Determine polarity of LEDs on PCB?

Sometimes there's printing on the board that will help. If there is printing, you may either see + and - or a symbol near the LED that looks like

--|>|--

If there's + and - you're done. If there's this (diode) symbol, then the equivalent is

(+) --|>|-- (-)

If not, then you'll have ot use a meter. The following applies to "modern" digital meters -- some old analog meters are backwards and I'm too lazy to tell you how to handle those if you don't need to know. If you have a digital meter with a "diode tester" then use that and you're all set. If not, set the meter to its lowest ohms scale -- typically 200 ohms and make sure that the red lead is in the ohms plug and the black lead is in the common plug. Test the diode both ways. You should typically see conduction one way and nothing the other way. When you have conduction, the red lead shows you the (+) side (anode) of the diode and the black lead shows you the (-) side (cathode) of the diode.

Norm

Sorry to be so uninformed, but what exactly does a person ask for when he goes shopping for a "digital meter"? And are they available at Radio Shack?

BTW, thanks for the instructions on what to look for - unfortunately, there are no markings on Kato's PCB. However, one lead of each LED has a trace running to a little red coil-like thing - could that be the diode? If so, would that indicate the + side of the LED?

Frank

Neither am I, but I have just put the leads to the traces while the track power and lights are on. If it lights, I have the leads correct, if it doesn't, I turn it around.

Digital Railroader LLC wrote:

"RL> I admit I'm no electronics expert. So, I'm asking the good folks on this "RL> forum: "RL> "RL> I'd love to see if there is a way to determine the polarity of LED leads "RL> that are already soldered to a PCB - is there something on the PCB that "RL> would give away the orientation of the plus and minus? Or do I have to "RL> invest in some kind of electronic device, which would be my last option, "RL> since I don't do this that often - the Kato SD70MAC comes with Golden White "RL> LEDs (the cases are colored orange) and I'd like to replace them with the "RL> new clear-cased LEDs, to give the lights a bit more punch...

You have three options:

1) *Commonly* LED cases are not truely round. They are round with a 'flattened' side next to one of the leads. Most often this is the larger ones, but sometimes the smaller ones have a flattened side.

2) You can trace the foil. There are a couple of possiblities - one lead is wired to the '+' or '-' supply. This will be a *common* foil area that connects to many other things on the PCB. Sometimes there will be a current limiting resistor between one lead of the LED and the '+' or '-' supply. Generally, this resistor will be right next to the LED. Once you find this 'common' side you need to figure out if it is the '+' or the '-' supply. For this, you need to do some general 'detective' work on the PCB. Since this is a fairly simple board this should not be too hard. Most likely you would be able to tell by wire colors coming on/off the PCB: red is generally '+' and black is generally '-'. The PCB might also have a printed marking and/or a square foil pad.

3) Your third option would be get a cheap little DVM (Digital Volt Meter) -- this is in fact a fairly standard 'must have' *tool* for any model RR (how are you going to find broken power feeder wires? Or shorts? Etc.). You can get a cheap little DVM at Radio Shack. LEDs are polarized. The LED will have inifinite resistance in one direction and almost 0 Ohms resistance in the other.

"RL> "RL> -- "RL> Frank Eva "RL>

"RL> "RL> "RL>

\/ Robert Heller ||InterNet: snipped-for-privacy@cs.umass.edu

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Something like at this link (this is a 'cheap' $20.00 model -- they have some others up to a $90.00 model -- print this page and bring it to your local RS and ask the sales person for something like this, maybe ask about various features -- you'll want this for other tasks around the RR so you should get something reasonably decent, so you'll want to look at a couple of models and ask the sales person some questions):

"RL> "RL> BTW, thanks for the instructions on what to look for - unfortunately, there "RL> are no markings on Kato's PCB. However, one lead of each LED has a trace "RL> running to a little red coil-like thing - could that be the diode? If so, "RL> would that indicate the + side of the LED?

There would not be a 'coil' but there might be a wire-wound *resistor*. What does the other lead of this thing go to? Is this a 'common' foil trace (with many connections to other things)? Does it end up connecting up to a wire? Red? Black? Does it go to a DCC connection? Which pin?

"RL> "RL> Frank "RL> "RL> "RL>

\/ Robert Heller ||InterNet: snipped-for-privacy@cs.umass.edu

|| snipped-for-privacy@deepsoft.com /\FidoNet: 1:321/153

I don't know if this applies to proprietary items if they are custom manufactured, like the headlights in some models - but I suspect it's true there as well - there is usually either a small notch or a flat side at the base rim, about 1/6 of the circumference of the LED (you may need a magnifier) and this indicates the anode or negative lead. The longer lead before cut-off is also the positive. Should note, though that some manufacturers of specialty items now identify the lead polarity only by the lead length - which doesn't help once it's installed.

Ian Mathers Yellowknife, NT Great Northern Sand & Gravel Company

That's no help once the LEd is installed in the board and he's trying to determine the installation polarity.

But, assuming that current is adequate limited in a properly designed circuit, if it's installed backwards then it simply won't light and it can be turned around with no damage to the LED or the circuit.

Norm

LOL! And when was the last time you were in a Radio Shack? The only questions the twits around here can answer is how much commission they make on cell phone sales!

JE> In article , Robert JE> Heller wrote: JE> JE> > "Digital Railroader LLC" , JE> > In a message on Sat, 10 Jul 2004 22:22:29 GMT, wrote : JE> >

JE> >"RL> Sorry to be so uninformed, but what exactly does a person ask for when he JE> >"RL> goes shopping for a "digital meter"? And are they available at Radio JE> Shack? JE> >

JE> >...so you'll want to look at JE> >a couple of models and ask the sales person some questions): JE> >

JE> JE> LOL! And when was the last time you were in a Radio Shack? The only JE> questions the twits around here can answer is how much commission they JE> make on cell phone sales!

Things like DMMs are somewhat more straight forward -- mostly it is a matter of ease-of-use issues: size of the numbers, useful ranges, and price issues, and warranties.

The on-line search I made of RS's web site turned up a number of models ranging from the $19.95 model to the $89.95 model. I have no idea which (if any) would be in stock at any given RS store. There is a non-trivial chance that the one in the link I posted would not be in stock, but one of the others might be -- there is a picture of a

*typical* DMM. The OP needs to be flexible enough to get a similar model. Maybe there might be a *local* sale item (maybe a discontinued over-stock model on hand).

JE> JE> -- JE> JE> Joe Ellis JE>

\/ Robert Heller ||InterNet: snipped-for-privacy@cs.umass.edu

|| snipped-for-privacy@deepsoft.com /\FidoNet: 1:321/153

Many times you can determine the polarity of an LED by looking at the LED itself. Often if you examine the base of the device, you'll see that one part of the rim is flat. This denotes the cathode or negative wire on the LED. For LED's that have not been soldered onto a PCB, the cathode is also denoted by the shorter of the two wires.

a really cheap and simple way to check polarity is to take a suitable battery, one cell for single colour LED's, higher for white and blue, with a current limit resister, typicaly 470 ohms will work, and apply this to the LED. because it is completely floating, unconnected anywhere else, it does not affect, and is not affected by surrounding circuits, exactly like a meter is not. Easy, simple and cheap.

John H.

Great! Thanks!

BTW, what is the "dial" for?

Frank

Are you sure? I have always treated the longer lead as the + lead... on fresh installs, that is...

Frank

That is the range selector. These are Digital *Multi* Meters. They measure various different thigs: AC Volts, DC Volts, AC Current, DC Current, and resistance. At different ranges. The meter (digital readout) can only display +/-199 (.199, 1.99, 19.9), so you need to select different ranges for different sorts of measurements. Say the

200 VAC range to check the voltage coming out of your power outlets (110VAC) or 20VDC range to measure the output of your MRR power pack or 200 Ohms range for measuring the LED in your original post.

"RL> "RL> Frank "RL> "RL> "RL>

\/ Robert Heller ||InterNet: snipped-for-privacy@cs.umass.edu

|| snipped-for-privacy@deepsoft.com /\FidoNet: 1:321/153

This is a very dependable method, in many ways more fool proof than using a multimeter. I use the multimeter method, since I have a multimeter. For a model railroader a multimeter is worth having. With it you can check for opens or shorts in wiring, locomotives, lamps, switch machines, push buttons, rail gaps, switch frogs. You can check for 12 volt power at the power pack, at the rail, at the block selecting switches, at the motor brushes. You can tell AC from DC. You can detect the short that is causing your DCC supply to shut down. You can measure locomotive current draw. The life of a multimeter will be prolonged if

1. It isn't dropped.
2. It isn't set to ohms or current and touched to a live circuit.
3. It isn't touched to 120 VAC when set to measure 12 volts DC or ohms.
4. The battery is changed before it leaks and corrodes the instrument to junk.

Sears Roebuck now has multimeters in the tool dept, as good as Radio Shack and perhaps better. For model railroading, any meter is sufficiently accurate and covers the ranges we care about. Choice between models is based on price, durability and readability. Simpson and John Fluke are the name brands in the multimeter business. New, they are probably too expensive for most of us. But should one encounter a Simpson 260 or a John Fluke anything on the used market for a reasonable price, you cannot go wrong.

David J. Starr

That sounds like the perfect solution for me - easy, simple and cheap! Thanks!

Frank

In my example, a reading of zero would indicate that the leads need to be reversed, and some varation of 199 would indicate that they are correct... correct?

Frank

Alternate method: with the shell removed, power up the loco until one LED lights up, then use your handy voltmeter to determine which lead of that LED is + and which is minus. Mark the circuit board indicating the

+. Repeat for the other LED. When you install the new LEDs, the longer lead is the one that goes to the + mark you made. Also, be careful to bend the new leads to match the old ones; aiming is important for good brightness.

Some kind person who already has an SD70MAC (I don't) might do this and post the results for those who don't now own a meter.