Why DCC inrush problem is solved with in-line light bulb?

[Anonymous]

When I upgraded from a Powercab to NCE's SB3 to get another amp, I experienced a behavior difference.  When taking trains on and off the track, I disable the track connection with a master switch (rather than turning off the power to the DCC unit).   With the SB3 when I restore the track connection, the unit turns itself off.  This happens when I have a number of cars on the track lighted with incandescents.   No problem if I cut and restore the DCC power supply.

Being I have around 800 ma of incandescents, I was suspicious of the inrush.  NCE confirmed the SB3 is more sensitive to inrush and said the solution is to put an auto light bulb in line with one of the track leads.

I'm curious why this works.

Thanks, Bob   

 

 

 

[JamesP]

Since nobody else replied to this yet, I'll put in my two cents worth, please correct me if I am wrong.  The incandescent lamps create an inrush problem due to their positive temperature coefficient - the hotter the filament is, the more resistance it has.  When you flip your toggle switch on, the lamps have been off, the filaments are cold, they have low resistance, so the initial current flow is higher than when everything is operating... apparently this is enough higher that it is tripping out the unit's overcurrent protection.  With an automotive lamp in series, there is some extra resistance in the circuit, but with a cold lamp it is low enough that it does not cause a substantial voltage loss during normal operating current.  Also, normal operating current will not be high enough to cause the auto bulb's filament to heat up, thus increasing its resistance and causing a voltage drop.  However, when everything is cold and the toggle switch is flipped on, the inrush current from your incandescent load is enough to cause a slight voltage drop across the auto bulb due to its initial resistance and this will tend to limit the inrush right off the bat.  If the inrush is high enough and continues long enough (I'm talking milliseconds), the filament in the auto bulb will also begin to heat, causing its resistance to increase, further limiting the inrush.  Since the resistance of the small bulbs is going to be much higher than the resistance of the auto bulb, they will cause the most voltage drop as things equalize out and you probably don't even see a hint of glow from the auto bulb even on startup.  This is a cool, simple solution to the inrush problem!  When I did a lot of audio electronics, we used to use light bulbs as a power limiter and indication in series between speakers and solid-state amps.  If the volume was turned up too much, enough current would flow to cause the bulb to light, giving a visible indication and at the same time increasing the total resistance of the circuit, thus limiting the current and saving the speaker.  We had to play around with light bulbs to get the proper resistance to make it work.  By the way, when you turn the power supply on, the unit won't have an inrush problem because many units like this have "soft-start" circuitry to prevent it from happening on powerup.  Anyone else want to weigh in on this?

[Anonymous]

I think your explanation is right James.  I also asked an electronics knowledgable friend and got the same answer. 

Interesting what you said about speakers.  I happen to have some old Pacific Stereo "Concept" speakers that have a pair of lights in them.  Green one lighted, no problem, but the instructions said if the red one is on solid, don't keep the volume at that level for very long.

Thanks, Bob 

 

[JamesP]

Thanks for the confirmation...two heads (or three) are better than one.  It's a neat idea having two lights in the speaker, my homebuilds weren't that sophisticated!  It also dawned on me that depending on the resistance of the auto bulb, the overcurrent protection may or may not trip out when the track is shorted.  If the resistance of the bulb is high enough to limit the fault current during a track short to an amount less than the overcurrent protection, it would just light up.  Not a bad thing, certainly won't hurt the unit, plus if you know to look for it, you could use it as a short indication.  However, I suspect that the resistance of the bulb may be low enough to trip the unit out when shorted.  If we knew the capacity of the unit and the wattage of the bulb, it could be calculated, but the easiest way to know is to lay a nail across the rails...

 - James

[Anonymous]

Interesting you mentioned the short circuit issue because NCE recommends the same in-line light bulb solution for "long term" short circuit protection.   The built in protection is just short term - it re-energizes the power every 2-3 seconds when detecting a short.   BTW, the bulb they recommend is a #1156 tail light bulb.  Another advantage of the bulb for the NCE SB3 is putting it where you can see it.   If you call the "front" of the SB3 the end that has the connector for the cab cable, they put the leds that show an overload on the "back" of the unit - not a great design.

[JamesP]

Just out of curiousity, how is the bulb connected?  If I remember correctly, it will have two filaments, one for the tail light (11 watts) and another for the brake light (56 watts).  The case is common and there are two contacts on the bottom, one going to each filament.  Is just one used, or are they wired together in series or parallel?  I'm guessing they have you use either just the brake element or both wired in parallel - one wire to the case, the other connecting to both of the contacts.

[Anonymous]

I think it's up to the user deciding what current they want to accommodate - evidenced by their manual showing the option of using more than one bulb if desired - the bulbs being in parallel with each other but the group being in series with the circuit.   I haven't put the bulb in yet so can't comment on which filiment to use.