How to connect LED lamps?

[faded_Glory]

I bought a set of cheap LED lamps on Ebay but I'm not sure how to wire these up.

 

Each lamp has two lights and there are 4 thin wires coming out of the base, 2 insulated and 2 uninsulated. A bunch of resistors came with the set but no instructions.

 

 

I want to place 6 of these lamps (12 lights in total) on my small layout (Paris 1889). I will power the layout with an old Fleischman controller which has 2 outputs, one variable DC up to 14V for the loco, and a second fixed 14V AC for accessories. I would not want to use the loco output for the lamps because obviously the light would then vary along with the loco speed. So it seems I would need to connect the lights to the 14V AC outlet. Will these LED's work on AC?

 

I don't know what the normal voltage is for these LED's, would it be 3V? So I'm thinking of connecting 6 of them in series, and these then in parallel with the second batch of 6 also in series. That ought to work on a 14V outlet I think. To wire them in series I presume that I need to daisy chain each time one of the uninsulated wires to one of the insulated ones?

 

I could of course just experiment with them but I rather not blow them up through ignorance so any advice would be welcome!

 

 

Edited August 6, 2019 by faded_Glory

[Guy Rixon]

Being diodes, they conduct current in one direction only, and light up when they are conducting. Therefore, a DC supply is best. If you connect them to AC, they will conduct and light up for only half of each cycle. This would probably be OK (50Hz is too fast for visible flicker), but not ideal.

 

All LEDs have a minimum voltage at which they will light, related to the band gap baked into the silicon. It's usually less than 3V and tends to decrease as the LED gets hot.

 

They also have a reverse breakdown voltage at which they start conducting in reverse. It's normally between 12V and 20V, so 14V AC might do extra fun stuff to your poor LEDs. Or they may be OK with it; hard to tell without a spec sheet.

 

They have a maximum current (not voltage) above which they overheat and die.  The point of the resistors is to limit the current so they survive. A resistor goes in series with each LED. See Wikipedia for a few more details.

 

It's likely, but not certain, that the insulated wires are the positive leads.

 

PS: I'm guessing that these particular lamps are designed such that one resistor per LED gives about the right current for a 12V input, that being the traditional voltage for accessories; they should then be OK on14V DC. But we don't know this for certain. 

Edited August 6, 2019 by Guy Rixon

[faded_Glory]

Thank you Guy for your reply, very helpful!

 

It sounds like it would be best to connect each lamp separately then? I guess I could rig up a small circuit board under the baseboard to connect everything to.

 

What puzzles me is that I have 10 lamps and 10 resistors, but each lamp has 2 LED's. Pehaps the LED's in each lamp are meant to be connected in series, and then the resistor goes in the combined feed wire? Or maybe the Chinese designer didn't think as far as that

 

I have a few more lamps than I need so perhaps I should just go and experiment a bit.

 

[Guy Rixon]

Yes, multiple LEDs can be connected in series. They don't have much resistance when conducting in the forward direction, so the current will be about the same, for a given supply voltage and resistor, for one or a reasonable number of LEDs.

[cliff park]

The resistors all look to be the same size, but not easy to decode. It could be they have supplied more than one so you can tailor them to different supply voltages. Can you write down the colour bands, are they all the same ?

[Right Away]

All my MAS signals, platform lighting, street lighting and building interior LEDs run off an old discarded power adaptor (transformer plug) which has a DC output of 12v. A cheap and cheerful "temporary" supply, now in its 5th year!

A "bank" of resistors allows every LED to have its own one (not strictly necessary if cumulative values are worked out, but useful for tracing faults should any arise.) There are spare resistors for future growth. 

 

As has been mentioned, LEDs appear to function on an AC supply but the continuous "off and on" can have a detrimental effect on their longevity.

 

[Sol]

Some info re LEDS   https://electronicsclub.info/leds.htm

[DCB]

The Fleichman contrloller is unlikely to control the voltage, it is much more likely to be a resistance controller which will give 14 volts off load no matter where the control knob is apart from "Off"  Check it with a voltmeter.  A proper voltage control unit should start at anywhere from 0.01 to 4 volts off load. I wouldn't run LEDs on pure AC.  I would put a bridge rectifier in circuit or a 1 amp diode in series and and a fairly large capacitor in parallel  to smooth the current. The singe diode will drop the voltage around 0.7 volts while the  Bridge will drop the voltage by about 2.8 volts at approx 0.7 volts each for the 4 diodes in the bridge.  That said the six LEDs in series might well work on half wave  rectified but smoothed AC, 13 .7 volts divided by 6 = 2.3 volts which is a pretty typical vf for LEDs, reds tend to light at 1.8 volts, yellows 2.1 volts or thereabouts.  If they are too bright you may need a resistor but it may well be that even with all the resistors supplied in parallel you still can't get the LEDs to light. Obviously(?) if you connect any LED "backwards" none of the string will light and if you short one or more out or feed it 12 volts it will instantly glow super bright and die.

[kevinlms]

First thing to do is work out the value of the resistors you have, BEFORE you connect anything up.

 

They will have either 4 or 5 bands. Can't tell from you photo, but if only a single bands, they are diodes and not resistors.

 

Take a look here for how to identify and post back here.

 

https://www.digikey.com.au/en/resources/conversion-calculators/conversion-calculator-resistor-color-code-4-band

 

Edited August 7, 2019 by kevinlms

[PatB]

In these days of ultra cheap multimeters I've pretty much given up interpreting resistor colour codes. It's much quicker and easier just to test them.

 

It also looks as if all the supplied resistors are the same, given that they appear to still be attached to their factory packaging strip. 

[John ks]

Here are some circuits for you lights

Most should be self-explanatory

 

 

If you use the 14VAC (with or with out rectifier) then I would wire 2 or more of the resistors in series to see how bright the LEDs are. Add or remove resistors until you get the desired brightness.

CAUTION Leave at least 1 resistor in series with the LEDs

My best guess would be at least a 1K resistor & maybe up to 5K

By wiring both LEDs (in each lamppost) in series you only need 1 resistor for each lamppost

If you want to wire more than 1 lamppost in series then the disadvantage is if one LED fails then all LEDs go out.

As mentioned earlier in this thread reverse voltage can destroy LEDs.

A diode across the LEDs will clamp the reverse voltage to a safe level or

In the bottom diagram each LED is limiting the reverse voltage to the other LED

Hope this helps

John

[faded_Glory]

Thank you all for your comprehensive replies!

 

I can't really make out the colours of the bands, the resistors are tiny and the bands all are very dark - possible one brown and the rest black? I can't really say.

 

John ks' diagrams are very helpful. If I assume that the LEDs are wired consistently (i.e. the insulated wires are the same polarity for both LEDs)  I can try to rig up one lamp using his third diagram.

 

 

Edited August 7, 2019 by faded_Glory

[smokebox]

Usually brown,black, red to signify 1000 ohms (1k). A magnifier helps or check with your multimeter.

[John ks]

A LED tester can be very handy 

This one I Got from Jacar  Electronics,(an Australian Electronics retailer) cost about AU$10.00 Quite a few years ago

They currently have a similar version at a similar price

A quick search found an updated KEMO version at about £15.00

A quick & dirty way to make a LED tester is to put a 1K resistor in series with a 9V Battery

John

 

 

[faded_Glory]

Right, inspecting the lamps through a magnifying glass it appears that the two uninsulated wires actually run through separate channels up the lamp post. So the two LEDs in each lamp are actually totally separate. I soldered one resistor to one insulated wire and connected that, plus its corresponding uninsulated wire to the 12V DC output, and with a twist of the knob it lit up I must have connected it the right way round!

 

Then I connected both wires to the 14V AC and it lit up too, with decent brightness and no flickering. It looks like I am overthinking this and I should just wire each LED separately with one resistor on each input wire. I can solder the whole lot to a small piece of circuit board that I can stick underneath the baseboard.

 

What type of 12V plugs and sockets do you guys use to connect such a caboodle to the controller? I obviously want to make that detachable for transport (and so I can use it on other layouts).

 

BTW my multimeter suggests that the resistors are 100 Ohm, does that make sense?

Edited August 7, 2019 by faded_Glory

[John ks]

When connecting to AC

Remember what Guy Rixon Said "They also have a reverse breakdown voltage at which they start conducting in reverse. It's normally between 12V and 20V, so 14V AC might do extra fun stuff to your poor LEDs. Or they may be OK with it; hard to tell without a spec sheet. "

 

A 100 Ohm resistor at 12V is about 120mA add the LED to the circuit with a forward voltage of between 1.5V to 5V then the current drops to between 105mA & 70mA

Both of those currents seem a little high to me

As I said earlier "If you use the 14VAC (with or with out rectifier) then I would wire 2 or more of the resistors in series to see how bright the LEDs are. Add or remove resistors until you get the desired brightness"

 

You could use your multi meter to measure the current drawn by the LED

 

If you decide to use my last circuit then to ensure the LEDs are connected correctly, first connect to DC, 1 LED should light, second reverse the connection & the other LED should light

 

John

 

[sharris]

14 minutes ago, faded_Glory said:

BTW my multimeter suggests that the resistors are 100 Ohm, does that make sense?

 

I couldn't tell what colours the bands were from the picture, but 100 ohms will either start brown-black-brown for a 4 band resistor (the 4th band is a tolerance, usually gold or silver) or brown-black-black-black for a 5 band resistor (the 5th being tolerance).

 

100 ohms sounds a little low for a modern LED and a 12V power supply - I would have thought it more suitable for a 6V supply. 

 

If you want to learn resistor colour codes, here's a web page that will help.

 

https://www.digikey.co.uk/en/resources/conversion-calculators/conversion-calculator-resistor-color-code-4-band

 

[faded_Glory]

I think they are brown-black-black-black then.

 

So here is my thinking now: I can group the lamps in 2 groups of 3, i.e. 6 LEDs per group. I connect each group of 6 to a small circuit board under the baseboard, with one resistor per LED. That puts those 6 LEDs in parallel so the current draw will be reduced, hopefully mitigating against too high currents?

 

I can then wire the two circuit board in series, so the voltage drops to 7V instead of 14V, right?

 

I hope this makes sense, as you can tell I'm not much of a sparkie!

[Crosland]

On 07/08/2019 at 13:27, John ks said:

When connecting to AC

Remember what Guy Rixon Said "They also have a reverse breakdown voltage at which they start conducting in reverse. It's normally between 12V and 20V, so 14V AC might do extra fun stuff to your poor LEDs. Or they may be OK with it; hard to tell without a spec sheet. "

 

Absolute maximum reverse voltage is much lower than that, typically 5V.

[Tricky Dicky]

On 07/08/2019 at 13:37, faded_Glory said:

I think they are brown-black-black-black then.

 

So here is my thinking now: I can group the lamps in 2 groups of 3, i.e. 6 LEDs per group. I connect each group of 6 to a small circuit board under the baseboard, with one resistor per LED. That puts those 6 LEDs in parallel so the current draw will be reduced, hopefully mitigating against too high currents?

 

I can then wire the two circuit board in series, so the voltage drops to 7V instead of 14V, right?

 

I hope this makes sense, as you can tell I'm not much of a sparkie!

Not according to Kirchhoff's Laws, 6 LEDs each with their own current limiting resistor,  all in parallel will result in the total current drawn equalling the sum of all the individual currents. So assuming each LED + resistor draws say 10mA then the total current drawn will equal 60mA.

 

If you then wire your circuit boards in series and connect to  a 14V supply then you are creating a voltage divider with 7V dropping across each circuit board.

 

Richard

[faded_Glory]

Well, I managed to get them working. I connected them as above, 2 groups of 3 lamps each in parallel. When I connected these 2 groups in series they lighted up, but very weakly. So I connected them in parallel (so all of them in parallel) and on the 14V AC they now light up very nicely, without any flickering.

 

 

As I said above I was overthinking it and too worried to blow something up. Thanks for all the help and my confidence in electrics has just gone up a tiny little bit

 

 

[DCB]

5 hours ago, faded_Glory said:

Well, I managed to get them working. I connected them as above, 2 groups of 3 lamps each in parallel. When I connected these 2 groups in series they lighted up, but very weakly. So I connected them in parallel (so all of them in parallel) and on the 14V AC they now light up very nicely, without any flickering.

 

 

As I said above I was overthinking it and too worried to blow something up. Thanks for all the help and my confidence in electrics has just gone up a tiny little bit

 

 

If they work on 14 volts AC it Looks like the lamps have built in resistors, Normally they would imitate a flash bulb and die instantly.

[faded_Glory]

I did solder in the resistors that came with them.

[AndyID]

On 08/08/2019 at 08:14, Crosland said:

 

Absolute maximum reverse voltage is much lower than that, typically 5V.

 

That is typical of the value quoted by the manufacturers but it's extremely conservative. In practice the value is often orders of magnitude greater.

 

The manufacturers are not about to put a lot of energy into specifying the reverse breakdown voltage for a diode that's only intended to operate in forward mode so they play it very safe and peg it at something like 5 volts.

[Dave John]

Hmm, glad you got them going faded glory. 

 

Useful advice about LEDs.

 

But 16 VAC , 1 k resistor , nice warm glow.  I think they are actually miniature incandescent lamps rather than LEDs.