How to check the operating voltage of LED indicators

Hi -These 3mm diameter LED indicators were picked up at an ME exhibition some years ago sold for use in model railway signalling applications. Only problem is that I am not sure now of the operating voltage (?12V) and whether or not they have an built-in series resistor. Short of trial and error what is the recommended method to establish their operating voltage?

Try first with an AA cell and if nothing happens both ways round, then two in series. If that still does not work, you could try with bridging your car battery.

If they have a built in resistor you can often feel it as a bump in the supply lead.

Don't try with just a battery. Put a 330 ohm resistor in line too. Then try the other way round unless there is something to tell you the polarity. Then if no joy increase voltage but don't remove the resistor until you are sure it has one of its own.

Well you have identify the positive and negative leads, which is which. If you hold one LED up to the light you will see a smaller leg that is Pos, the other looks a bit like a chicken head the Neg, unfortunately all are quite not the same shape. To work them on 12 volt DC not all are quite he same diameter either. Try putting a 470 ohm resistor in series with one leg and go from their. I've found different diameter LED need different current, so might need less resistance or more., not scientific here, as you say, a bit of trial and error. By connecting a diode in inverse parallel with the LED and a series resistor, you can use LED on low voltage AC. I have a lot of used ones and the above is how I identify the resistance and working voltages. John

They look like a wire ended lamp to me, how sure are you that they are LEDs?

Mike

Looking at what you show, I am not convinced that these are LEDs. Here are my clues:

The filament is almost clearly visible;
The outside layer (glass or plastic?) looks like glass, and is strongly coloured. With LEDs the colour is produced within the diode itself, and does not work by having a white light and strong colour covering.

And Mike Poole seems to have come to the same verdict ... Sorry

Tim

Not sure that they are LEDs, it is just my assumption. No obvious sign of any 'bump' in the leads either,indicating a series resistor, just a slight mismatch which I take to be the soldered joint between lamp and their connecting leads. Have not considered the possibility of them being wire ended lamps.

Further close inspection with a magnifying glass seems to confirm that they are in fact filament bulbs which I had not appreciated existed in this size. Question still applies of course as to the operating voltage but perhaps it does suggest that the original understanding of them being a 12v device might be correct.

Before I read the other posts I'd identified them as filamen lamps. You can see the filament supports.
Ideally put them on a variable supply and bring the voltage up slowly until reasonable brightness is acheived. Probably 6 or 12V.
if you only havea 12V supply connect two lamps in series and try that first. Saves blowing them up if they are 6V.

Robert.

They are probably 'grain of wheat' bulbs, surprisingly tiny filament bulbs. I used to buy them for physics and electronics projects when I was a school technician. Since LEDs became ubiquitous, they are rather expensive.

Bill

Go carefully! Many such lamps were 5 volts, and these appear to have the appropriate short filaments for that. Other common voltages were 12 volts and 28 volts, but the latter had an obviously long filament.

I didn't spot they were filament bulbs! Now they're identified as such, I wouldn't use them for new work unless I had a large stock of spares. They're getting hard to replace, causing trouble when one goes pop. Correctly wired LEDs last for ever, are cheap as chips, and widely available.

Someone else may be desperate for them : replacing just one bulb in an existing installation with a LED looks terrible.

Dave

Edited By SillyOldDuffer on 08/05/2023 16:43:38

To be sure what they are a simple test with a multimeter would give equal resistance each way round with filament bulbe and different readings with a diode.

Not necessarily so, we get odd failures in our signals, nearly always the greens. They are running well below max rating, when they stop emitting light they still conduct, so as we have 2 in series the other one still shows.

Probably of cheap chinese origin - sub-standard production runs of good LEDs. The lifetime guarantee only lasts until they are connected and powered up, in a circuit!

The 'cheap Chinese' theory is impossible to prove, leading to bad engineering decisions. Better to look for solid evidence instead. It reduces the risk of replacing a lot of LEDs only to find they go pop as well.

Ordinary folk can guess if they like, but engineers must strive to avoid fixing the wrong problem!

A possibility that can be checked is that Duncan's LEDs aren't "correctly wired" in that there may be a vulnerability due to their environment. What's fine indoors may not be adequate on an outdoor railway system, where wet, static, and capacitive and inductive coupling have to be considered. For example, does Duncan's circuit include an ordinary diode in parallel with the LEDs to protect them from reverse spikes? If missing, Duncan's circuit isn't as "correct" as it could be.

Dave

They have a series diode to protect from being connected to the battery the wrong way round. The series resistor limits the max forward current to less than their rating, then the brightness is further controlled by feeding a mark space waveform. There is nothing in the neighbourhood to produce spikes on the feed. It's always the green LEDs, and most of them are fairly shaded by trees. The boxes are IP65, but one of them did manage to get flooded, think I didn't tighten the screws properly. It started performing really oddly, but after I dried it out it recovered. Can't remember whether this was one of those with failed LED. My record keeping isn't that good.