i have some bulbs marked R3 6v,70mA they dont look to be leds. i can get to 6v but am at loss as to limiting the amps. my power supply does not rate amps, please help an old duff.
Eneg,
Which power supply/power pack are you using?
Tom
https://tstage9.wixsite.com/nyc-modeling
Time...It marches on...without ever turning around to see if anyone is even keeping in step.
tyco pak 1
Amps are irrelevant here. You need to know the voltage output.
Once you know that, you need to lose enough voltage across a resistor to have 6 volts available to the bulb.
For example, if the output is 16V, you need to drop 10 volts. At 70mA, I=.07=10/R, where R is the resistor size needed. So you need to divide 10V by 70mA, which equals ~142. So a 150 ohm resistor is what you need in series with the bulb. (V=IR, or .07*150, = 10.5V)
If the output is 16 volts, three bulbs in series would work fine. If 12V, two in series. As long as the total equals the voltage supplied.
Betamax is right. You don't need to worry about the amps rating when setting up your voltage.
There's a bit more to it, though. The amps rating will tell you how much total current your bulbs will draw. So, if you have a 6 volt, 1amp power supply, and you use 10 of those bulbs, then you will be using 10 x 70 milliamps, or 0.7 amp. That's getting close to the limit for that supply, and you might want to consider a bigger one.
The other thing we like to inform people about is the idea of running bulbs at less than their rated voltage. For example, I use mostly 16-volt bulbs on my layout, but I power them at 12 volts. The result is a dimmer bulb with a warmer, more yellow glow. I prefer this look, as it better matches the lighting used in the past eras that I model. Also, this practice greatly extends bulb life.
It takes an iron man to play with a toy iron horse.
70ma is .07 amp, so 10 of those bulbs would be near the limit of the pack, if not exceeding it.
You need to knwo output voltage of the pack to calculate the resistor, and you need to know that 70ma rating for the bulb, because current and voltage are proportional. You do not drop amps, you drop volts.
Asusme the output voltage of the pack is 12V, you need to drop 6V. At 70ma.
Ohm's Law says Current x resistance = volts.
We know volts, and current. So volts/current = resistence.6v/,07 amp = 128.6 ohms
Nearest standard value without going under is 150 ohms.
Now the kicker - the resistor has to have a wattage rating capable of handling the power flowing throughit. This is current squared times resistence. .07 x .07 x 150 = .74 watts, so you would need a 1 watt resistor. And it would get hot.
Bottom line: do not use such high current bulbs for lighting. Use LEDs, or lower current 15ma bulbs that are 1.5 volts.
--Randy
Modeling the Reading Railroad in the 1950's
Visit my web site at www.readingeastpenn.com for construction updates, DCC Info, and more.
i agree that LEDs would be more efficient, use less current, provided you have the color you want.
but it may be difficult to use a resistor to control the voltage since the drop across the resistor will depend on the sum of the current drawn by all the lamps. what happens if a couple blow out ?
another approach would be to use a regulator to drop the voltage down to what you want, 6V. an LM317 is adjustable with a couple resistors. It will always output the regulated voltage as long as the source has sufficient current.
greg
greg - Philadelphia & Reading / Reading
gregcbut it may be difficult to use a resistor to control the voltage since the drop across the resistor will depend on the sum of the current drawn by all the lamps. what happens if a couple blow out ?
That's one reason you use a seperate resistor for each lamp.
using resistors, while simple, seems awkward. but if it works well enough, it's fine.
but if the supply is not regulated, or well regulated, its voltage will vary with the number of lamps. and the resistor values will depend on the actual output voltage.
using a regulator for the lamp voltage is one simple approach that, while perhaps a bit more elaborate than a few resistors, will maintain a constant voltage hence brightness, for as many lamps that the supply can handle.
understanding how to use a adjustable regulator can be useful elsewhere
LION uses LEDs for lighting. Each one has a resistor.
Here you can see the resistors and the LEDs. I mounted the roof supports first, took some steel welding wire and did a basket weave among them near the top, and then soldered a resistor to an LED, and then soldered the assembly between the two steel wires. The platform decorations were then installed, and the roof was applied. The roof is corrugated cardboard painted red on top and green across the corrugations to represent the lattice iron work of most NYCT stations.
Under construction:
The finished canopy...
ROAR
The Route of the Broadway Lion The Largest Subway Layout in North Dakota.
Here there be cats. LIONS with CAMERAS
Just keep in mind that bulbs/motors/LEDs draw the amps they require. The power supply can be 20 amps and it won't be a problem. That's why you use the amps for the LEDs(for example) to do the calculations for the resistor requirements.
Richard
LEDS are very forgiving. Nice light from 5 to 24 volts, a 1000 ohm resistor will protect it across all of those voltages.
Forget the resistor and KILL the LED.
Train ModelerJust keep in mind that bulbs/motors/LEDs draw the amps they require. The power supply can be 20 amps and it won't be a problem. That's why you use the amps for the LEDs(for example) to do the calculations for the resistor requirements.
An LED is not a normal device in this context. See below.
BroadwayLionLEDS are very forgiving. Nice light from 5 to 24 volts, a 1000 ohm resistor will protect it across all of those voltages.
lamps are usually just a piece of wire that has very little resistance when cold but heats of very quickly and increases its resistance to limit the current before it burns out. The greater the voltage the greater the resistance and the current increase is less than proportional.
Motors are also just a wire, which will spin within a magnetic field and generates an opposing voltage (BEMF) to limit the current through it. Under greater load, it spins less, generating less BEMF and drawing more current to apply more force. The maximum current though a motor is when it is stopped (i.e. stalled).
But light emitting diodes (LED) are just diodes. The voltage drop across an LED can be from 1.8 to 3.3V. The current through a diode exponentially increases with voltage. Unlike lamps, where current does not increase much with changes in voltage, LED current can change significantly with voltage. The use of a resistor helps regulate the current to protect the LED, but the brightness may vary.
A 1000 ohm resistor across a 5-24V source limits the current to 5-25ma. 24 ma is a reasonably safe current for LEDs. But wouldn't you want to use a smaller resistor if the voltage were only 5V. Would a ~133 ohm ((5-1.8/0.024) be a better choice.
Using a regulated voltage source and a suitable resistor allows the maximum brightness from an LED without worrying about unregulated voltage fluctuations. If the regulator were adjustable, it can be used to safely adjust the brightness of all LED.
gregc A 1000 ohm resistor across a 5-24V source limits the current to 5-25ma. 24 ma is a reasonably safe current for LEDs. But wouldn't you want to use a smaller resistor if the voltage were only 5V. Would a ~133 ohm ((5-1.8/0.024) be a better choice.
LION buys resistors by the thousand. One size fits all. Well ok, 1K ohm resistors for LEDs. LION has been using 5.1 ohm to control train speed into the stations. Him will try re-working the layout using rectifiers for this purpose, with about 1.2 volt drop across each device.
Him will try it out and let ewe know what him of this thinks!
thank you, all.
now i have enough info to get in trouble.