Resistor wattage rating. How important is it?

[Vaderta]

Just curious as too how important wattage rating are on resistors for LED lights DCC? I was trying to get 1/4 watt 2K resistors.  Unfortunately I couldn't find any until I got up to the 1206 size resistor.  The 0603 all I could find was 1/10th watt and the 0805 was 1/8th watt. I am using them for 3mm and 0402 warm white LEDs . Just wondering if I will be ok using these for the purpose stated above? Thanks for any replies.

[mmagliaro]

You probably do not ever need 1/4 watt resistors for an LED dropping resistor.
Here's how you can calculate this.. (just because people ask questions like this all the time, and maybe this will
help pass it on).

There is a little variability in that LED's drop different amounts of voltage, but
it's still easy to get close enough to the ballpark that you don't care.

1. White LEDs tend to drop around 3v, while red/yellow/green drop more like 2v.  It might be a little more or less, but that's close enough.  I'll guess you are using a white LED for this.
2. You need to know your supply voltage.  I'll guess this is going to be something like 12v in your case.   (Even if it is PWM, what we care about here is average, so I'll call it 12v).
So for a white LED, we have 12-3 = 9v that will be dropped across your resistor.

3. Current.  You are using a 2k ohm resistor, so the current can be computed via   V  = I * R, or rewriting it
for current, I = V / R.     V = 9.  R = 2000, so I = 9/2000 =  .0045 amps   (4.5 mA)

And now... drumroll, the basic power equation: P = I x V.   I = .0045 (20 milliamps)   V = 9 volts
So your power dissipated by that resistor is .0045 x 9 = .0405 watts
or... about 40 mW.

So an 1/8 watt (125 mW) resistor is plenty big enough, as is a 1/10th watt (100 mW).

[Vaderta]

You probably do not ever need 1/4 watt resistors for an LED dropping resistor.
Here's how you can calculate this.. (just because people ask questions like this all the time, and maybe this will
help pass it on).

There is a little variability in that LED's drop different amounts of voltage, but
it's still easy to get close enough to the ballpark that you don't care.

1. White LEDs tend to drop around 3v, while red/yellow/green drop more like 2v.  It might be a little more or less, but that's close enough.  I'll guess you are using a white LED for this.
2. You need to know your supply voltage.  I'll guess this is going to be something like 12v in your case.   (Even if it is PWM, what we care about here is average, so I'll call it 12v).
So for a white LED, we have 12-3 = 9v that will be dropped across your resistor.

3. Current.  You are using a 2k ohm resistor, so the current can be computed via   V  = I * R, or rewriting it
for current, I = V / R.     V = 9.  R = 2000, so I = 9/2000 =  .0045 amps   (4.5 mA)

And now... drumroll, the basic power equation: P = I x V.   I = .0045 (20 milliamps)   V = 9 volts
So your power dissipated by that resistor is .0045 x 9 = .0405 watts
or... about 40 mW.

So an 1/8 watt (125 mW) resistor is plenty big enough, as is a 1/10th watt (100 mW).

Thank you so much. That was an excellent description. Now that I know the equation on how to figure that out it will make me more informed going forward. I appreciate your break down. That makes me feel more confident picking the correct resistor for my project.  I decided to try these style resistors as opposed to the barrel style resistor in the hope of saving some room. The resistors I was using especially in N scale using several different auxiliary wires shrink wrapped can get tight quick in an F3A. So hoping to be able to use these with the DC boards and maybe save a little space. Thanks again for your help. Much appreciated.
                Brian

[peteski]

Yes, making your own resistors by adding wire leads to SMD resistors does save space.  Here is an example of a 1206 resistor with decoder wire with 30 AWG wire leads.



Of course  even more compact resistors can be made by using enameled magnet wire and smaller SMD packages (like 0805, 0603).  INstead of heat shrink you can coat it with some liquid electrical tape.

Do remember that the resistor does have to dissipate the power as heat, so it can get warm.  That is why it is important not to exceed the resistor's power rating.  Doing that doesn't necessarily mean the resistor will get damaged, but it can get hot (since the power it is dissipating will be concentrated on its small size package).  As Max showed, it is safer to use a resistor rated higher than the estimated power it will be dissipating.  Also keep in mind that  (especially with SMD resistors) the rated power is for when they are soldered directly to copper pads on a PC  board.  The copper pads and the PC board itself act as heat sink for the resistor.  In our specific application (used as current limiting resistors in very low current LED circuit) we don't have to worry too much about the power dissipation, but when I use free-standing SMD resistors I like to use ones rated at 200% of the calculated power dissipation.  Again, in Max's write-up, that just happened to work out to what I do.

I often make (etch) small PC boards to hold the resistors and LEDs in my models.

[Vaderta]

Yes, making your own resistors by adding wire leads to SMD resistors does save space.  Here is an example of a 1206 resistor with decoder wire with 30 AWG wire leads.



Of course  even more compact resistors can be made by using enameled magnet wire and smaller SMD packages (like 0805, 0603).  INstead of heat shrink you can coat it with some liquid electrical tape.

Do remember that the resistor does have to dissipate the power as heat, so it can get warm.  That is why it is important not to exceed the resistor's power rating.  Doing that doesn't necessarily mean the resistor will get damaged, but it can get hot (since the power it is dissipating will be concentrated on its small size package).  As Max showed, it is safer to use a resistor rated higher than the estimated power it will be dissipating.  Also keep in mind that  (especially with SMD resistors) the rated power is for when they are soldered directly to copper pads on a PC  board.  The copper pads and the PC board itself act as heat sink for the resistor.  In our specific application (used as current limiting resistors in very low current LED circuit) we don't have to worry too much about the power dissipation, but when I use free-standing SMD resistors I like to use ones rated at 200% of the calculated power dissipation.  Again, in Max's write-up, that just happened to work out to what I do.

I often make (etch) small PC boards to hold the resistors and LEDs in my models.

Thank you Pete. That is pretty impressive that you make your own boards. 

I appreciate the tip on heat dissipation. My F7 and F3 installs I have not been milling the back of the frame.  Instead I have been fitting the speaker long way for clearance on the windows and using either a micro or Nano. Needless to say space for wiring gets tight. When I splice into the blue wire and then adding a large barrel resistor with heat shrink in the headlight lead it starts getting tight. So if I can just cut the circuit in the board add one of 0603 resistor and solder the blue wire on the board that should clean up some space. I am getting the Kato Milwaukee Road F7A and B set this week. So I will get to try it out and see if things fit in place better. Appreciate your insight as always

[mmagliaro]

What size barrel resistors were you trying to use?   1/4 watt?  Yes, those would be pretty huge to try to fit up near the headlight.
But there are 1/8w conventional barrel resistors, and while they aren't as small as SMD, they are a LOT smaller than 1/4w, so
they might work for you if you have the room. 

[Sumner]

I use these ....



.... flexible PCB strips a lot where room is tight as they are very thin....



..... great for lights and....



... other circuits and ...



... as solder pads.

Only problem if you are in the states or not Europe it is hard to get them shipped to you.  I had a friend over there order and ship to me and then found them in England for a while but now not there either.  But if you can get them they are very handy to use.

More on their use here and possible sources for them.....

https://1fatgmc.com/RailRoad/Decoder-1/page-24.html

Sumner

[Vaderta]

What size barrel resistors were you trying to use?   1/4 watt?  Yes, those would be pretty huge to try to fit up near the headlight.
But there are 1/8w conventional barrel resistors, and while they aren't as small as SMD, they are a LOT smaller than 1/4w, so
they might work for you if you have the room.

Yes, I was using the 1/4 watt resistors.  I will have to get some of the smaller wattage ones to add to my arsenal.  Thanks for the input

[Vaderta]

I use these ....



.... flexible PCB strips a lot where room is tight as they are very thin....



..... great for lights and....



... other circuits and ...



... as solder pads.

Only problem if you are in the states or not Europe it is hard to get them shipped to you.  I had a friend over there order and ship to me and then found them in England for a while but now not there either.  But if you can get them they are very handy to use.

More on their use here and possible sources for them.....

https://1fatgmc.com/RailRoad/Decoder-1/page-24.html

Sumner

Thanks for the tip. These could definitely come in handy with N scale. I will have to try to find some to have on hand. I have just been using scraps up until now in these situations.

[peteski]

Here are couple of resistor's physical size comparisons for future reference.





My advantage in this hobby is that I have been involved with electronics as a hobby and on a professional level.  I have, over several decades accumulated all the standard value resistors (1 Ohm to 10 Mega Ohms) in 0.5, 0.25, 0125 W, and SMDs in 1206, 0805, and 0603 sizes.

[Vaderta]

Thanks for the pictures Pete. Definitely helps me. I appreciate you sharing your knowledge that you have accumulated throughout the years. For me being relatively new to N scale I have some things to learn. I look at the 1/4 watt resistor and thought that is pretty small. Then I try to squeeze 3 of them in a small space and they don't seem so small anymore.  The 1/8th watt barrel size resistor will definitely be more manageable. Appreciate everyone's help.