Capacitors/lighting

[Anonymous]

Hi all, I am in need of some help. Does anyone know what size of a capacitor I would use on a 5000 mcd LED. I will be using this on a modified caboose w/a snowplow attached to the front of it Aristo snowplow). Do I still need a resistor also, and is there a certain way this/these must be soldered.............thanx.

ED

[Tom The Brat]

Um. What do you expect a capacitor to do for you?

[cacole]

Any capacitor with a voltage rating at least double the highest applied track voltage. If you're going to use an electrolytic capacitor, be very careful of polarity.

Since you're wanting a capacitor for an LED circuit, you obviously already have a bridge rectifier circuit to insure that the LED gets only the corrrect polarity regardless of the direction of travel. Depending on the available space, All Electronics has a 4700 mfd 100 Volt electrolytic that would be a good choice. Catalog Number EC-4710.

I put a VCR clock backup capacitor into an HO scale caboose and the two surface-mount LEDs on the rear stay lit for about 10 minutes after it is removed from the track or the power is turned off.

http://www.allelectronics.com

[grandpopswalt]

What Cacole says is right on. I'd add that the rule of thumb for sizing a filter capacitor
(electolytic) is about 15,000 microfarads per amp. So if the only load on the rectifier is one LED rated at 30 ma, then a 100volt/500mfd electrolytic cap would work just fine.

Walt

[Greg Elmassian]

Is this a capacitor to power the led, or to filter a dc supply or what?

You did not state track power or not.

Curious.

Greg

[piercedan]

Since the track voltage is only 24 to 28 volts, a 35 volt capacitor will suffice. You can go bigger, but it will not make a difference.

For an LED, 100 microfarad will work. Do not forget the diode bridge or if a directional light, the diode across the LED so the back emf from the motor does not light the led. This is where a bigger capacitor can help.

[Capt Bob Johnson]

You guys are talking Greek to the electronically challanged!

I don't know the difference between a capacitor, resistor, diode, transistor, or what have you; let alone what each is for!

Need somebody to point me to some sort of beginners book that will explain those things or you guys have to tell me!

[Tom The Brat]

Careful! Most electrolytic caps are polarity sensitive! Makes a big pop and brown fuz[:-,]

[grandpopswalt]

The capacitor's voltage rating should be at least twice the highest supply voltage. So if your track power can reach 28 volts, then the cap needs to be rated at least 56 working volts. Since there is no such rating, I'd advise using a 100 volt cap.

Walt

[Greg Elmassian]

Tom, your post reminds me of something funny I saw years ago when I worked for Hewlett Packard.

We sold digital plotters internationally, and in those days, you still had to have a 110v/220v switch on the power supply. HP was concerned what would happen if it was set to 110v and plugged into 220v.

The test engineer had the cover off and was peering at the power supply when he plugged it in. Bang! Not only was the brown fuzz all over his face, but the capacitor (radial, i.e. tubular with both leads at one end) blew the rubber plug out of the end and hit him right in the middle of the forehead. He had a perfectly centered red spot on his forehead!

The next morning, someone had brought a motorcycle helmet and left it on his bench!

Regards, Greg

[Anonymous]

Thanx all for the wisdom! I will be using track power to run the LED. I am not sure if I still need a resistor on the circuit. The cap is just being used for temporary loss of track power. Can someone tell me how to hook everything up? Thanx again!!!!!!!! This is going in a snowplow.

P.S. I could really use all your know how relayed in laymans jargon............

ED

[Greg Elmassian]

You know, you have several options.

(we are assuming you are running DC on the rails, not DCC)

1. Run track power to a full wave bridge, then to your lamp (which if it has a led, then it already has a resistor, it might have a resistor if incandescent).

Now add a capacitor, only needs to be a few volts over track voltage, not double. If you run 24 volts max, then get at least 30v. Higher is ok, harms nothing.. Now the problem becomes how long do you want it to light without track power. My recommendation is just to eliminate flickering out completely.
You will have to experiment a bit but I think 4,000 mfd would be fine.

Remember that it will still dim a bit when you hit a dead spot, since you switch from track power to the cap. You might be tempted to get a bigger cap, but then you will probably have to complicate your circuit by putting a resistor in to limit the inrush current when "charging" the cap, but have to also add a diode so you don't have the resistor in the circuit to the led/lamp.

I'd be tempted to put a 5 volt regulator after the full wave bridge, and then a big cap on the output and whatever appropopriate dropping resistor for your led/lamp (since you would now be running from 5v all the time, not 24 v max)

This way, you would have a constant brightness lamp (as long as track voltage was over about 7 volts most of the time) that would never vary in brightness, instead of changing with the throttle.

I realize this seems more complex, but Radio Shack sells the 5v regulator chips cheap, and there is very little extra circuitry.

Regards, Greg

[grandpopswalt]

One more word on capacitor working voltage. A lot of electronic application textbooks recommend sizing capacitors such that the applied voltage is 50 to 70% of the of the capacitor's voltage rating. There’s a good reason for that. For example, if the maximum track power arriving at your snow plow is nominally rated at 24 volts DC, there could also be an AC component to that voltage which can be significantly greater than 24 volts, particularly if the power supply is an unfiltered ½ wave type. Also, there are always voltage disturbances, which can generate electrical “noise”. This noise is very high frequency but can have very high peak amplitudes. High voltage spikes can and do puncture the dielectric (the insulator between the capacitor’s plates) and can, over time, cause the capacitor to start leaking and eventually to short out.

Walt

[Anonymous]

Thanx again, will keep everything in mind when putting this together!!!!!!!