LED Selection for DCC Specialties Hare

[elnscale]

Hi

I have picked up some DCC Specialties Hares (NG/New Generation/II), attached them to some tortoises and am putting together a small track panel with LED indicators from the Hare.

The documentation on the Hare is sparse when it comes to details of the specifications of the LED to use. By testing the Hare both on my 13.4vac Digitrax Zephyr and my 11.5vac Digitrax Super Chief, I have determined that the voltage output for the LED (pins 6, 7 & is always 5vdc.

The Circuitron site (http://www.circuitron.com/index_files/AN/AN-6000-07.pdf) for the Tortoise states that if you wire the LED in series with the motor (pins 1 & 8 on the tortoise) for a typical LED you don't need an additional resistor since the motor has a resistance of 600 ohmns.

But I cannot find anything on the hare which suggests that what its LED outputs (pins 6, 7 & 8 on the Hare) are connected to and what resistance it offers so that I can properly size the LED (or include additional resistance).

I have prototyped some Radio Shack LEDs that work fine with no additional resistors (each has 2.1v forward voltage) and am looking to buy some bulk LEDs online so I need to make sure I am not buying the wrong thing (btw, I know what luminosity I want so that's not an issue).

If the 600 ohm tortoise is the resistance offered across the Hare's LED connections, then I have no problem because I have (5v - 2.1v) / 600 ohms = 5ma approx. current.

Also, let's say that the above is correct and the current flow is 5ma, is it ok to buy a 10ma LED or should I get one that will take 20ma for additional margin?

Does anybody have any experience on this that can help me on this?

Thanks

Steve

p.s. I am powering this from an assessor bus that is fed from an output of a Digitrax PS42 configured for 1.5am max just like my track power districts. I'd expect this to be a pretty consistent power supply. Is that true?

[DKS]

FWIW, I have always been reluctant to connect an LED in series with a motor. DC motors do not have a fixed resistance; it varies according to RPM. Also, DC motors tend to introduce current spikes in the circuit that can damage an LED. Granted, a Tortoise is pretty benign when it comes to fluctuations in current, but I'd still be very leery of this practice.

[elnscale]

David,

I do not know if the LED outputs of the Hare connect to the tortoise motor or not. I was just pointing out what documentation on the tortoise says regarding connecting LEDs directly to the tortoise. I do not know where the Hare derives its LED output. There's no information that I can find and no circuit diagram. It's possible that it has nothing to do with the motor connections but is controlled by what it knows of the DCC state of the position of the tortoise.

I've also sent an inquiry to the manufacturer of the Hare to see what they say.

Thanks, though for the general comments on the direct connection of LEDs to a motor. I am new to LEDs and so everything I can learn is appreciated.

Steve

[peteski]

Steve,
I understand DKS' concern with wiring LEDs in series with Tortoise motor. However, the motor's low current winding has enough resistance to safely power LEDs. As far as BEMF voltage spikes go, the indicator LEDs are hooked up in such a way that they protect each other from being zapped by reverse voltage.   For practical examples, my friend's layout is wired that way. He has dozens of such circuits and in the 20 years of operation, we have not lose even a single LED.   I would consider this a safe method.

The added benefit of this method is that while the tortoise is in the process of throwing the points, the current flow is at its lowest.  That is because the motor acts as an inductive load. When the motor stalls, the current jumps higher (stall current) because the motor now becomes a resistive load.  This results in the LED being dimmer during motion, and getting brighter when the points are fully thrown.  That is helpful in a noisy room where you can't see the switch (some hidden area or the switch being too far from you).

Also remember that Tortoises consume most current while at rest. Make sure to use a power supply which can safely provide that constant high current.

As far as running LEDs from 5V without resistors, I would not recommend that.  To be safely powered, LEDs need a current limiting resistor.

[elnscale]

Hi Pete,

The Hare is a board that fits onto the tortoise and acts as a stationary decoder. It has support for LED panel lighting to indicate the aspect of the turnout.

This may or may not be related to the capabilities of the tortoise to support LED display. I do not know that because I have no details of the circuit to tell how the LED circuit on the Hare is architected.

I posted the very same question as this post to the manufacturer.

The response I received was "The Hare outputs are designed for >5v/20ma LEDs".

I am going to respond again because I don't think that helps me.

Back to the tortoise for a moment, without getting into the appropriateness of using LEDs in series with the motor, at least Circuitron states there is 600 ohms of resistance and so the math for the current flow is possible since for a given LED, the forward voltage is known, the voltage to the tortoise is known (or measurable) and the resistance is known. (Voltage minus forward voltage) / resistance = current. So, for example, (5 - 2.1) / 600 = 5ma.

Now I am totally new to LEDs and all of their specifications etc. So I am sure to have it wrong somewhere.

But for the hare, I still do not know how to right size a LED. The resistance built in to the LED circuit is not known to me (BTW, can I measure this with my multimeter which has an ohms setting, and if so, how?). If I don't know the resistance, I cannot even add an extra resistor accurately, can I?

If the LED is meant to support > 5 volts and 20ma, without knowing the resistance, how can I know what to get. It's frustrating.

I did tests yesterday with two different LEDs that I picked up from Radio Shack. First there was a green/red two lead LED #276-012. This is 6.3mcd, 30ma and it says "Typical Voltage is 2.0, with a maximum voltage of 2.8V". I don;t know if that means supply voltage or it means forward voltage. Anyway when connected to the hare, these seemed to be what I would consider a normal brightness (which was not very bright). Then I tried a green LED, #276-304. This one is much brighter at 620mcd, 30ma and "Typical voltage: 2.1V, with a maximum of 2.8V". Again what voltage this refers to is not specified.

Clearly,  I don't know if these LEDs were receiving too much current. They "seemed" to be ok.

Like I said frustrating.

Steve

[peteski]

It is frustrating, because you simply don't have enough information (yet).

Couple of issues ago, the Electronics column in N-Scale magazine had a lengthy article educating model railroaders about LEDs.

Without going into it too deeply, LED is a diode. While is does have a resistance, it is non-linear and voltage dependant. Regular resistors are linear.  The non-linear characteristic of the LED is why it needs some external device which will limit its current.

Radio Shack specs seem a bit broad.  The forward voltage of the LEDs depends on their chemistry.  White, blue and "true-green" LEDS have forward voltage of around 2.8 - 3.4 V (usually in the lower end of this range).  Other color LEDs usually have forward voltage of 1.7 - 2.1 V.  Red are usually on the lower end, and green and yellow on the higher end.

Your math is correct.  To calculate the current-limiting resistor's value you subtract the forward voltage of the LED from the supply voltage, then divide that by the desired LED current.  The usual current for LEDs is between 5 and 20mA (or 0.005 and 0.02 A).  The brightness of the LED depends again on the chemistry of the LED and on the current. The larger the current, the more light will be emitted.

Without seeing the hare circuit, I do not know whether the 5V outputs already have a series connected resistor installed in the circuit or is it raw 5V.  I wouldn't hurt to add a resistor if you aren't sure.

I'm also confused by your calculation which seems t indicate that the tortoise is powered from a 5V output.  Usually those machines use 12V.

Assuming that the tortoise is powered from a 12V source (and the motor has a DC resistance of 600 ohms) then the current going through the LED (when the motor is stalled) could be calculated as 12 - 2 (assuming a green LED) / 600 ohm = 0.017 A (17 mA).  That would be pretty safe for the LED.

If the tortoise is actually powered using 5V, then your calculation is correct.

LEDs seem daunting in the beginning, but once you know the basic theory of how they can be utilized, it  becomes quite easy. Just like most other aspects of our mulch-faceted multifaceted hobby.

[DKS]

Just like most other aspects of our mulch-faceted hobby.

Didn't know we were doing gardening...

[peteski]

Didn't know we were doing gardening...

LOL!  No help from the spell checker on this one.

[elnscale]

Hi Pete,

I did imply that the tortoise operates on 5v. I didn't mean to do that. I do agree a 12v supply would cause about 17ma through that LED.

The Hare documentation in one place I found does say that the LED circuit is protected by a resistor but it never says the value of it. I wish it would. I checked some of the digitrax devices. For example, the BD164 (block detector). For it's LED circuit it tells you it has a 1k resistor and a 5v supply so you can do the computations. Shame the Hare does not.

I just checked the actual device and I can see that there is a 130 ohm resistor on both the pin 7 and 8. This can be seen in the image at http://www.dccspecialties.com/products/hare-ng.htm, resistors R10 & R11 (center of left edge) are marked 131 = 130 ohms. You use 6-8 (red/green 2 lead bicolor) or 6-7 and 7-8 (two mutually exclusively lit) for the LEDs so you have at least 130ohm resistance.

Ok, so now I have (5 - 2.1)/ 130 = 22ma. That means I am in business. Of course, the resistance could be more elsewhere in the circuit but that would simply mean that the current would be less.

Thanks for the lessons.

Steve

p.s. Could not measure the resistance with my multimeter because it requires not power to the Hare and with no power, the LED circuits are open.

[peteski]

See what a bit of knowledge can do?  You are no longer a novice when it comes to LEDs! 

Sounds like you have it all figured out - your writeup makes sense to me, and it seems correct.  I agree that it is too bad that the manufacturer does not give more precise instructions, or specifications, as far as the LED outputs go.

I'm not sure what you mean by not being able to measure the resistance.  You are supposed to measure resistance with the circuit power being off.  With the power off, place your ohmmeter leads on both sides of the resistor to measure its value. If one side of the resistor only goes to an unconnected terminal then the meter will give you true reading.  If the traces on both sides of the resistor go to other components then the reading can be skew by those components.

But no need to do that since you correctly identified a resistor labeled "131" as 130 ohm.  One side of the resistor should then go to the terminal (7 or 8 ) and the other side will go to some other circuitry on the board.  You can verify that by visually tracing the metal traces on the circuit board.  If for some reason you cannot visually follow the traces, then you can use ohmmeter to verify if a trace goes between point A and B.  If it does, then the reading should be 0 ohms.

[elnscale]

Pete,

You misunderstood me.

I tried measuring the resistance of the LED circuit from the pins 6 to 8 and then 6 to 7 and then 7 to 8. I did this before I thought about looking for resistors. So it was not that I could not measure the resistance of a resistor (because I did not try), it was that I could not measure the resistance of the whole LED circuit (e.g. between 6 and because the circuit was open when no power was applied.

Anyway this is close to a dead horse now! Close, but not completely there, because I have asked the manufacturer explicitly what the resistance of the LED circuit is. And I will post that if I get it.

Steve

p.s. I order a bunch of LEDs tonight. They are WAY cheaper that buying them one or two at a time at the Shack.

[peteski]

Hopefully the manufacturer will provide you a meaningful answer (even though you already reverse engineered that circuit, and know the resistor value). 

As for LEDs, I haven't bought them at Radio Shack since the 80s.    Nowadays, there are plenty of excellent LED deals on eBay (usually directly from China).  If I need specific size or type of LEDs, with well documented specs, I get them from Digikey or Mouser Electronics.  The wide range available from those distributors can be overwhelming, but that is not a problem.