Diodes for LEDs

Consolidated Leo posted:

Those are certainly diodes and 1000 volts is certainly overkill. Can you explain how you want to use them in a circuit?

Go from the LED (whichever type needed for project listed above) to 14v accessory line. To give them DC power instead of the AC I'm running them off of now. 

Matt 

An AC to DC buck converter is probably your best bet. They come as modules with various output voltages including variable (settable). If you have different voltage requirements, you could hook up a number of these around the layout all attached to the same 14 volt AC line from your transformer.

Here's a sample:

 

Or you could use a bridge rectifier plus smoothing capacitor:

Image result for ac to dc converter circuit

Also available are fairly inexpensive wall-wart products:

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Consolidated Leo posted:

An AC to DC buck converter is probably your best bet. They come as modules with various output voltages including variable (settable). If you have different voltage requirements, you could hook up a number of these around the layout all attached to the same 14 volt AC line from your transformer.

Here's a sample:

 

Or you could use a bridge rectifier plus smoothing capacitor:

AC to DC Converter Circuit Schematics Diagram

Also available are fairly inexpensive wall-wart products:

Alright. Thanks. 

Matt 

  Ultimate protection maybe a tvs and or isolatiing chip ciruit driving them. Few go that deep for led.

  That big a diode is better for motor mods or big tranformer whistle/bell, stand alone whst./bell, etc.  Maybe some in the In4007s (? Check me) could handle two I'd bet. Extras more versatile really.

The rectification is more about steadyness and flicker from the 60hz timing of the deep voltage drops vs a steadier voltage.   Voltage is just a warning about a maximum voltage. Turn on is usually lower, not usually too much brightness variance from turn on to burn out voltage, but that can vary.

   Brightness reduction with resistance would ad service hrs and is often a better way.  

But back to flicker and rectification. A LED is a diode .  So, if you add near any normal diode (or close matching led) in the opposing direction to an LED, you can provide the missing wave to the correct LED input and have full rectification with less diodes/led total e.g. you can use 2, light 2....or...one bridge rectifier + one LED = 5 diodes total, just one lit.

So a Bridge rectifier (or converter) suitable as a big universal supply to many led, but there are other more compact and simple ways too. 

"Still trying to not shoot my eye out"

 

"Nursing insomnia one railcar at a time"

My aroma therapy? Smoke Pellets.

 





most asc techs just install a 220 ohm resisters in series with the led and works just fine and 1/8 watt resisters are very cheap!

you can experiment with higher and lower resister values according to how bright you want your leds!

why complicate things make it simple . buck converters works great also as stated above too! 

All you need is the diode if you aren't sensitive to the 60hz flicker that can happen.  If that bothers you, you could consider a capacitor, though for a 5 meter strip, it would be a whopping big capacitor.

gunrunnerjohn posted:

All you need is the diode if you aren't sensitive to the 60hz flicker that can happen.  If that bothers you, you could consider a capacitor, though for a 5 meter strip, it would be a whopping big capacitor.

Cool thanks. The flickering doesn't bother me. So I can use a capacitor to give me flicker free LEDs in my passenger cars? Just want to be sure. 

Matt 

One thing to keep in mind - if you use a single diode as @Consolidated Leo posted above, you can still use you outside rail as ground. If you use a rectifier bridge or some other device (refer to a second post by @Consolidated Leo), please take careful note that the rectifier bridge or other device generates its own ground that you must use, and can not use the outside rail. 

As an old rule of thumb I was taught many moons ago, for a full wave rectifier, you use 1000uf per 1amp being supplied. For half wave, I would double. For 18VAC type layouts, a full wave rectifier would yield about 25VDC, so I suggest the cap should have a working voltage rating of 50VDC or higher. Note that these thousand uf caps are polarized, the case these days normally has a segmented bar/arrow with a minus sign in it pointing to the negative terminal of the cap.

BTW - I know this if TMI, but - to get an idea of what VDC a full wave rectifier will create from VAC, you multiply the VAC by 1.4 (officially, use the square root of 2, but 1.4 is close enough for government work). This is due to the fact that AC is spec'd not as a peak to peak value, but as an energy equivilant to DC. So for example, you house has 120VAC, but the peaks are + and - 168v, and a full wave rectifier would yield about 168VDC.

I found an online calculator to determine the value of a series resistor to limit the current through a LED. This calculator is for a single resistor in series with a single resistor. You simple enter the DC supply voltage, the forward diode drop (2 to 5 volts - I normally use the 2 value), and the LED max forward current, which is normally 20ma. My thanks to @gunrunnerjohn for catching my decimal point mistake, which I have corrected above.

https://www.digikey.com/en/res...-led-series-resistor

 

50V is a bit of overkill, the 35V rated caps are fine for 18V rectified power.  No reason to make the caps physically larger than needed.

The voltage drop across an LED is way more than .2 to .5 volts.  For common LED's, the IR and red end of the spectrum starts around 2 volts and they work up to 3 or a bit more for white and UV.

FWIW, my "rule of thumb" for LED's is simple, I use 50 ohms for every volt I want to drop, that gives me 20ma of current in the LED.  Another point is frequently it's beneficial to run the LED at less than rated current, sometimes even a lot less than rated current.  You can use any value per volt more than 50 ohms, just don't go below it for common LED's.

gunrunnerjohn posted:

Yep, for passenger cars with an 18" segment, a 330uf or 470uf cap does the job. 

There seem to be several conversations going on.  I believe you are referring to the situation with a (a) 4-diode-bridge and (b) some type of regulator between the capacitor and the LED strip. 

(a) if you only have a 1-diode AC-to-DC converter, you only charge the capacitor 1/2 as often and hence you'd need a larger capacitor for the same effect

(b) the regulator meters out the stored energy in the capacitor for times when there is no incoming power - the benefit being constant or "regulated" brightness.  Without the regulator, you demote the effectiveness of the capacitor as an energy storage device.

c = it/v where i is the discharge current, t is the charge interval (17 for half wave), and v is the allowed voltage droop on the capacitor.

milliamps, milliseconds, volts, and microfarads are the units

Chuck TCA LCCA ARRL BMWMOA

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