I am installing a new Tsunami TSU-AT1000 decoder. The instructions tell me that if I wish to use 1.5V incandescent bulbs, that I simply connect one lead to the “1.5V Common Terminal”, and the other lead to the appropriate headlight terminal. They don’t mention using any resistors. Does this “common terminal” act as a resistor? The same goes with using 12V bulbs. LED’s, on the other hand, are required to use a resistor in series. Just want to be sure… JRP Great Southwest and Pacific RR
Not all tsunami’s are the same. The AT1000 is made with the 1.5 volt out and does not need resistors. Other models are 12 volt and need them.
I don’t know anything about that particular decoder, but when it comes to 1.5 volt incandescent bulbs, if a resistor is not required it is because there already is a resistor built into the decoder.
Rich
Do not “assume” when it comes to DCC installs. There are “Gotchas”. Those who smoke a LED or decoder usually do assume. The different decoder manufacture sites have decoder manuals and the decoders come with instructions. With the Internet, you can find the manuals easily .Ignorance is not bliss.
A digital multimeter to check the install can be had for about $5.00. I have three of them and cheap to replace if I misuse it.
Rich
The AT ( TSU-AT 1000 ) version of the Tsunami decoders do have a pad that provides a 1.5 volt output.
Not sure it is a resistor on board. I think the AT has a voltage regulator for the 1.5 volt output.
Ar any rate, if you use the 1.5 pad for your 1.5 bulbs you will not need a resistor.
The 1.5V bulb IS a resistor. OTOH, an LED needs a resistor to limit the current flow. It’s easy to confuse the requirements, since both the bulb and the LED produce light - but in entirely different ways.
Chuck (Modeling Central Japan in September, 1964)
That is true. Put a 1.5 V LED across a 1.5 volt battery and watch how fast it burns out.
Huh? [*-)][%-)]
You don’t use a resistor on an LED when there is a 1.5 volt output.
How the device produces light has nothing to do with the voltage that is feeding it.
Go ahead, try it.
Well that would depend on the LED would it not? Some LED’s will light up with 1.5 volts.
Yes they will, but there is nothing to limit the current. The 1.5V output of a circuit board may have the current limited on the board, but a battery does not…
There are some LEDs that have a resistor built in.
You don’t need anything to limit current. If a device is listed for say 1.5 volts and 20 miliamps, and you apply 1.5 volts, it will draw 20ma.
By the way, I installed one of he GN1000’s and it put out 1.5 volts as advertised. No resistors needed.
If you’re nervous about doing it, you won’t hurt anything by trying it with a resistor. You will probably just get little or no light.
Here’s a copy of the manual. Note it say that when using LED’s to use the 12 volt wiring. This would require resistors. The reason they say this is because many LED’s require more thant 1.5 volts for full brightness. If you use the 12 volt then you would use a 1k resistor. You could go down to 680 ohms or a little lower but the LED will last much longer with 1K.
http://www.mrdccu.com/graphics/jpg/TSU-AT1000Instructions.jpg
I made an error in my last post ( Some LED’s will light up with 1.5 volts). should have said - Most of the LED’s will not light up with 1.5 volts. So with a 1.5 volt output you will never get most LED’s to work.
It has a lot to do with it. LED means Light Emitting Diode. When forward biased, it is almost a short, with a small amount of voltage drop across the junction. They are current operated, and without the series resistor to limit the current (and provide a voltage drop) they burn out. Really quickly.
An incandescent lamp, like used in many household fixtures, has 120V across it at all times while operating. A fluorescent lamp requires a lot of voltage to strike the arc, and once the arc is going, the ballast limits the voltage and current across the lamp. Ohms Law says they are all related. The difference is one increases in resistance when lit, the other decreases. An LED is a much different animal. It will not tolerate excessive voltage or current.
to expand on what betamax had already said.
a lamp is just a piece of wire. the more voltage, the hotter it get producing more light and increasing its resistance to limit the current, within limits. Hewlett-Packard got started using a lamp to regulate current to make a very stable oscillator.
an LED is a diode, a semiconductor, a device normally used to restrict current flow in one direction (think A.C. to D.C.). Once the forward voltage is exceeded, the current increase exponentially (1.5 is not 1.4). Without a resistor, the current limit can easily be exceeded and it will burn out.
The forward voltage drop across many LEDs was typically 1.4 volts. The resistor value is the source voltage - 1.4 divided by the desired current (2.6k for 12v and 4ma).
I’m not familar with the types of LEDs available today, but i’m guessing that there are probably LED packs that operate at higher voltages.
Resistors are only used to drop the voltage to the rating of the device. By doing so you limit the current. If the supply voltage is already at or lower than the LED’s rating you do not need a resisitor. The led will only draw it’s rated current at it’s rated voltage. I have an LED flashlight. Open it up and guess what. No resistors. It has 3 double A batteries that feed directly into the LED.
LEDs don’t work if the supply voltage is less than Vfwd. They are current devices. A dry cell can also supply more current than needed to fry the junction.
A 4.5V battery is a lot more than the typical Vfwd of an LED. There is more to your flashlight than meets the eye.
Current is part of the PIE in OHMS law.
If yo have 1000 amps available at 3 volts and you supply a 3 volt 30 miliamp LED you get the same light as if you have 3 volts and 50 miliamps available.
Dropping resistors are used to bring the voltage of a circuit down to divide the supplied voltage with the LED so that the LED has only it’s rated voltage or lower available.
If you have a 3 volt LED and a 12 volt supply, you have to add the resistor required to remove or drop 9 volts. If the voltage is already at the LED’s rated level there is no resistor needed.
Also An LED Will light at a voltage lower than what it is rated at. The Result will simply be a dimmer LED.
This is why we often recommend a 1K resistor instead of the 680 ohm value that works mathmatically.
The LED is a little dimmer but it will last longer since it won’t be running at maximum brightness.
Obviously there are several here who have a basic understanding of the subject but dont understand Ohms law. I can follow up with a couple of pictures if needed of an LED hooked directly to different sized batteries with no resistor and having the same brightness and not burning out.
I can use a pair of AA batteries directly to a 3 volt, 30ma LED with no resistor. The AA batteries have a 2 amp hour rating and can produce over an amp when shorted. (over 1000 miliamp)
I can also go further and show the same LED with 3 batteries in series. Neither of them blowing the LED.
It would be magic for some but no LED’s would be harmed in the test I promise [swg][(-D]
I’m bored tonight so I’ll go ahead and do a show and tell. We can do an ohms law explaination later if anyone is interested. By the way I’m just having fun with it and not at anyone’s expense.
First up is a single LED with a pair of AAA batteries. These babies have a 1200mah (1.2 amp hour) capacity. Surely they’ll blow the 30 miliamp led without current limiting resistors.
Neat Huh. Note the brightness and it’s not smoking or burning up.
Next up the AA battery. Twice the size of the AAA and carries a 2700mah (2.7 amp hour) punch. Close your eyes if you can’t bear to watch.
Wow It survived, and guess what? Same brightness. Applause in the background.
Now we’ll go even further. Whoooooooooooo. A pair of D’s with a 12000 (12 amp hour) capacity. Those dirty minded people thinking Double D’s give it up.
Whooo Ho Ho. The LED is the same brightness and still burning. Amazing.
But Wait. He goes even further. Adds a AA battery in series with the Double D’s for a total of 4.5 volts.
And the crowd goes wild. The led is a little brighter but still survives. and with NOOOOOO resistors.
Note: the led will eventually fail at this voltage.
Randy usually chimes in about now but the fact is NO you don’t need resistors if the supplied voltage is the same or a little lower than the rated voltage of the LED.