Constant Lighting & SCR Throttle


J

JimmyG

Member
Here's one for you electronics wizards. :D Many years ago I built an SCR momentum throttle from "Electronics Projects For Model Railroaders." The throttle works great and I use it all the time (AC input, pulsed DC output). I recently built the bright white LED headlight circuit from April 200 MR. When I tried it the LED blew out immediately. #1) Could this be from the pulse throttle? :confused: #2) Is my pulse throttle hard on electric motors? It is filtered with a large capacitor. (As you can tell, I'm not an electronics wizard.)
 
If I understand the design you have correctly, it is essentially a PWM (pulse width modulated) output which means that it doesn't actually "regulate" the voltage level, it pulses full power very quickly and the motor "averages out" the voltage to go a specified speed. For example, if you were to set 30% throttle, the SCR will pulse full voltage for 30% of the time and zero volts for 70% of the time. The motor response is slow enough that it will average the voltage out and go slow. Increase the speed and the full voltage pulse gets wider, putting out more "average" voltage.

That being said, it is surprising that an LED would blow immediately assuming the LED is properly current-limited by a resistor. If not, then for sure it would blow - an LED is a lot quicker than a motor and is unable to average the voltage out. Plus it can only take a small amount of current (generally 20mA or so). If you were to install at least a 500 ohm resistor (prefer 1000 ohm), it would limit the current to the LED and prevent the blow out. The LED would run somewhat dimmer though. The only other way would be to install a 3.3 volt regulator between the LED and the power - that would stabilize the power and reduce the voltage to the LED.

Mark
 
The circuit I used included a 3.3v voltage regulator. Perhaps I did not assemble it correctly. You are correct about the pulse width modulation in the throttle. The controls in the throttle clips the AC pulses at different points depending on the settings. It then holds the voltage at a given level for some time. I am able to accelerate or decelerate the loco using a spring-loaded SPDT switch. It works great for moving locos very slowly, including balky locos. I still want to use bright light LEDs. Q) Can I do this without the voltage regulator circuit, using just resistors? Even a little on the dim side would be great. LEDs last forever and the brightness remains constant with just a little voltage. Would it need to be wired in parallel with the motor?
 
The PWM setup is a great one especially for older motors that don't like to get going off the line very cleanly. In fact, a number of DCC decoders actually have something similar to this as a selectable feature - on my NCE decoders it is called "voltage kick". It basically goes PWM at low voltages then evens out to "pure" DC at higher speeds. The cool thing is they are configurable for the frequency and voltage level (but I digress :) ).

LEDs have a max current rating - generally speaking it is around 20mA for most small LEDs. As voltage is increased to the LED (and it gets brighter), the current draw also increases. It gets to a point however where the current draw goes up sharply - for a white LED that is usually at around 3.1 to 3.4 volts. The 3.3v regulator (one piece TO-92, like a LP2950CZ-3.3) is ideal because it sets the ideal voltage for the LED plus the input only needs to be over about 4 volts or so for full regulation.

To answer your question, a resistor should do it but the brightness would likely fluctuate with throttle speed and it might flicker too due to the PWM throttle. You might be able to stabilize it by adding a capacitor to the LED, protected behind a blocker diode. In other words, use a 1N4007 style diode in series with anode on the power feed and cathode to + a cap (- goes to ground), then on to the resistor/LED combo. That way the cap should stabilize the LED and the diode will prevent it from affecting the PWM throttle.

Darn, after reading that I hope I'm helping you not confusing you more. :D

Mark
 
Last edited by a moderator:
I sort of understand your answer, but not being a real electronics person, I don't understand all of it. Any chance I could get you to draw the schematic? Also, could the diode be a bridge rectifier so that the LED would always get voltage regardless of polarity (i.e., it would light up regardless of loco direction)? The diode(s) would also draw a little current too, right? Please pardon my ignorance on this subject.
 
You must log in or register to reply here.



Back
Top