Flashing 12v LEDs. Help please with weird effect.

[Crosland]

"Thanks, Richard.

 

They have a resistor fitted but wrapped in heat shrink. I haven't bothered to find the value.

 

Dave"

 

Does anyone else find this slightly worrying, shouldn't the current limit be internal to the led package?

 

Yes

[kevinlms]

Correct but this chaps 12 volt supply was  was nearer 21 volts.   More than enough to fry many "12 volt" electronic components. 

Computer 12 Volts is 12 volts   Car 12 volts is or was 13.8 to 14.2 volts in my day and Model Railway 12 volts is typically 19 volts.

 

You really need to size an LED Resistor to keep the LED within maximum forward Voltage limits when the power supply is off load, not just use the nominal rating as LEDs are fairly unforgiving if too much voltage is applied.

Haven't I seen threads of yours where you DON'T use any resistors on LED's at all?

[kevinlms]

 

You do not need AND SHOULD NOT ATTEMPT TO USE a resistor with a flashing LED.

 

There's no such thing a "computer 12V". 12V is 12V is 12V,... Anything else is 12V +/- something or "nominally 12V", just like our mains is "230V" In fact it is 230V +10%/-6% and is, in practice, 240V.

 

We are now well in the 21st century, just use a properly regulated power supply.

Agreed, a '12 volt PS' that puts out 'near to 21 volts', isn't a 12 volt supply at all, but something else entirely.

 

As the OP has described, a higher voltage than designed for, will cause weird effects. I'm not a scientist, but I think that's because its opening another light spectrum of the LED. However, that's if his lucky, because it could easily let the smoke escape, which of course is a once only event.

 

While it is possible to add on a circuit to something like a Duet to regulate it to a lower voltage, I have to wonder what the point is, when a small plug pack can be obtained for the small quantity, the OP plans on lighting at once.

A 'free' old controller, isn't that cheap, if it blows up modern electronics.

[sharris]

I wonder if the effect of sunlight falling on it isn't actually a photoelectric effect, but just the sunlight itself being reflected back off the LED chip and drowning out the light that the LED is generating.

[Suzie]

The circuitry of flashing LEDs is unlikely to work with a resistor in series. I am getting the impression that they are not quite what they should be and perhaps someone has got a wheelbarrow full of 5V flashing LEDs and is trying to move them on by adding dodgy circuitry. Try removing whatever has been put in line and see if it runs from 5V DC (from a USB charger or similar).

 

Suzie x

[dasatcopthorne]

GUYS.

 

Thanks for the help some have given and no thanks for the others who have taken my first simple query in other directions.

 

May I please ask you to read messages 1 & 13 before searching for a tangent to take.

 

The LED'S are working as are the signals on the layout without smoothing though I still may go down that route. Thanks cpman for the circuit and to dhjgreen for the links.

 

So thanks again for the positive posts.

 

Cheers

 

Dave

[Crosland]

May I please ask you to read messages 1 & 13 before searching for a tangent to take.

 

Well, some of it wasn't a tangent, from post #1 "I have spent many hours trying to discover the correct resister to fit so they flash on 12v."

 

That's approach is wrong, wrong, wrong.

 

Flashing LEDs are not like normal LEDs that require a resistor. Flashing LEDs contain an integrated circuit to make the LED flash. Using anything above 12V (plus whatever margin the datasheet allows) is likely to result in long-term damage.

 

Andrew

[sharris]

I'd use CPMan's regulator circuit from post 18.

 

The problem as I see it is that you've got it kind of working on a Clipper by tweaking things with a series resistor, and the system you actually want to use it in has a different power supply - it's possible that the values you've selected will only make it work with the Clipper set up in a specific way (I think it's the same as the Duette on which there are 4 combinations of rectification and resistance switches as well as the knob setting to contend with!). If you want to get some commonality between it working on one and on the other, then sending power through CPMan's regulator circuit will give you a smooth 12V DC to power your LED on whichever power supply you are using.

[KalKat]

I agree with the above. Use the regulating circuit and it won't really worry about what the voltage is as long as it is greater than 12v. It will also not worry your other LED devices on the same supply

 

 

Emma

[dasatcopthorne]

Thanks for all that.

 

As I said in the later post, I know the voltage of the supply I was finally to use.

 

Thus I adjusted the Clipper to give the same following the discovery that it was actually putting out 21v odd.

 

As I said they are working fine.

 

Thanks again.

 

Dave

 

Ps the LED signals have been working fine for 3 years without smoothing although I will probably built the circuit along with an LED dimmer.

[kevinlms]

Thanks for all that.

 

As I said in the later post, I know the voltage of the supply I was finally to use.

 

Thus I adjusted the Clipper to give the same following the discovery that it was actually putting out 21v odd.

 

As I said they are working fine.

 

Thanks again.

 

Dave

 

Ps the LED signals have been working fine for 3 years without smoothing although I will probably built the circuit along with an LED dimmer.

I think you're missing the point. Modern electronics work much better with stabilised power supplies. You got it to work, but strictly speaking your approach isn't right.

 

If you had a regulated 12 Volt supply in the first place, this thread wouldn't have been required.

[sharris]

Something doesn't quite add up here.

 

 

Post #1

I have now, just by chance, found that they are effected by ambient light. So cutting the light level brings them on at 12v with a 10k resisted in line.
I've now found that if direct sunlight falls on the lens, the LED shows a steady light!!
 

 

 

Post #7

Optimum Supply Voltage : 9v - 12v for highest brightness
Minimum Supply Voltage : 5v with much reduced brightness
Maximum Supply Voltage : 14v
Current : 20mA @ 12v

 

Ignoring questions about whether the supply is smoothed, the use of a series resistor in the first place, etc.

 

The specifications say current 20mA (it doesn't say whether that's a typical or maximum rating), but that seems in line with a lot of visible light indicator LED forward current ratings. Your power supply is approximately 12V (you don't say whether your measurements are RMS or Peak - I suspect 12V RMS with a peak of 20V. If we ignore any voltage drop across the device (i.e. if we just put your 10k resistor straight across the power supply without the LED in series) you'd be getting about 2mA current - this is the maximum that you could possibly deliver to anything connected via a 10k resistor and your power supply. Of course, there will be some forward voltage drop across your LED (a few volts - as the datasheet says the minimum working voltage is 5V), so you're actually feeding your LED with something less than 2mA (probably 1 - 1.5mA).

 

 

Datasheets that quote a 20mA maximum forward current will often show useable illumination levels for 10mA, but with that 10k resistor less than 2mA forward current will leave a seriously malnourished LED!

 

It is possible that the LED may be dimly lit - I wonder if this is the 'photoelectric' effect you are seeing. With normal light levels in the room, it may be that the LED is just too dim to be particularly visible, but as you dim the lights you can see it glowing a bit - I've already suggested a possible reason for the sunlight effect as a reflection of the sunlight from the LED chip or its substrate.

[AndyID]

I would power it from a car battery with no resistors. If it didn't work properly, it would be consigned to the trash.

 

(My guess is it's intended to be used in a car to simulate an armed security system.)