Interesting phenomenon as regards using an LED as a warning light?

[MalcolmInN]

My meaning of "in-line" is in the outgoing power line, not across between the negative and positive lines.
,
Was that the wrong way to do it?  Peter.

Ah now then this may be it
or maybe I am misunderstanding, words are such poor things for this !!
"in-line" with one of the wires IN the outgoing power line
or between the two wires OF the power line ??

If the first then that is the problem,

Cant write more at the mo. I am supposed to be cooking the evening meal ! and the domestic staff will soon be complainig,
perhaps Bob or someone can pick it up and run with it meanwhile ?

Ah, I see Steve already has !
more later , , ,

[keithfre]

If the LED was in line and on one of the wires from a 16 VAC power supply then only half of the AC current would pass through, at most. The LED is a diode which is a sort of valve that only lets current pass in one direction.

Assuming it is in series, in other words in one of the two wires from the AC supply to the controller (see below), wouldn't it also limit the current? Say it has a working current of 20ma, that will be the current passing through it, and as it completes the circuit there's nowhere else for any current to pass.

AC output 1 --- LED ----- controller input 1
AC output 2 -------------- controller input 2

[Steve.T]

I didn't really want to go into it too deeply as Peter has said his electronic knowledge is basic.

Assuming the led did not blow then it would limit the current by at least half due to blocking half the current as it is AC.
On top of that it would lower the voltage to the controller by 2 to 3 V (maybe more if a blue led) due to the forward voltage of the led.
However it will not limit any current to say 20mA as the current is normally limited by adding a series resistor. To be honest without knowing the circuitry of the controller it is difficult to say what current would be passing through the led, it could be more or less than 20 mA.

Or that's how I am picturing what will be happening.

Steve

[austinbob]

Peter,

If the LED was in line and on one of the wires from a 16 VAC power supply then only half of the AC current would pass through, at most.
Which is why it appeared to sap so much power.
The LED is a diode which is a sort of valve that only lets current pass in one direction. Thus for 1/2 the AC wave it would be blocked.
Also without a resistor then I would imagine after a short while the LED will blow.

The LED could have been used by putting it across the 2 wires from the power supply with an appropriate resistor to limit the current. In this configuration current would still pass in only one direction but the flickering would be so quick that you would still just see a lit led.

Steve

Steve - from memory Peter is using an led with a resistor in series. So if he's put the led and resistor in line from his power supply to his controller he would have only a few mA available to power his locos.
Your suggestion to connect the led (with series resistor already incorporated) looks good.
If you understand any of that Peter then give it a try.

[Steve.T]

Steve - from memory Peter is using an led with a resistor in series. So if he's put the led and resistor in line from his power supply to his controller he would have only a few mA available to power his locos.

True, that would reduce any current to almost nothing.

[keithfre]

However it will not limit any current to say 20mA

Surely it will, Steve? Being a diode it will block half of the cycle completely (converting the AC into DC), and the DC half that it is allowing through cannot be much more than the working current, otherwise it will blow and there will be none.

[petercharlesfagg]

Peter,

If the LED was in line and on one of the wires from a 16 VAC power supply then only half of the AC current would pass through, at most.
Which is why it appeared to sap so much power.
The LED is a diode which is a sort of valve that only lets current pass in one direction. Thus for 1/2 the AC wave it would be blocked.
Also without a resistor then I would imagine after a short while the LED will blow.

The LED could have been used by putting it across the 2 wires from the power supply with an appropriate resistor to limit the current. In this configuration current would still pass in only one direction but the flickering would be so quick that you would still just see a lit led.

Steve

Thank you, do you think this why I have experienced so many problems with the other LED's because I had wired an LED in-line for their power too?

I did not know how to wire in an indicator, it was pure guesswork but it worked, albeit for a short period!

Many thanks, Peter.

[Steve.T]

However it will not limit any current to say 20mA

Surely it will, Steve? Being a diode it will block half of the cycle completely (converting the AC into DC), and the DC half that it is allowing through cannot be much more than the working current, otherwise it will blow and there will be none.

Okay I agree the current will more than likely be less than 20 mA.
But the led will not actually limit the current to 20mA by virtue it is a 20mA led.

I think we have all established that the current will be greatly limited by fitting the LED in series and so long as Peter is aware of how to use an LED with current limiting resistor by fitting in parallel and not in series then I hope that has helped him out and is sufficient.

[austinbob]

Peter,

If the LED was in line and on one of the wires from a 16 VAC power supply then only half of the AC current would pass through, at most.
Which is why it appeared to sap so much power.
The LED is a diode which is a sort of valve that only lets current pass in one direction. Thus for 1/2 the AC wave it would be blocked.
Also without a resistor then I would imagine after a short while the LED will blow.

The LED could have been used by putting it across the 2 wires from the power supply with an appropriate resistor to limit the current. In this configuration current would still pass in only one direction but the flickering would be so quick that you would still just see a lit led.

Steve

Thank you, do you think this why I have experienced so many problems with the other LED's because I had wired an LED in-line for their power too?

I did not know how to wire in an indicator, it was pure guesswork but it worked, albeit for a short period!

Many thanks, Peter.

That is a possibility Peter.

[Steve.T]

Thank you, do you think this why I have experienced so many problems with the other LED's because I had wired an LED in-line for their power too?

I did not know how to wire in an indicator, it was pure guesswork but it worked, albeit for a short period!

Many thanks, Peter.

If that is the case Peter then I would say there is a good chance.
And for short periods leds can stand higher currents than they are rated at but after a while they will blow, generally running very brightly until they do.

I hope you know how to put them in parallel across the two leads now (with the correct size resistor in series with the led).  Then they will only get the current that is allowed by the resistance of the led plus the resistance of the resistor. If not then we can help further.

If it is AC then the polarity does not matter - any lead can go to either lead of the power supply.

Steve

[keithfre]

I hope you know how to put them in parallel across the two leads now (with the correct size resistor in series with the led).  Then they will only get the current that is allowed by the resistance of the led plus the resistance of the resistor.

The best solution, as I understand it, is to use two LEDs wired in parallel, with the cathode of one LED connected to the anode of the other. A single LED could otherwise blow if the reverse voltage (the voltage the LED is trying to block) gets too high.

[MalcolmInN]

Oh dear !
what a load of,,
noise !
Peter does not need all this theory, debate, forward-reverse ( we did all that elswhere ! in many other threads interminably) all he needed to know was which wire to glue to what terminal, simples !

Having identified a possible - then became probable, area of misunderstanding about the words 'in-line' I leave y'all (whilst I go cook ) to determine what Peter actually did - -  and then to tell him which wire to move to where, to put it all right, nurse.

!!!!!