If there is not a nameplate then you need to know where to put your test leads on the output plug to verify the supplied output voltage. Normally -the outer metal material of the output/plug with hole- will be negative and inserting your test lead inside shall we say the hole of the output will be the positive. Black lead Negative, Red lead positive,should your meter give you a reading with a Negative Sign this will tell you the polarity is the opposite of what you thought it was just invert or change around your leads to verify that in this instance the outer sheath is positive and the inner is negative, unlikely but it could happen. Should you set your meter to DC and test an AC output you will more than likely get No Reading from your meter.If your meter is an auto-range style you will not even have to adjust the voltage range only AC or DC. Other wise you might set it at say the 20 VDC setting and begin testing the output from there working your way down. You really just need to look at this link I will submit about halfway down the page with text and pictures it will show you graphically where to put your leads and what to set your meter it is specifically dealing with Wallwart power supplies and how to test them. Hopefully you have an Auto-Range multimeter if not like I said any auto parts store or a Radio Shack has them for $20 bucks.

    Please just scroll halfway down the link's page and look at the pictures along with text this guy does an excellent job of instructing the layman on how to use a meter on Wallwarts
    http://www.ladyada.net/library/metertut/voltage.html

    Pete
    Eighty 1 Fourever

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    Ok, I got to thinking, Surely I have a couple old adapters for cell phones or something around here! I found an adapter that went to my old router that bit the dust. It says that it has 100-120v and output is +5v (solid line over dotted line) 1A then it shows that the inner part of the connector on the other end is positive and the outer is negative. Now, is this thing DC? ANd is there a way to determine it?

    If so, would this be good to power LED lighting with? If a LED light take sonly 15-20 MA to power it, am I correct in thinking that this thoung should power my entire city??

    Be the kind of person your dog thinks you are.



    Ron

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    Quote Originally Posted by REM37411 View Post
    Ok, I got to thinking, Surely I have a couple old adapters for cell phones or something around here! I found an adapter that went to my old router that bit the dust. It says that it has 100-120v and output is +5v (solid line over dotted line) 1A then it shows that the inner part of the connector on the other end is positive and the outer is negative. Now, is this thing DC? ANd is there a way to determine it?

    If so, would this be good to power LED lighting with? If a LED light take sonly 15-20 MA to power it, am I correct in thinking that this thoung should power my entire city??
    Yup. That'll do. If you put a 500Ω resistor on each LED, you should be able to drive about 100 LEDs at 10mA each.
    Never mistake a guy who talks a lot for a guy who has something to say...

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    I'm gonna figure this out if it kills me! Thanks TD I do appreciate you! So, my slow moving bacan fat burning brain tells me that the size of the resistor is directly related to the size of the LED being burned AND the amount of power being supplied. Believe it or not, the explanation you gave over in the LED thread I started a while back just now popped into my head. I sort of get it! For some reason I was thinking that the only thing that mattered was the size of the LED being lit. Don't mean to derail my own thread. Thank you kind sir!

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    Ron

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    Quote Originally Posted by REM37411 View Post
    Ok, I got to thinking, Surely I have a couple old adapters for cell phones or something around here! I found an adapter that went to my old router that bit the dust. It says that it has 100-120v and output is +5v (solid line over dotted line) 1A then it shows that the inner part of the connector on the other end is positive and the outer is negative. Now, is this thing DC? ANd is there a way to determine it?

    If so, would this be good to power LED lighting with? If a LED light take sonly 15-20 MA to power it, am I correct in thinking that this thoung should power my entire city??
    In your post "solid line over dotted line" that is telling you as well that it is DC
    Eighty 1 Fourever

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    I agree with everyone Rem about having an inexpensive multimeter in your tool box. It is the single best trouble shooting tool you can have for electrical issues. If you don't want to get one right now, here's another way to tell which is positive and which is negative. Get a 16 volt light bulb with wire leads made for lighting building structures. Go back to Radio Shack again and buy a 1 amp DIODE. This little cylindrical shaped thing will have a silver band painted on one end. Solder this end to one wire from the light bulb. Touch the other end of the diode along with the remaining wire from the light to whatever your are trying to figure out. If the light bulb lights up, then the diode is touching + positive and the other wire is - negative. If it doesn't light up reverse the positions and it will. All for $.79 and a light bulb.

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    I do have a couple of theres:



    I am familiar enough with it to measure voltage on something if I already know it is ac or dc. Like checking household circuits. I can do that. I can also check voltage on a circuit on my train if I know it is ac or dc. I know that you are supposed to set to to the lowest possible setting for checking a circuit. say I am going to check an outlet on my house. I know that the circuit should be below 200 so I set it to 200ACV and test. Usually if I am checking a circuit on my train, and I KNOW it is ac or dc I use the lowest possible setting for whichever I am checking and test. The whole evolution of DCC has confused me a bit bbut I think I am starting to get it. For any DC circuits I check on my train I usually et it to DCV and usually set it on 20 because I know that most of these circuits should be below that. I also know that if I choose a higher setting then it will simply show a lower reading exponentially. Instead of getting a reading of 16, I will get a reading of say .016. That makes sense to me because I am adding a zero to the reading parameter by using 200 versus 20.

    I use it all the time to check circuits on my Nova. Those are all 12vdc so it is easy.

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    Ron

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    Next time you are reading a known DC voltage, turn your meter over to AC and note the unusual display. Like wise, when reading a known AC voltage, turn it to DC. When you become familiar with these displays you will know to change the position on your meter. Knowledge of electrical things is accumulated, like building blocks. As you understand more complex circuits the things you struggled with today will seem simple. Hang in there, we'll all help you make sense of it.

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    Did you realized this is REM's thread and Ron's multimeter. Wate up Michael and go to bed.

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