Replacing old 12V bulbs with LEDs

[kgstones]

I'd like to replace burned out bulbs with LEDs but I'm finding LEDs are made for AC or DC depending on what they are used for.  I thought it would be simple, buy a 12V LED bulb that fits into the socket and I would be all set.  Particularly, I'd like to replace the bulb in the Lionel rotating tower with an LED.  I was going to buy a 12V bulb used for vehicles but they are for DC.  Would that work?  What LED should I use?

Thanks in advance, Mickey

[KRM]

Mickey,

 I have had great luck with these for use in engines. Already for AC operation.  Not sure about your tower, what model is it? You could also just add a bridge rectifier in front of the DC bulb to run it with the AC transformer.

http://www.modeltrainsoftware.com/bl-212.html

[kgstones]

Thanks for the information.  The tower just has the standard bayonet base with a incandescent 12V bulb.  I was hoping for an LED with the same bayonet base that would work in the tower.  I power it with an old transformer that supplies variable 12V AC current so I can adjust the brightness and how fast the beacon rotates.  The base is a mini base, smaller than those found in car and truck lights.  I don't know the model number on the tower as it's over at the grand-kids home.

Thanks again, Mickey

[trestrainfan]

I've never purchased from this company:

Town & Country Hobbies

[kgstones]

Thanks for the supply information.  I've also found them on ebay.

I'm still confused on a couple of points.  Does anyone know:

Does it matter what type of current (AC or DC) is used to power an LED?

Will an LED work at various voltages, (0V to 16V) to make it dim or bright?  Will an 18V LED work at 12V?

In other words, does an LED work just like an incandescent bulb?

Thanks again, Mickey

[lionelsoni]

Light-emitting diodes (LEDs) are very different from incandescent lamps.

An incandescent lamp is electrically a resistor, that is, an element for which Ohm's law applies:  The voltage across it is proportional to the current through it.  It's resistance is the ratio of voltage to current.  For the short time--one sixtieth of a second--of a cycle of AC voltage, its resistance is nearly constant; and it operates just the same on AC or DC.  Over the time of a second or so, the resistance increases enormously with temperature; so it has a low resistance at low RMS voltages and a high resistance at high voltages.

An LED puts out light only when current flows through it in the forward direction, anode to cathode, or in the direction of the arrowhead in the diode symbol.  The brightness of the LED increases with the current.

When current is flowing in the forward direction, the voltage across the LED is nearly constant.  This makes it a very poor match for a voltage source such as we use to power toy trains.  If you power it directly from a DC source, the source must be closely matched to the LED's forward voltage:  A little less and it doesn't light; a little more and the current goes way up and destroys the LED.  Furthermore, the forward voltage is only a couple of volts.

If you try to power it directly from an AC source (like a transformer), the instantaneous voltage is all over the place, making it impossible to match the LED's voltage for more than an instant.  Furthermore, the LED can stand only about 5 volts in the reverse direction.  This is the peak voltage of a 3.5-volt AC sine wave.

This is why LEDs used as incandescent replacements have other circuitry between them and the power supply and why that circuitry must be designed with the type of power supply in mind.  Without knowing what that circuitry is for a particular replacement light, you just have to trust the manufacturer's specifications (if any) to know whether the light will work for you.

[kgstones]

Thanks Bob,

I think what we'll just have to do is try a couple to see how they work.  One supplier states the bulbs are used to replace 6V - 18V bulbs so I'm assuming they will work.  Whether they can be dimmed or the brightness can be controlled is another question.  They look like regular bulbs so it doesn't look like they have any circuitry in them.  We have sources for both AC and DC current, old Lionel and HO transformers.

Thanks again to all for the help, Mickey

[lionelsoni]

I'm pretty sure there's more in there than just an LED.  The circuit needed can easily be smaller than the miniature incandescent it's meant to replace.  For example, a simple circuit for AC operation can comprise the LED, a diode (or another LED), and a resistor.

By the way, note that any incandescent replacement using an LED is unlikely to produce anything like the amount of heat that an incandescent does.  So an application that depends on that heat (smoke generator, rotating beacon, bubbler) is not going to work.

[kgstones]

That's interesting.  We wondered what made the beacon rotate.  We'll have to keep an incandescent bulb in it but perhaps when it burns out use an 18V bulb.  We can experiment to see how much heat each voltage bulb produces at different brightness to make sure the beacon rotates.  Actually it rotates at fairly low voltage with the 12V bulb that's in there now.

Thanks again for your help.

[lionelsoni]

Your beacon may use a different principle.  Does it make any noise?

[kgstones]

You are right.  It vibrates and there are small barbs on a ring around the top and bulb so I don't think in this case heat causes it to rotate.  But that's something to keep in mind for other accessories.  I've learned a lot, thanks.