DCC Function Output Question

[Railfan Alex]

I know this is stupid and the answer might be obvious, but what do I use to mesure the function output on a DCC decoder. I think I can use a multimeter or some fancy device, and if so, on what setting? I am thinking the DCV set on 20, because this is what I read most often, but am still not sure and confused.

Thanks!

EDIT: I don't really know if my "multimeter" is what I have. All I know is that it measures a bunch of different things called: DCV, ACV, OHM, DCA, BAT. I think I know what is means, but any help would be good! All this research is stressing me out, so I came here!

[richg1998]

I use the 20 volt DC scale also. Works fine. I have measured about 13 volts DC for the functions in my decoders. I use LEDs for lights. One for the front headlight and one for the tender light. The reading will be dependant on the transformer voltage for the DCC controller. My NCE Power Cab has a 13.5 volt DC wall wart.

Does you meter come with an instruction manual? 

Make sure you do not have the ohms scale selected when reading voltage or current. It sounds like you have not used a multimeter very much. You can search using Google for using multimeter or using digital voltmeter. Both will both get a lot of hits. The 'Net is loaded with useful information.

 Rich

[rolleiman]

Set your meter for DCv, if it has an auto setting use it. With some meters, a setting too low sometimes will read as O.L. (overload) as it does on my Fluke which means the measured exceeds the scale. If your decoder has a wire harness, use the blue wire as the common (black meter lead) and use the the red meter lead for the function outputs, turning each on as you measure it.

Be aware, that these are NO Load voltages (if unused) and May decrease when you connect a load (light bulb) to them. It wouldn't be a bad idea to take something like a 1000 ohm resistor (Brown Black Red) and a 12 volt light bulb and connect it across the outputs (blue and white on the harness for instance) to get a more accurate reading. The lamp may not light beyond a faint glow but it will provide enough of a load to get current flowing (which does not flow in an open no load circuit) so you can get a true reading of the voltage output. Most decoders that I've dealt with have a Total function output capacity of about 1/2 amp. That should be plenty for your loco lights, ditch lights, number boards, etc.

[Vail and Southwestern RR]

Look at your documentation, too.  One of the new Digitrax decoders provides a current source, rather than a voltage source.  The voltage it reads with no load my be completely misleading.

[jasperofzeal]

...All I know is that it measures a bunch of different things called: DCV, ACV, OHM, DCA, BAT...

I'll take a stab at these:

DCV - use to measures volts in a DC circuit.

ACV - use to measures volts in a AC circuit.

OHM - use to measures Ohm value for resistors.  Use this mode in case you can't tell the value by the color bands or if they're surface mount types.

DCA - use to measure amps in a DC circuit.

BAT - use to measure battery charge.  Good for those rechargeables or if you find a battery and you want to know if it may still be good to use.

[Railfan Alex]

Thanks! I'll try these things soon and I'll let you know what happens.

[Vail and Southwestern RR]

Don't use an Amperage setting across a load!  In fact, unless you know what you are doing, it is better not to use the Amperage setting at all.

[Railfan Alex]

Thats the DCA?

[Vail and Southwestern RR]

yep

[Railfan Alex]

Hello again.

I have done my tests, and found that the track voltage (measured with RRampmeter) is steady everywhere at 13.9 volts. I have found that function outputs (measured with voltmeter on DCV 20) is between 13.3 to 13.7 volts. Now I guess I need to get some resistors and do some more tests to get 1.5 volts!

[rolleiman]

You can calculate that.. Look here..

http://cs.trains.com/forums/1443077/ShowPost.aspx 

[cacole]

I've never needed to perform any calculations for LEDs connected to a decoder -- I always just use a 1,000 Ohm resistor and the LEDs work okay.

[Phoebe Vet]

Thats the DCA?

That's the ICAO designator for Reagan International Airport in DC.

... or Direct Current Amps on your multimeter.

You are looking for Direct Current VOLTS

 

[Railfan Alex]

Thanks so much! That should be everything I need to know.

By the way, I did mean DCV, not the DCA I had said.

[richg1998]

If you ever come across a loco with 1.5 volt mini lamps, a 560 ohm 1/4 watt resistor is a nominal value.

Rich 

[Railfan Alex]

I thought that I would need resistors in the 500-600 olm range, but according to the math, I need one in the 800 olm range.

A FEW MORE QUESTIONS!

Where do I locate the resistor?

I've heard of having the resistor like so:

Blue (common +), Resistor, Bulb, Function Output.

And like this:

Blue (common +), Bulb, Resistor, Function Output.

Also, I'm sure there is a direction to resistors, but I don't know. I did some tests, and the resistor seemed to work in both directions.

Thanks again!

[cacole]

Resistors are not polarized and can be in either lead.

[richg1998]

Locate the resistor anywhere convenient. Blue lead to resistor to LED. White to LED. If it does not light, reverse the connections to the LED. I can never remember which lead is cathode or anode. Your mileage may vary.

Most use a 1k 1/4 watt resistor. The LED intensity normally will not have much different between 850 ohms and 1k ohms. 

There is no direction for resistors.

Here is a lot of info on DCC. Store it in your Favorites folder. 

http://www.members.optusnet.com.au/nswmn/index.htm

Rich 

[Stevert]

I thought that I would need resistors in the 500-600 olm range, but according to the math, I need one in the 800 olm range.

A FEW MORE QUESTIONS!

Where do I locate the resistor?

I've heard of having the resistor like so:

Blue (common +), Resistor, Bulb, Function Output.

And like this:

Blue (common +), Bulb, Resistor, Function Output.

Also, I'm sure there is a direction to resistors, but I don't know. I did some tests, and the resistor seemed to work in both directions.

Thanks again!

Alex,

  As is explained in the thread Jeff previously pointed to, the amount of resistance needed is also dependent on the current draw of the bulb/LED.  I didn't see you mention your current draw, so whatever current draw you used when you did the math may be the cause of the difference.  Either way, when in doubt, go with the higher resistance.

  Also, the resistor can be on either side, as long as it's in series with the bulb, so both of the two scenarios you outline will work equally well.  However, I think many folks follow the second scenario since it saves them from having to connect multiple resistors to one place (the Common +).  It doesn't usually matter electrically, it's just easier to make the physical connections.

   Finally, no, resistors aren't "directional".  There are many other components that are polarity-sensitive, but plain ol' resistors are not.

HTH,
Steve 

[Railfan Alex]

Thanks very much! I appreciate all your help and the links are excellent resources!

If you did want to know what I did, here's my math:

• Miniatronics 1.5 volt 15mA bulbs.
• 13.7 volt function output.
• 13.7-1.5 = A drop of 12.2 volts.

R=V/I

R=12.2/0.015

R=~813 olm resistor.

----------

P=VxI

P=12.2x0.015

P=0.183 Watts.

So, 1/4 watt ~813 olm resistor should work.

I guess, as you said, a 1000 olm resistor won't hurt anyone, it will only extend the life of the bulb!

Now, if I have two bulbs in series, what is different in the calculations?

[Vail and Southwestern RR]

Two bulbs in series, same current, double the voltage, so you have 3V across the bulbs.  Two bulbs in parallel, double the current, but the same voltage.

[rolleiman]

Thanks very much! I appreciate all your help and the links are excellent resources!

If you did want to know what I did, here's my math:

• Miniatronics 1.5 volt 15mA bulbs.
• 13.7 volt function output.
• 13.7-1.5 = A drop of 12.2 volts.

R=V/I

R=12.2/0.015

R=~813 olm resistor.

----------

P=VxI

P=12.2x0.015

P=0.183 Watts.

So, 1/4 watt ~813 olm resistor should work.

I guess, as you said, a 1000 olm resistor won't hurt anyone, it will only extend the life of the bulb!

Now, if I have two bulbs in series, what is different in the calculations?

I think you've got it. Another Jeff answered your series question. AS to where to put the resistor, put it on the Output lead. Reason being, as discussed at some length in the linked thread, Each one should have it's own. If you are only operating one output (just a headlight), then it really doesn't matter. However, if operating more outputs (headlight, beacons, ditch lights, firebox, etc), the power disapation of the resistor will increase (if used on the common).  

[richg1998]

Click on the below link and go to page 28. You will see lighting examples. Store the document in your PC. I told you incorrectly to put the resistor on the blue lead.

http://www.soundtraxx.com/documents/manuals/tsuinstallation.pdf

Always use one resistor for each function.

Rich 

[Railfan Alex]

Thanks a lot guys. I'm glad you could all help.