Question for Jim Banner and Others

[Bill Baker]

Jim,

Over the past several weeks I have been putting lights in my town.  I have connected about 20 lights in parallel, all of which run under my board to my 12v power source. The power is a dedicated transformer to which I plan to use the AC terminals.  Although I have not connected the circuit to my transformer to test it, I am worried that I have not placed any resistors in my circuitry.  Based on other posts you have made about the need to place such resistors, I am concerned about burning out my bulbs and/or excessive heat build up. Most of the bulbs are miniatronics grain-of-wheat 12v. I don't recall the mA value since I have thrown away the package in which they came.

Question 1: What is the danger of operating the circuitry without any resistors, and
Question 2: If I definitely need resistors, would it be possible to place one in the feeder lines so that the whole circuit would be protected?

Thanks, Bill

[richG]

I would find a 9 volt wall wart. My way. Check the Miniatronics web site. They will say what the current draw is.

Rich

[Jim Banner]

Powering 12 volt bulbs with a 12 volt supply should be just fine.  When running at their rated voltage, they should last their rated life, which is often around 1000 hours.  Now for the fine print:

If you had just one or two lights connected to a power pack, the output voltage of the power pack would likely be more than 12 volts, maybe as high as 14 volts.  This is because of internal resistance in the power pack.  The more current you draw out of the power pack, the more voltage is lost across that internal resistance.  So the manufacturer will often rate a power supply as, say, 12 volts at 1.0 amp.  If you draw less than 1.0 amp, the voltage will be higher than the rated voltage.  If you draw more, then the voltage will be lower than the rated voltage.  This happens in your house too where the voltage at your wall socket rises and falls a bit depending on the load you and your neighbours put on your transformer (the one on a pole or in a vault.)  This is not something you need to worry about unless you find your bulbs burning out too fast.

If you draw much more than the rated current out of a transformer or power pack, it will overheat.  If it overheats too much, it will burn out.  Model railroad power packs generally have a built in circuit breaker to prevent burning out the transformer if we draw too much current.  For other power sources we can add a fuse.  The rating of the fuse should be the same or lower than the current rating of the power source.  Actually, adding a fuse to a power pack is not such a bad idea either.  Then if you add too much load (too many lights in your case) the fuse will blow before the power source is damaged.

Another alternative is to use a meter to measure how much load we are putting on the power source.  Yampa Bob has posted links to good sources of digital multimeters in the past and perhaps he will do so again.  The best ones for this job include 10 amp dc and 10 amp ac scales.  I bought one a year ago from a local supplier here in Canada for 14 Canadian dollars.  Bob has explained many times in his posts just how useful such a meter can be, and I agree with him.

A meter is also very useful if you would like to have some of your lights a little less bright.  Then you can measure how much current an individual bulb is drawing.  From that, you can easily calculate how large a resistor you need to reduce the voltage by a couple of volts.  Let's go through an example.  Suppose we have a lighted house that we would like to have only dimly lit, as if the owners had the lights turned down watching television, or maybe a romantic evening.  We measure the bulb current by cutting one of the wires to the bulb, then connect our meter leads to the two cut ends.  With the meter set for milliamps, we read, say, 65 milliamps.  That is .065 amps.  And let's say we decide to drop the voltage by 2 volts as a first try.  Then we apply the formula

Resistance = voltage / current.

That is, we divide the voltage we want to drop by the current through the resistor, and we get 2 volts / .065 amps = 30.7 ohms.  The closest commonly manufactured value is 33 ohms.  But we are not finished yet.  That resistor will get rid of the extra power by dissipating it as heat.  And the amount of power is has to dissipate is:

power = voltage X current.

So multiplying the volt times the current, we get 2 volts x .065 amps = .130 watts.  It is good electronics practice to use a resistor of about double this rating to keep the temperature down.  So we would select a 33 ohm, 1/4 watt resistor.  In practice, we would buy four or five of them so that we could add more than one in series with the bulb if one did not make it dim enough.  Where do we add them?  In practice, the place where we cut the one wire to make the current measurement is usually a good choice.

Suppose we ended up using 3 of our 33 ohms resistors all in series with one another and with the bulb, and we had several more identical bulbs that we wanted to dim to the same level.  We could buy a whole bunch of 33 ohm resistors but we could also use 100 ohm resistors if we wanted (3 resistors of 33 ohms each in series makes 99 ohms - close enough.)  But we need to recalculate the power rating.  If each 33 ohm resistor dropped 2 volts, then 3 of them in series would drop 6 volts.  And so would our 100 ohm resistor.  (We could check this with our volts function of our digital multimeter to be sure.)  So the power would be 6 volts x .065 amps = .39 watts.  So if we used a 100 ohm resistor, we would use one rated at 1 watt.  It might well be that a single 1 watt resistors cost more than the price of three 1/4 watt resistors, in which case we might decide to stick with multiple 33 ohm 1/4 watt ones.

Now what does all that do to bulb life?  The life of a bulb is increased by the 13th power of the ratio of the voltage decrease.  The decrease from 12 volts to 10 volts is a ratio of 12 / 10 = 1.2  And the 13th power of that is 1.2 x 1.2 x 1.2 x 1.2 x 1.2 x 1.2 x 1.2 x 1.2 x 1.2 x 1.2 1.2 x 1.2 x 1.2  =  10.7.  So our bulb rated for 1000 hours at 12 volts can be expected to last 10,700 hours at 10 volts.  This works the other way too.  Way up the page we talked about only 1 or 2 bulbs on a transformer.  Suppose the bulbs were rated at 12 volts and the transformer was rated at 12 volts but was actually putting out 14 volts because it was no where near its rated current.  Then their life would be shorted by a factor of 1.167 exp 13 ( a short hand way to saying to the 13 power) which is 7.42.  These poor bulbs can be expected to last only about 1000/7.43 = 135 hours.  If they were bulbs that were hard or impossible to replace, we would want to put a resistor in series with each one just to keep them from burning out prematurely.

One last comment.  In these calculations, we used the bulb current that we measured at 12 volts, even when the voltage was reduced.  This is not quite true.  If bulbs were resistors, we would expect the current to drop in proportion to the voltage drop, that is, if the voltage dropped from 12 volts to 10 volts (a factor of .833) we would expect the current to drop from .065 amps to .054 amps.  In practice, it drops to maybe .062 because bulbs are not resistors.

In the examples, I have used a bulb current of .065 amps because that is the rated current of one particular kind of bulb from one particular manufacturer.  This is not to imply that the same bulb from a different manufacturer would draw the same current, or that any of your bulbs draw that particular current.

Jim
^1:39

[Yampa Bob]

Hi Bill,

In case you don't have a digital multimeter:
http://www.harborfreight.com/cpi/ctaf/displayitem.taf?Itemnumber=90899

Description is in error, it does have a fused 10 amp DC ammeter. Similar types are available from many sources.

Regards

[Bill Baker]

Thanks guys for a very informative reply.  I've printed this out and will keep it as a part of my "Bachmann Book of Instructions".

Jim, I feel like Plato sitting at the feet of Aristotle....or was it the other way around?  Wait, I know....it's like Charlie McCarthy sitting on the knee of Edgar Bergin!  I wonder if our younger members of this forum know who they are???  Anyway, thanks, Jim, for your help.

Bill

[Jim Banner]

Plato??  Aristotle??  Socrates??  I don't know.  That's all Greek to me.

Now Edgar Bergin and Charlie McCarthy I understand.  Saturday night around a big old radio.  Switch it on a few minutes early to let it warm up.  The whole family listening.

Anyway, you are more than welcome.

[jsmvmd]

Dear Jim,

I'll ditto the good remarks! 

Those ancients probably did not wash their feet too often.  Is that why there were open air forums?

Sure wish I were closer to you guys than eastern PA.  Sure could learn a lot!

Best Wishes,

Jack

[Yampa Bob]

FWIW, Edmund Scientific has 12 volt GOW bulbs in bulk, 100 clear for $39.95, or 100 assorted colors for $42.95.

http://scientificsonline.com/search.asp?t=ss&ss=grain+of+wheat+bulbs&x=7&y=10

[Jim Banner]

Bob, is it my eyes or do those bulbs look a little smaller than the usual grain of wheat bulbs?  Or maybe the wires are a little fatter?  Still a good deal compared to the usual price at the LHS.

I have been buying rope lighting from Walmart for around $12 a string with no deliver charge.  One lengthwise slit with a knife and out come 100 grain of wheat sized bulb.  They are rated 10,000 hours at 3.6 volts which should rise to about 35,000 hours at 3.3 volts.  While 3.3 may seem like an odd voltage, it is one that is available at high current from most computer power supplies made in the last 10 years or so.  (If you use one, don't forget - fuse each circuit at no more than 5 amps.  Each 3.3 volt, 5 amp circuit can run close to 100 of these bulbs.)  Alternately, you can run sets of 4 bulbs in series from 12-14 volt supplies.  Life will rise to about 120,000 hours at 12 volts (or about 14 years if left on continuously.)  At that kind of life expectancy, the bulbs can be installed with no worry about ever having to replace them.  If you want brighter lights, you can run a string of 3 off 12 volts and still expect a life of 3800 hours.  At 13 volts, the expected life of a string of 3 drops to 1000 hours which is about the same as generic grain of wheat bulbs.

Jim   

[Yampa Bob]

Jim,
Maybe they had a dry year and the "grains of wheat" didn't grow full size. 

Edmund Scientific and I go back a ways. It's nice to see companies I grew up with are still in business.

I'm glad you mentioned the light rope, last night I was trying to remember what you wrote about them. I'll check them out next visit to Walmart.

[Bill Baker]

Rope lights? You mean he kind you put on your shrubbery at Christmas time?  I have yard and yards of the stuff....and they don't twinkle!  If that's the case I wish I would have known that weeks ago.

[Tylerf]

I believe he means the rope lighting that starts off in a clear tube.

[Jim Banner]

Those mini Christmas lights work well too.  The ones that come 50 in a string are rated at 2.5 volts each.  Strings of 100 or 200 bulbs are actually the same bulbs wired as 2 or 4 strings of 50.  Sources of 2.5 volt power are pretty rare but you can use "mini strings" of 5 in series with a 12 volt power source.  For a bit less light and a lot longer life, you can use 6 in series in a mini string at 12 volts.  Or you can run them in pairs from the 5 volt output of a computer power supply (don't forget the fuse!)  These bulbs are about 1" long and the socket adds some more length, so they would be too big for your platform lights where the bulbs are visible.  But they are great for in buildings.  You might, for example, light up five small houses using one bulb each on a 5 five bulb mini string.  You might use all 5 bulbs from another string to light up a larger building.   You might be tempted to use a whole string of 50 and just plug them in the wall, thus eliminating any low voltage power supply.  I strongly recommend against doing this.  Having line voltage wiring mixed in with the low voltage wiring under the train table is an invitation to disaster.  Besides, who wants all the lights coming on and going off at the same time?  It is much more fun to have the lighting on several circuits, each with its own switch.

The rope lights I was talking about are clear, flexible plastic about 3/4" in diameter with the bulbs embedded in it.  The manufacturer starts off with a clear core about 1/2" in diameter.  This core has holes for the bulbs molded into it every 1" along its length.  Each bulb sits in its own hole and has its wires connected to its two neighbours.  Then the whole assembly is covered with a molded on plastic tube.  Carefully slitting the outer tube lengthwise (VERY carefully, please!) lets you peel it off and then the bulbs can be pulled out and cut apart at the joints.  The bulbs are much smaller than the mini Christmas lights - they are about 1/8" diameter by 1/4" long.

Jim

[Yampa Bob]

Jim
Some links for rope lights.

http://www.1000bulbs.com/Rope-Lights/?gclid=CLePrJGNrZoCFQJhnAodelVecw

http://www.1000bulbs.com/12-Volt-Rope-Light/

http://www.centrix-intl.com/details.asp?productid=499

[Jim Banner]

Those are great links, Bob.  And they are 12 volt bulbs which is an advantage if you don't like messing around with series strings.  Do you know how they are constructed?  Is it easy to get the bulbs out?

I will see if I still have a set of the ones I use.

Jim