Calculating Resistor Values

[NWsteam]

Hi all,

I hope everyone is enjoying the Holiday season. My question is on calculating resistor values. I'm doing a HO scale brass locomotive installation but figured this topic can cover a wide assortment of scales and applications.

I have a Digitrax DH123D decoder. It is a 1.5 Amp/2 Amp peak decoder. I purchased incandescent lamps made by Miniatronics Corp. They are 1.5 Volt 15mA lights. How do I calculate the value of the resistor I need to install on the front/backup lights?

I figure this information can be used across a wide range of applications so I hope Bachmann will forgive the other product references.

Thanks for the help,
Brad

[richg]

http://www.members.optusnet.com.au/nswmn1/Lights_in_DCC.htm

This fellow has a wide range of DCC info also. Check out the complete site.

Rich

[jbrock27]

Brad, what is the maximum voltage your power source puts out?

[richg]

Most DCC systems run between 12 to about 14 volts. Check the power pack that feeds your DCC controller. Subtract about 1.4 volts. The full wave bridge of decoders drops the voltage 1.4 volts.
With NCE systems, I measure about 12.3 volts DC at the function outputs on my decoders.

Rich

[richg]

As an example, I have a Miniatronics 1.5 volt bulb, 2mm diameter for a beacon on my Spectrum 44 ton. The controller is the NCE Power Cab.The wall wart measures about 13.8 volts DC.  Operated by a LokSound Micro decoder.  I used, 390 ohms, 1/2 watt resistor. Measured, 28 ma 1.4 volts 0.3 watts. That gives me a little buffer.

Rich

[NWsteam]

I forgot, I'm running Bachmann's EZ Command. I'll look at the links when I get the chance. Most likely not till tomorrow. I'm electrically handicapped so any any all explanation would be great.

-Brad

[richg]

Look at the power pack with the EZ Command. What is the voltage that it sends to the controller? Figure the decoder DC voltage for the lights will be at least 1.4 volts less.
I strongly suggest that you get a digital multimeter from Home Depot, Lowe's, LHS, a Local Hardware Store or even Harbor Freight if there is one around you. There is a Harbor Freight that sells a digital multi mer for a few dollars. I have three of them for some years. Very useful if you want to work at what I call the component level.

Rich

[NWsteam]

Got it figured. Needed a 1000 ohm resistor. I do own a multimeter. I have no idea how someone could get through a brass decoder install without one...great tool. Found this article to be helpful: http://www.wiringfordcc.com/gorhlite.htm. Runs like a dream and now I have forward and reverse lights too.
Thanks all!

-Brad

[jonathan]

Brad,

If you have a photobucket or flickr account, I would love to see a photo of your work.

I recently rebuilt a brass loco using surface mount leds (smd) for the lights.  I used 1K ohm resisitors which seem to work just fine.





Congrats.

Regards,

Jonathan

[richg]

I did the below measurements a few years ago.

Some Bachmann locos have about 2.5 k for a resistor.

Measured with 12.2 VDC supply using 20 ma max current LED's. Values will vary a ittle depending on actual DC supply value.

Resistor      Current
1000 ohm    9.0ma

750 ohm     12.45ma

680 ohm     13.12ma

510 ohm     16.25ma

Rich

[jward]

it should also be noted that resistor values will vary somewhat from their stated value, but are within the tolerance indicated by the 4th colour band (gold 5%, silver 10%, no band 20%) 
it should also be noted that these differences will not affect things in a lighting circuit, but if you are using the meter to measure the current in the circuit you will notice a small discrepancy.

[Len]

LED current limiting resistor formula for a DC power supply and a single LED:

       V t – V led
R = -----------
           I led

Where:

V t = total voltage (power supply voltage)
V led = operating voltage of LED
I led = operating current of LED (in amperes)

For example, with a 12-volt power supply and
a 2-volt, 20mA LED, you'd get

R = (12 - 2) / 0.02 = 500 ohms.

The closest standard resistor above or below the calculated value is generally okay to use if you get an oddball number.

Len

[richg]

No calculations needed with a multimeter.
It should also be noted that super bright LED's are available today and require over 1k for a normal brilliance. I have seen this mentioned in different DCC forums at times. Not everybody will buy these LED's though.
I have a 2.5k potentiometer with a 510 ohm resistor soldered to one terminal. That way if I intolerantly turn the pot the wrong way, I will not wipe out the LED.
Using the pot gives me a good idea of the difference in brilliance at different current levels.
I also have a good selection for 5% 1/4 watt resistors.

Some people in the Yahoo SoundTraxx Group have bought a nifty LED tester that is quite nice. Under $10.00.

I found out years ago, if you want to work at what is called the component level, you should have a multimeter. Make things a lot easier. Real world testing helps a lot.

Rich

[Len]

No calculations needed with a multimeter.

I have multimeters, along with several other pieces of electronic test equipment, but I like to have some idea what value resistor to use before I apply power to the LED. Less smoke that way.

Len (Retired Telecommunications Electronics Engineer)