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Discussion Starter · #1 ·
I may be starting something that has no definitive answer but over the years I have heard many discussions (sometimes heated) on the use of head light globes as short circuit protection in DCC systems.

I personally won't do it but for every electronic guru that says don't do it I also find one that says it is OK.

So, at the risk of asking an unanswerable question, can any one enlighten me/us.

Ian
 

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Just another modeller
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*** Hi Ian

There are no "experts" that dismiss bulbs totally. Those who do cannot be experts as they do not understand the difference in applications :) :) ...and therefore don't really understand the issues they are good for in DCC.

Lamps as a form of short circuit protection have been with the hobby as long as electricity has. They work in their intended manner very well.

They are not in reality a replacement for a short protection device - they are good for many things BUT the use is similar but NOT always the same.

** A short circuit device will cut off the power intantly.
** A bulb will temporarily buffer a momentary short so sound decoders don't reset, so there is no voltage spike, so there is no shut down, so there is no hesitation in the loco.

they are not short circuit protection so much as buffering, for which they work perfectly. The best example of positive use is for momentary shorts at peco insulfrong points.... peco turntables, etc etc...

They are a totally valid answer to specific problems.

Joe Fugates video mentioned by Poliss has a good treatise on their use- Joe uses lots of them on his own layout.

Richard
 

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Discussion Starter · #4 ·
QUOTE (Richard Johnson @ 29 Mar 2009, 05:20) <{POST_SNAPBACK}>*** Hi Ian

There are no "experts" that dismiss bulbs totally. Those who do cannot be experts as they do not understand the difference in applications :) :) ...and therefore don't really understand the issues they are good for in DCC.

Lamps as a form of short circuit protection have been with the hobby as long as electricity has. They work in their intended manner very well.

They are not in reality a replacement for a short protection device - they are good for many things BUT the use is similar but NOT always the same.

** A short circuit device will cut off the power intantly.
** A bulb will temporarily buffer a momentary short so sound decoders don't reset, so there is no voltage spike, so there is no shut down, so there is no hesitation in the loco.

they are not short circuit protection so much as buffering, for which they work perfectly. The best example of positive use is for momentary shorts at peco insulfrong points.... peco turntables, etc etc...

They are a totally valid answer to specific problems.

Joe Fugates video mentioned by Poliss has a good treatise on their use- Joe uses lots of them on his own layout.

Richard

As usual, Richard, your answer is very informative however: I must query your first paragraph
which was the reason for this post.

We have two electronic engineers in our DCC club,one is retired the other is still earning his living with his profession. The retired gentleman says bulbs are OK, the other advocates strongly against it.

I am also friends with a young man who is nearing the end of an electronic engineering course at Uni and he goes of his nut if globes are mentioned for use with DCC.

All of the above have been using DCC for some time.

After reading your reply and looking at Joe Fugates video I am certainly rethinking my attitude on globes and DCC.

Ian
 

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*** There are many "electronic experts" who get the wrong end of the stick in relation to DCC issues.... but its not technical error, its simply that the inerpretation of use is often not clear.... so I don't doubt their qualifications in other areas, but the bulbs are a safe and pragmatic means of buffering which have been used with no negative issues for years.

The problem comes in the interpretation of their use - Some misinterpret them as being all thats needed, but that isn't the case. In the case of the "Soon to be a graduate" he needs to live in the real world for a wee while :) :).

They are as I said, a useful, helpful and economic supplement but not in real terms a substitute or a replacement for short circuit protection.

They certainly do save the bacon of many of who change existing layouts from DC to DCC, or those who use insulfrog points.... or those who are being driven mad by sound resets.... the alternative for those is often significant changing of existing layout wiring or pointwork.

The hardest thing here in AU is getting an economical supply of 30~35 watt 12v bulbs.

Incidentally you will also find Marcus a firmly committed "bulb person".

regards

Richard
 

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Can I add my 2 cents here.

Firstly would you define a light globe as a resistive element? If yes why not use a resistor?

But a light globe is not just a resistive element, it has wire wound in a coil so now we have an inductor in there. Have a think about it guys I would be very interested to hear what you have to say on this.

By the way Richard and Marcus are right, you have your clue....but why?

martin
 

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QUOTE (Martin71 @ 30 Mar 2009, 11:00) <{POST_SNAPBACK}>Can I add my 2 cents here.

Firstly would you define a light globe as a resistive element? If yes why not use a resistor?

But a light globe is not just a resistive element, it has wire wound in a coil so now we have an inductor in there. Have a think about it guys I would be very interested to hear what you have to say on this.

By the way Richard and Marcus are right, you have your clue....but why?

martin
It's noting to do with inductance.

The key is that the bulb's resistance changes drastically between cold (dark, low resistance) and hot (lit, high resistance). You choose a bulb that stays dark (low resistance) under normal conditions. You also chose the bulb so that when excess current flows (you define what an excess is) the bulb illuminates. This causes the resistance to increase which limits the current. An equilibrium is reached where the bulb glows but only limted current can flow in the circuit.

The other point to note is that the voltage across the bulb increases and the voltage at the track decreases when the bulb is lit.

Thus a bulb is a current limiter.

Andrew
 

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QUOTE (SPROGman @ 30 Mar 2009, 13:35) <{POST_SNAPBACK}>The other point to note is that the voltage across the bulb increases and the voltage at the track decreases when the bulb is lit.
That could have been clearer. In the case of a dead short, obviously the track voltage collapses to (near) zero.

Andrew
 

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Hi

Ok, no disrespect bit I disagree with you on this Sprogman and I will put a very simple argument across.

Firstly I will agree the light bulb is a resistor and it is this resistance which creates heat which makes it glow.(By the way the old incandescent light globe just for info produces about 80% heat 20% light, not very efficient)

My point.

The wire in the coil has much lower resistance (it's just wire,), so what you would expect when you turn on the switch is for the bulb to glow very dimly. Most of the current should follow the low-resistance path through the loop. What happens instead is that when you close the switch, the bulb burns brightly and then gets dimmer. When you open the switch, the bulb burns very brightly and then quickly goes out.

The reason for this strange behavior is the inductor. When current first starts flowing in the coil, the coil wants to build up a magnetic field. While the field is building, the coil inhibits the flow of current. Once the field is built, current can flow normally through the wire. When the switch gets opened, the magnetic field around the coil keeps current flowing in the coil until the field collapses. This current keeps the bulb lit for a period of time even though the switch is open. In other words, an inductor can store energy in its magnetic field, and an inductor tends to resist any change in the amount of current flowing through it.

The inductor play a key part in the light bulb, and like most I always believed the light bulb was a purely resistive circuit.

Lets say I am totally wrong, if it is a resistive only circuit what limits the current (it's just wire) and how do we achieve equilibrium (balance)?

Martin

Can someone please tell me how I take a quote form this post and post it in my post, like what Sprogman has done.

Thanks
 

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Ian Wigglesworth
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Martin to 'quote' another post simple click on the quote button at the bottom of the post it will then be added.

Also gentleman I do believe this has now gone way over the top of what anybody really wants.

I have used the CP6 which has 6 light bulbs on it, for 6 different zones.
By selecting different wattage bulbs I can limit the current that is applied to the track in each of these zones, I have used low power bulbs to limit the current in each zone to just under 1amp.

This seems to work fine and using the PowerCab is(not sure if it's since changed) the only way to add some sort of protection.
The Power from the Powercab was too low for other type circcuit breakers to work.

I know that a circuit breaker will do exactly that, break the circuit in the event of a fault so no voltage or current flows through the circuit.

The bulb on the other hand will allow voltage and current to flow still but limits this current to a level of my.......well the bulb manufacturers choosing!!
 
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