Resistor Value Voltage Question AC vs DCC 12 volts Atlas Signal System

[nscsxcrrailfan]

I'm wiring up an Atlas signal system on my layout. I use Digitrax DCC. I am in N Scale and use the 12 volt setting on my booster. Now, I know a little bit about resistors, electronics, etc... I am using some Caboose Industries ground throws with resistors attached to report turnout position to the NCE BD-20 detectors that I am using. I found an online resistor calculator where you enter your voltage and resistor ohm value, and it reports the milliamps and watts that are consumed. I use the milliamps value to know how many resistors and the resistor's ohm value I will need to activate the BD-20 and report a red signal. Now, here's my question. I know that DCC signals are not exactly the same as AC signals. AC signals are generally reported in a true RMS value 120 volts cycling between 0 and 170 volts (something like that). I also know that resistors should generally not carry a load more than 50% of what they are rated for. For example, a resistor that is 0.25 watts (1/4) should not carry any more than 0.125 watts. The area I am uncertain of is choosing the right resistors to use with DCC. Does 12 volts DCC have the same effect on a resistor as 12 volts AC/DC? If I size the resistor according to the online calculator I'm using, will the DCC 12 volt signal push through more watts than a regular AC 12 volt signal or will it be less? What I'm concerned about is the possibility of selecting a resistor such as a 1/10 watt that supposedly at 12 volts AC or DC will draw 0.05 watts when actually under DCC 12 volts it will draw 0.15 watts, or something like that. I want to avoid burning up any resistors. Also, I am using these resistors in a resistor only circuit. Basically, I run a wire from one leg of the DCC main bus wire to the ground throw, from there I run a wire to the resistor, on the other side of the resistor I run a wire back to the other leg of the DCC main bus wire. Along this path, I loop the wire running to the resistor through a BD-20 detector. I am NOT using the resistor in a light bulb or LED circuit. When the ground throw is in a certain position, it will complete the circuit (like a switch) and the resistor will consume electricity (DCC signal), which the BD-20 will detect and report to the Atlas signal board, which will drop the signal to red.

[rrinker]

 You're overhtinking it (something I am sometimes guilty of as well). It will work exactly the same - the calculation is exactly the same. The BD20 needs to see at least 8ma to detect, so for reliable oepration you should base your calculation on something a bit higher - 12-15ma should give enough of a safety margin.

 Now - why the BD-20? If it's just to interface the ground throw to the Atlas signal board, that's a waste. If there is ALSO a track block wire runnign through the BD-20, just wire the contacts in parallel to the BD-20 so it switches the logic pin to ground. The BD-20 output is an open-collector circuit and can handle being logically ORed with multiple circuits or simple switches in parallel. Think of the output of the BD-20 as a switch to ground (less the junction drop inthe output transistor, that's EXACTLY what it is). You can put as many 'switches' in parallel as you want, if any or all of them are closed, the signal would be red. No harm will come to the BD-20 if it switches and the contacts on the ground throw were already closed.

 DCC Decoder functions work like this as well - you can connect 2 or more function leads to the same light bulb because the function leads simply act as a switch to ground. The blue common wire is actually +.

                                                    --Randy

 

[nscsxcrrailfan]

Yeah, I understand the basic idea behind open collector (I posted a question about that a few weeks ago). My main concern is with the resistors and the effect that DCC 12 volts will have compared to AC 12 volts. From what I've read, AC and DC are for the most part equal in watts and amps as long as the AC volts and amps are in sync with one another as they cycle back and forth. A simpler way to ask the question is, does 12 volts of DCC current across a resistor equal 12 volts of AC current across a resistor? From what I understand from your post, there will be no difference, correct?

[rrinker]

Correct. It's exactly the same. If your track voltage is 14.5 volts and you put a 1K resistor across it, it will flow 14.5ma. We're talking about a purely resistive load, so under AC it has a power factor of 1, meaning it's the same as using DC voltage.

                           --Randy

 

[nscsxcrrailfan]

O.k., good, that answers my question. It took a while before I found that AC (wall outlet) has same effect as DC on resistor, as long as the RMS (volts and amps same cycle) is uniform. I still don't fully understand these terms, but what had me concerned was when I began reading how DCC's waveform was totally different. I'm glad to hear that it will not make any difference on my resistors. As a side note, I do want to add that from my personal experience, DCC seems to be easier with resistors than DC. For example, I connected a 100 ohm resistor across the rails a few weeks ago with the 12 volt setting. The resistor was 1/4 watt, so according to the calculations, it should have consumed 1.44 watts. It should have burned up fast, but it did not. I left it on there for 5 minutes and it was only lukewarm. On the other hand, I took a different 100 ohm resistor and connected it to the DC on my power pack, cranked it up to about 12-15 volts and it smoked and got hot fast. From this, it makes me think that DC is harder on resistors than AC.

[Vail and Southwestern RR]

If you had the DC voltage up at 15, you had 2.25W disappated by the resistor, half again DCC system. Your experiment may have been a bit flawed.

[rrinker]

Resistor behave the same, AC or DC or any shape wave. It gets tricky when you itroduce capacitors and inductors - they cause shifts in the timing of the wavefrorm. For regular sine wave AC, think of TWO sine waves superimposed but not exactly lined up, so you see to lines - they are out of phase. So the peak of the load either leads or lags the peak of the source. This is the power factor. A resistor - or a light bulb in your house - has a PF of 1. A 100 watt light bulb left on for 10 hours uses 1kW-hour of electricity. Things like the motor in your fridge have a power factor less than 1 - they use more power than they 'should' if they had a PF of 1. If you're really interested, read this (if you can manage to not fall asleep): http://en.wikipedia.org/wiki/Power_factor

                                             --Randy