Another DC Volt question

Well, I think the comment that it "draws what it needs" refers to amperage, rather than voltage. Applying too high a voltage can still cause damage, like if you were to plug a regular household toaster or vacuum cleaner into a 220v supply. As for an LED, a 3v LED needs to be matched to the appropriate DC supply voltage, or be fed through an appropriate resistor.

B. Smith has it right. A motor will draw only the current (amps) it needs when supplied by the rated voltage. If you connect a 6-volt motor to a 9-volt source, all bets are off.

LED's are current mode devices, unlike that dc motor.  If you connect a 9V source to a 3V LED, it'll smoke the LED in a fraction of a second.

Consider a heavy-duty system like the New York City subways. The cars' motors are specifically designed to operate on 625v DC from the third rail. Increasing this voltage above their design limits would lead to excessive speed of rotation, with reduced electrical efficiency, bearing failure, and failure of the insulation and other components due to excessive heat. Therefore, the motors do not simply "draw what they need" with respect to voltage. Same thing with a 12v DC Pittman in an O-gauge model.

The third rail supplies 625v DC at a maximum current of about 10,000 amp. The subway cars can draw the amperage they "need" to get the train in motion (around 1600 amps at starting) but the voltage potential between rail and ground remains at 625 v DC. The high amperage capacity of the third rail electrical supply allows several trains to be starting, and running, at the same time within a given power district, which is critical in a densely occupied system like a subway with stations very close together. There are limits, of course, and if the motors draw too many amps at 625 v for too long, then the total wattage (and heat generated) exceeds design limitations and things can burn up.

@Mannyrock posted:

But, . . . what'a'bout dc powered led lights?     If I hook a 3 volt led light to a 9 volt battery, will the light burn up?

Most definitely, trying to do its best imitation of an old-fashioned flash bulb!

No offense intended, but I think you need to seek out a basic explanation of the relationship of voltage (measured in volts), resistance (in ohms), and current (in amps). As a visual pneumonic representation of the relationship between them, I like the classic Ohm's Law pyramid:

Cover the term you're trying to calculate ("V" for voltage, for instance), and the pyramid tells you the calculation you need to do ("I" for current times "R" for resistance, in this example).

Spec's differ from LED to LED, but most 3 volt LEDs are intended to draw about 20 mA, or 20/1,000th of an amp. In your example, you need to figure out a resistance large enough to drop the 9 volt supply voltage to 3 volts across the LED (or, stated differently, what resistance will allow a current of 20 mA when exposed to 6 volts, which is the amount by which it must 'drop' the voltage and split it with the LED). Since R is the term sought, cover the R, and the other side of the equation is V/I, or 6 (volts) divided by 0.02 (amps), which yields a value of 300 ohms for the current-limiting resistor. Simple, right? Even easier, just use the DigiKey calculator to do the same thing!

Even easier, for a single 20ma LED, just multiply the number of volts you have to drop by 50 and you have the lowest value resistor to be used.  Obviously, you can use higher values, many times I go much higher to get the brightness I desire from the LED.

Thanks for all of this advice.

Here is some extra info I just read that may change everything.  The Burning Fire Circuit itself says that it runs on 7 to 9 volts, AC or DC.     So, the 7 volts coming out of my motion detector line should not burn out the circuit.   The circuit itself controls 4 LEDs, with each bulb being "max 3 volts".

So, I guess the circuit itself is designed to reduce the power to the bulbs so they don't burn out.

Does this seem correct?

Mannyrock

Mannyrock, I just looked up the Burning Fire Circuit on East Coast Circuits website. It says 9-12 Volts AC or DC Supply Voltage. You can try it with your Seven Volt Power Supply but it may not be enough voltage to power it. With the Circuit Board you will probably not need a Resistor to keep your Current down to 20 ma or less. The Circuit Board should do that for you. Good luck.

The correct spelling is "mnemonic" -- "an aid to memory."  

Thanks to everyone for your advice.

Gary, you are right, it does look like the circuit board itself must cut down the juice for the 3 volt LEDs.  So, I think that if I can get a motion detector that takes 9 to 12 dcv to operate, and then it sends this juice down the line to the circuit board, then things will be OK.

The description says the LEDs must be "max 3 volts."    And, it uses 4 bulbs.

So, I am going to buy a dedicated used dc transformer from an HO set, which I can get for about $10.00 at the flea market, and slowly increase the volts until everything works, and then just leave it at that setting.

Thanks again to all.

Mannyrock