Best viewed using:
Application example for the 50-volt 0.1μf capacitor
A typical application where this capacitor is useful is to absorb short-duration voltage spikes that can be emitted by a DC motor during certain momentary power-loss conditions.
In model railroading this can occur with a model locomotive motor operating in a non-DCC environment that momentarily looses power due to a dirty or gapped track. If LED lighting is being directly powered from the the same track pickups that the motor uses, the LEDs have a very fast response (unlike filament-type bulbs that have to heat up an element) and can react to these short voltage spikes by producing an unwanted flicker of light. This can be especially undesirable in situations where the ambient light level is low such as exiting a tunnel or with generally low room lighting.
Figure 1 shows a non-DCC circuit for forward and reverse lighting using our N1022C high-intensity Incandescent color LEDs. The resistor in the circuit is chosen to provide the correct voltage drop and current protection for the LEDs.
For example, with a maximum track voltage of 12-volts we would calculate the resistor to be 440-ohms and need to dissipate 170 milliwatts of power. Our NB4530 (453-ohm tolerance 1/4 watt) will work perfectly for this application. Since this is a 1% tolerance resistor, its worst case resistance value will be at least 448.5-ohms, which is still above the calculated requirement. Also, since it can dissipate 1/4 watt (250 milliwatts) it will be operating at only 68% capacity, so it will get slightly warm but not at all hot.
Now, LEDs are after all diodes, just the kind that emit light. Being diodes, they have a reverse breakdown voltage limit. That is, they will block the flow of voltage up to a certain limit. All of our LEDs have a reverse breakdown voltage of 5-volts DC. If an LED's reverse voltage limit is exceeded, the LED will break down (and likely be destroyed). With our circuit example above, a model locomotive's engineer's side track pickup motor wire is assumed to be the red (+DC) wire. The fireman's side is the black wire and is assumed to be -DC. Since we have a resistor in the circuit that at a maximum track voltage of 12-volts, will only allow 3.2 volts (and 20ma) to flow through either LED, when the loco runs forward, the headlight LED will light and the rear light LED will block that flow (because it is below the 5-volts reverse voltage limit). Since LEDs are polarity sensitive, when track polarity is reversed (loco going backwards) the rear light LED will be on and the headlight LED will block voltage flow (and be off).
The problem with this circuit is that the loco's motor will also be connected across the track pickups and during those intermittent moments that track power is lost (dirty track or gaps in the track), the spinning motor acts a bit like a tiny generator and actually creates little pulses and voltage spikes in out lighting circuit. This "generator" effect is know as back-EMF (back-electromotive force). Since LEDs are very fast-acting light emitting devices, they can respond to these small spikes of voltage. Placing one (or two, in parallel) of these 0.1μf capacitors in the circuit as shown above will absorb these spikes and prevent the problem.
Note: This type of light flicker is not to be confused with the flicker that occurs with the momentary loss of power to passenger car or caboose lighting as that rolling stock crosses dirty track or gaps. That is caused by tiny power interruptions and can usually be fixed by adding sufficient capacitance 600-1000μf to their lighting circuits. These much larger capacitor(s) act as little "storage batteries" and drain into the circuit to eliminate or minimize flicker. For those situations, please refer to our N3068 or N3100 capacitors on the Lighting Accessories Page.
© 2008 Ngineering