For LEDs, I see no reason to use anything besides a resistor with the LED. I do like using a simple constant lighting circuit for 1.5V mini-bulbs though. Instead of bridge rectifiers, I use half a dozen 1-amp diodes, and I wire it all like this:
There's a 2.1V drop, so the motor only gets 9.9 out of 12V, but it's no big deal as long as the whole thing can still run at realistic speeds. The results have been very good for me!
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On a BB Athearn motor, putting the diodes between the top clip and the frame results in a direct short around the motor limited only by the voltage drop of the diodes. To do what you are showing on the schematic you would put the diodes in series with the motor, say between the truck blades and the motor, so that the power to the motor runs through the diodes.
PennStation i had temporarily wired the PC board to the copper clip on top of the Athearn motor..and to a screw installed on the chassis. Previously hard wired this Athearn...if you're familiar with them you know what I mean. It didnt stop it from running but it definitely reduced the power...so, assume i had something wrong..I'll redo all of it just to be sure...and report. If wired accurately your saying the power drop should be minor? And I also assume no overheated bridge rectifiers?. Paul @ Penn Station
i had temporarily wired the PC board to the copper clip on top of the Athearn motor..and to a screw installed on the chassis. Previously hard wired this Athearn...if you're familiar with them you know what I mean. It didnt stop it from running but it definitely reduced the power...so, assume i had something wrong..I'll redo all of it just to be sure...and report. If wired accurately your saying the power drop should be minor? And I also assume no overheated bridge rectifiers?.
Paul @ Penn Station
Just in case anyone is interested this is a good link covering the circuit:
http://www.trainelectronics.com/LED_Articles_2007/LED_104/index.htm
Well, I think I've beaten this subject to death....but if you have a better way..sure like to hear about it...
Cheers
This circuit will work fine, EXCEPT... those two diodes that are in series with the LEDs really should be resistors. The two bridge rectifiers will get you 2.8 volts across the LED. But as others have already pointed out, LEDs are not to be thought of as "voltage" driven devices like light bulbs. They are current-controlled devices.
With no resistance in series with the LEDs, the only thing that is keeping them from smoking is that the rectifiers are clamping the maximum voltage across them to 2.8v, and while many LEDs can tolerate a forward voltage that high, it's not the way to control current through an LED.
Let's assume you want to limit the current through the LED to no more than 20 mA. If the LEDs themselves have a forward drop of around 2v, then we have .8/.020 = 40 ohms. But LED forward voltage drops are quite different, anywhere from 1.7 up to something like 3.7 for the white ones, so you should experiment. Somewhere between 10 and 56 ohm will get you a brightness you like and will protect the LEDs.
The other thing I don't like about this circuit for LED use is that the motor current is being pulled through the rectifers AND through the LED path. While the rectifiers end up carrying most of the current, there is no real guarantee that this will happen, especially if there is no resistance in series with the LEDs. If you put a resistor in there, then you really do limit the current through the LEDs and all the rest of it HAS to flow through the rectifiers.
To folks who said to just put a resistor in series with an LED and be done with it... that is not as effective as a constant lighting circuit. It "sort of" works because LEDS put out a decent brightness over a wide range of current, but they still do glow dimmer and brighter as you turn up the throttle when you use a simple resistor scheme. The circuit in this thread actually makes sure that a very rigid, fixed current flows through the LEDs at all times, so their behavior is much more constant. Giving up 2.8 volts to the motor is no big deal. Engines run way too fast on a full 12 volts anyway. There is no harm at all in having the engine not start to move until you get up to 5 volts. You'll just be running the engine between 5 and 12 (slow to fast), instead of 0 to 7.
So... yes, this is an "old school" circuit, but if you've got the room for the rectifiers, and you wire it right so they don't get hot (which it sounds like you've figured out), this will work perfectly... I would put some resistance in series with those LEDs, though.