small resistors?

[tstage]

Hi Tom,

10+ years ago I was an electronics technician for about 4 years with a medical device company.  Part of my job was making changes in prototype PCBs for the electrical engineers - e.g. cutting traces, unsoldering old/solder on new components (i.e. resistors, capacitors, ICs, diodes, crystals, etc), and skywiring 30 AWG and smaller pickup wire to said components.

I regularly dealt with 0402 and 0603 resistors and capacitors.  (I even successfully soldered on a few 0201 resistors - No kidding.)  While I have good hand skills, what was a helpful and an absolute necessity was a VERY good quality 7x-50x zoomable stereo microscope.  Otherwise, there would have been NO way to do those things - especially hand-soldering 128 & 256-pin ICs.

Soldering 34 AWG magnet wire onto 0603 SMD LEDs is doable with a pair of 2.5X Optivisors, pointed locking tweezers, and a metal weight heavy enough to hold down the back end of the tweezers so that it doesn't move when I'm soldering the wire onto the LED contacts.  That way both hands are free for holding the soldering iron and solder in each hand.

Tom

[IC_Tom]

Having manually soldered SMD PCBs for over 10 years, I'm shocked (no pun intended) at the encouragements for soldering wires to 603 and even smaller SMD sizes.

If I had a choice in the matter, I always required 1206 resistor sizes or 805 minimum, if absolutely needed.  I could get away with handling 805 capacitors because they're typically a lot thicker.  805 metal film resistors air-wired without a PCB would scare me to death.

Tstage - I give much respect to your abilities if you can handle air-wiring 603 and even seriously consider 402. Wow!

[tstage]

As mentioned, skip the PCB and just solder directly to the SMD resistor.  And insulate the SMD so that the contacts don't inadvertantly short against anything.  The Testors Clear Parts Cement that I recommended earlier works really well for that.

If space is NOT a premium, I would just use a conventional banded resistor "somewhere down the line".  You can find ones that are small enough to fit just about anywhere.  Just use the same precautionary measures to ensure the leads don't short against anything.

Tom

[Overmod]

As Tom pointed out, you can solder fine wire to the pads on a SMD device.  But remember these were intended to have conduction cooling through the board, so enclose them 'accordingly'.

My recommendation for soldering these is to use solder balls or paste, and just use a 'reflow' or heat gun as you would for SMDs or BGAs on a board.  Life's too short to wangle an iron on the connection between #34 and the pad on a small SMD...

[hon30critter]

ChrisVA

if I wanted to use one of those, it sounds like I would have to use a small piece of PCB with 2 small contacts and use the resistor to bridge the connection?

Hi Chris,

You don't need a PC board. That would just take up space. You can solder the wires directly to the SMD resistors. That can be tricky to do without the right equipment.

You will need a good quality pencil tip soldering iron, solder designed for electronics, 30 gauge wire in two colours, and something to hold the resistors.

I use an XYtronic iron with adjustable heat settings. It is by far the best iron I have ever used. Beware of cheap copies that have similar names. Well worth the few extra dollars.

http://www.xytronic-usa.com/shop/item.aspx?itemid=253

I use a pair of locking tweezers held in a vise. You can use folded over masking tape but the tweezers are much easier to use.

Kester make electronic solder. I would also suggest using some non-acid liquid flux. Ngineering sells both. Scroll down the page.

https://ngineering.com/soldering.htm

You have to be very quick when soldering to an SMD. If you apply heat for too long you will destroy them. To make the connections as quickly as possible I pre-tin the SMD contacts and the wires before attempting to solder the wires on. The fresh solder will flow very quickly. Theoretically the contacts shouldn't need to be pre-tinned but I find that doing that makes the whole job much easier.

Be prepared to smoke a few resistors before you get the hang of it.

Good luck,

Dave

 

[ChrisVA]

I did see surface mounted resistors while I was searching. I have never used those before. if I wanted to use one of those, it sounds like I would have to use a small piece of PCB with 2 small contacts and use the resistor to bridge the connection? Are there prefab PCBs for just that purpose?  Electronics newbie here.
Thanks

 

[ChrisVA]

ChrisVA

I'm trying to find very small resistors, e.g. 400K-1000K ohms for DCC conversions. I want to use them for the front and reard headlights, which will be surface mounted LEDs. The issue I'm facing is finding a resistor small enough when space is tight. I'm not sure how to tell the physical dimensions of  a resistor just from its OHM rating, and how to find "micro" sized resistors? Any websites or links appreciated!

Thanks!

 

 

 

Correction to original Post:

I meant 400-1000 ohms. 

[mvlandsw]

I have had 1/8 watt resistors get hotter than I like when running LED headlights from a decoder. If space is too limited for a 1/4 watt resistor I use two of the 1/8 watt ones in series with each one about 1/2 of the total resistance value needed. This spreads out the heat enough that neither one gets too warm.

  The two resistors can be placed anywhere in the circuit that space permits.

Multiple surface mount resistors could easily be mounted in series on a small strip of circuit board to spread the heat generated.

[hon30critter]

Hi ChrisVA,

I get my Surface Mount Device (SMD) resistors from eBay. I used to wire them myself, but I got lazy and now I buy the pre-wired ones. I usually use the 0603 or 0804 size because I found that the really tiny ones could get hot.

I get my regular resistors either from eBay or Digi-Key. I use 1/4w, again because the smaller 1/8w resistors can get hot.

You can also buy pre-wired 3mm LEDs with the resistors already installed, but they might not fit if there is very little space behind the headlight.

Cheers!!

Dave

[gmpullman]

1000KΩ ? Seems a little high. I generally use 1K to about 2.2K for various locomotive lighting assuming you have about 12V. at your U+ output.

I haven't had the need to use them but you might want to look at resistor arrays which group them in a nice package:

https://a.co/d/6JXm2hX this is a 1KΩ

You can find SMD resistors and solder your own but in reality the usual 1/4 watt axial resistors really don't take up that much more room.

Here are some 1K SMD resistors:

https://a.co/d/6yUyWKa

DigiKey and Mouser are another source.

You can get assortments but for model RR use you wind up with a bunch you'll probably never use.

https://a.co/d/2cqYmlq

Good Luck, Ed

 

[tstage]

Hi Chris,

Do you mean 400Ω-1,000 (1K)Ω resistors?  A resistor as large as 400KΩ wouldn't even illuminate an LED.

I've used 10KΩ resistors with "warm" SMD (surface mount) LEDs for the front & rear headlamps on a few brass locomotives.  I think it gives them a very realistic glow, as shown here:

For SMD resistors, the resistor value is usually printed on the top.  The 1st and 2nd (and sometimes 3rd) number represent the resistor value; the last number is the multiplier.  For example:

• "400" = 40 x 10^o = 40Ω
• "401" = 40 x 10¹ = 400Ω
• "402" = 40 x 10² = 4,000Ω (or 4KΩ)
• "403" = 40 x 10³ = 40,000Ω (or 40KΩ)
• "404" = 40 x 10⁴ = 400,000Ω (or 400KΩ)

The physical dimension for SMD resistors is represented in inches using a 4-digit number:

• "1206" = 0.12 x 0.06"
• "0805" = 0.08 x 0.05"
• "0603" = 0.06 x 0.03"
• "0402" = 0.04 x 0.02"
• "0201" = 0.02 x 0.01"

These you will see listed first from a vendor's webiste then the resistor value:

0805    470Ω   

The SMDs I used in my brass locomotive headlamps were 0603s.  Here's what one looks like in the front headlamp of an HO Alco Models 0-10-0 switcher:

If I were to do them again I would probably go smaller and use 0402s.  However, I think the 0603s look fine.

I soldered my own wires to the 0603s using 34 AWG (0.006" OD) magnet wire.  However, you can get SMDs from vendors with the wires already soldered on.  I prefer to solder the wires slightly different than how they come presoldered.

Lastly, for adhering an SMD LED to a lense or inside of a headlamp, I would recommend Testors Clear Parts Cement rather than CA.  The Testors holds well, remains pliable after it dries, and can be removed, if needed.

Let me know if have any other questions...

HTH,

Tom

[ChrisVA]

I'm trying to find very small resistors, e.g. 400K-1000K ohms for DCC conversions. I want to use them for the front and reard headlights, which will be surface mounted LEDs. The issue I'm facing is finding a resistor small enough when space is tight. I'm not sure how to tell the physical dimensions of  a resistor just from its OHM rating, and how to find "micro" sized resistors? Any websites or links appreciated!

Thanks!