I am getting into electronics so I can build circuits for various model railroad projects. My background is mechanical engineering, so please bear with me if my questions seem simplistic.

I bought a breadboard and an assortment of 3mm and 5mm LEDs so I can experiment with some battery powered circuits. An LED needs a resistor to limit the amount of current flowing through it. Assuming I will be stringing about 4 LEDs in series, what values of 1/4 watt metal foil resistors should I buy in an assortment package? My thoughts are an assortment of 200 to 976 Ohms with a 1% tolerance.
Original Post

It depends on the voltage you will use in the circuit, the LED specs, how many LEDS in the circuit and whether they will be series or parallel wired. Here is a site with a calculator that will help LED Calculator
Ron
Forget about the 1% resistors, that's overkill. LED's aren't that critical. I'd just buy the 5% steps from 220 through about 1500 ohms.
For experimenting, use the series
100
150
220
270
330
390
470
560
680
820
1000
etc.

These values will get you close enough for most work. Either 1/4 watt carbon film or metal film. The tolerance of the resistors can be 5%, 2% or 1%, depending upon your source and pricing. It is getting harder to find small bulk quantities at reasonable prices. The distributors want to sell you a reel of 10,000 pieces!

There are some nice protoboards with 3 built-in power supplies for about \$120 like this.
Stock some Cl2-N3 chips then there is no need to guess at resistor values.

For resistors,stock up to 1200 ohm for LEDs. Stock some 100 ohm,240 ohm 510 or 560 ohms,330 ohm,1k ohm. Combine values to make other resistor values if you do not have the right one. 2, 510 ohm in parallel equals 255 ohm. 2, 510 in series equals 1020 ohms. Always go equal to or over the value needed in the calculation for LEDs. If a capacitor is used on pulsed current add 1.4 times the input voltage. A little over is not critical. Use larger values to dim LEDs. Also sometimes a 1/2 watt resistor is called for so 2, equal 1/4 watt resistors in series gives you a 1/2 watt one for example at 2 times ohm value.

The LEDs I buy come with 510 ohm resistors.

Dale H
quote:
Also sometimes a 1/2 watt resistor is called for so 2, equal 1/4 watt resistors in series gives you a 1/2 watt one for example at 2 times ohm value.

You lost me on this one. It has been a long time since I studied ohms law back in school so bear with me on this as my brain doesn't always function in the right direction. I always thought that regardless of how many resistors were in series they were still only rated for what their rated value was, as 1/4 watt rated resistor stayed 1/4 watt rating. If you put 2 1/4 watt reesistors of the same value in parrallel wouldn't each carry have the load and give you 1/2 watt protection with half the resistance of the reated value of the resistors. Can you show the formula to calculate this.

Forest
If you parallel two 100 ohm 1/4W resistors, you get a 50 ohm 1/2W equivalent resistor. Resistors in series add in resistance and the power dissipation doesn't change, if they're 1/4W, they're still 1/4W.
quote:
For experimenting, use the series
100
150
220
270
330
390
470
560
680
820
1000

Radio Shack may still stock a blister pack of assorted 1/4 watt resistors in these values and 10x, 100x, 1000x these values as well.
There were 5 or 10 of each size depending on popularity of each.
I don't recall the RS # or the price, but it was quite reasonable.

Rod
quote:
Originally posted by gunrunnerjohn:
If you parallel two 100 ohm 1/4W resistors, you get a 50 ohm 1/2W equivalent resistor. Resistors in series add in resistance and the power dissipation doesn't change, if they're 1/4W, they're still 1/4W.

The individual resistors are still 1/4 watt but the voltage is divided in half,thus using Ohms law each resistor uses half the power.

Dale H
quote:
Originally posted by Dale H:
quote:
Originally posted by gunrunnerjohn:
If you parallel two 100 ohm 1/4W resistors, you get a 50 ohm 1/2W equivalent resistor. Resistors in series add in resistance and the power dissipation doesn't change, if they're 1/4W, they're still 1/4W.

The individual resistors are still 1/4 watt but the voltage is divided in half,thus using Ohms law each resistor uses half the power.

Dale H
That's what the word "equivalent" indicates.
Kind of splitting hairs here with semantics. I meant you could substitute 2, 1/4 watt in series if you did not have the right value in place of a 1/2 watt resistor.. 2, double the ohm value in parallel or 2 half the ohm value in series. Either way gives you double the wattage rating in the circuit or is safe to use. Equivalent means substitution with the same results in the circuit.

Dale H
Thanks all of you experts for your expertise. I am learning about electronic / electrical things from you all.

I bought an assortment of 0.25 watt carbon film resistors with 5% tolerance in the range of 10 to 1K ohms on the bay last night, so I can learn by doing on a breadboard and then take what works to my layout.

My layout needs lighting, and it annoys me that I have a mix of things with incandescent bulbs and LEDs that have differing power needs. My goal is to have a common 10 to 12 volts to all of the lighting circuits on my layout. To minimize the number of lighting transformers, I intend to use LEDs to the maximum extent possible.
quote:
Originally posted by Dale H:
Kind of splitting hairs here with semantics. I meant you could substitute 2, 1/4 watt in series if you did not have the right value in place of a 1/2 watt resistor.. 2, double the ohm value in parallel or 2 half the ohm value in series. Either way gives you double the wattage rating in the circuit or is safe to use. Equivalent means substitution with the same results in the circuit.

Dale H
True, you would spread the power over the series resistors as well, point to Dale.
quote:
Originally posted by Bobby Ogage:
Thanks all of you experts for your expertise. I am learning about electronic / electrical things from you all.

I bought an assortment of 0.25 watt carbon film resistors with 5% tolerance in the range of 10 to 1K ohms on the bay last night, so I can learn by doing on a breadboard and then take what works to my layout.

My layout needs lighting, and it annoys me that I have a mix of things with incandescent bulbs and LEDs that have differing power needs. My goal is to have a common 10 to 12 volts to all of the lighting circuits on my layout. To minimize the number of lighting transformers, I intend to use LEDs to the maximum extent possible.

Bob

What you may want to do is dedicate a DC circuit just for LEDs. Use 1 bridge rectifier 10 amp or more from any AC tap,then color code 2 wires + and -. Or you can even share a common with the AC circuits feeding bulbs. Just hook one of the leads from the bridge,such as the minus to the common bus. This would require a separate transformer for the LED circuit if you do this. Then you do not need a rectifier on each LED circuit. This would simplify your wiring a bit. Putting a 2200 uf condenser across the + and - of the bridge in proper polarity would give you filtered DC at 1.4 times the AC voltage output. That way you could get a few more LEDs in series per building.

Dale H
quote:
Originally posted by Dale Manquen:
For experimenting, use the series
100
150
220
270
330
390
470
560
680
820
1000
etc.

These values will get you close enough for most work. Either 1/4 watt carbon film or metal film. The tolerance of the resistors can be 5%, 2% or 1%, depending upon your source and pricing. It is getting harder to find small bulk quantities at reasonable prices. The distributors want to sell you a reel of 10,000 pieces!

There are some nice protoboards with 3 built-in power supplies for about \$120 like this.

Thanks for the info. I really like the links fun with LED
TT -

Thanks for the excellent tutorials regarding wiring LEDs. The graphics and photos made it even more informative. Your clear explanations were right on target. Thanks again!

Dale -

Also appreciate your continuing input, and sharing your wealth of knowledge about electronics. Your hand written note book sheets are great. They remind me of my years as an electronics model maker, when the engineers would come back from lunch with diagrams, schematics, or hardware designs sketched out on the back of napkins with the instructions "here, build this".

Mike
OGR Publishing, Inc., 1310 Eastside Centre Ct, Suite 6, Mountain Home, AR 72653
330-757-3020

www.ogaugerr.com
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