mA rating of transformer

Thread Starter

camner

Joined Jul 3, 2017
20
I'm thinking of using these LEDs as indicator lights (https://www.idec.com/language/english/datasheet/AP22_datasheet.pdf). As far as I can see, the specs say nothing about current requirements. I can buy a 24v transformer rated at 20mA, and I found another one rated at 40mA.

I remember reading that unlike incandescent lamps, which have fixed current draws determined by the load resistance and the applied voltage, LEDs respond to the current applied, so an excessive current will harm the LEDs.

The spec sheet shows the circuit built into the device. Does it matter what the current rating of the transformer is, and if so, how do I determine that?

And, since the circuit contains a rectifier, I presume I can run this off of a 24VAC transformer?
 

Thread Starter

camner

Joined Jul 3, 2017
20
I sure did (miss Rated Current)!! Don't know how, though, and I appreciate your taking the time to reply.
 

Thread Starter

camner

Joined Jul 3, 2017
20
No problem, I’ve done the same sort of thing often enough.
Perhaps you can help me with one other piece of this project. I installed per the circuit diagram below, and I'm getting "ghosting" on the LED indicator lamp that is NOT energized (when in unenergized state, the LED assembly glows faintly).

The cable is 18/3 thermostat wire with a run of about 25 feet total. I don't know enough about LEDs to know whether that kind of cable run is enough to generate a large enough induced current to faintly energize the "unenergized" lamp.

Is there something that can be done about this ghosting?
2019-04-02_garage.png
 

Yaakov

Joined Jan 27, 2019
1,535
Perhaps you can help me with one other piece of this project. I installed per the circuit diagram below, and I'm getting "ghosting" on the LED indicator lamp that is NOT energized (when in unenergized state, the LED assembly glows faintly).
If that schematic is accurate, I can see no reasonable way to get the behavior you describe. Your idea of induced current is about the only thing that would make any sense at all, but since the LED has a current limiting resistor that is designed to handle 24V, I can't see how that would work.

Is the switch 25 feet away from the LEDS? Are you sure it is wired the way you've drawn it? Are you sure there are no breaks in insulation?
 

wayneh

Joined Sep 9, 2010
16,102
Odd. I was going to suggest a resistor across the power leads to the LED as a quick (but inelegant) fix. The idea is to pull down any ghost current. But the "equivalent" circuit implies there already is one.

If your switch is capable of it, you could consider shorting the power leads of the "off" LED together. You obviously need to be very careful you don't create a short while power is applied.

Like @Yaakov, I'd be suspicious of how things are wired. I wouldn't expect ghost currents large enough to light an LED even dimly.
 

Thread Starter

camner

Joined Jul 3, 2017
20
If that schematic is accurate, I can see no reasonable way to get the behavior you describe. Your idea of induced current is about the only thing that would make any sense at all, but since the LED has a current limiting resistor that is designed to handle 24V, I can't see how that would work.

Is the switch 25 feet away from the LEDS?
Yes, the switch is about 25 wire feet from the LEDs.

Are you sure it is wired the way you've drawn it?
I'm pretty sure...I don't know how else I could wire two LEDs to a DPST switch and have only one on at a time.

Are you sure there are no breaks in insulation?
Well, since there is an outer jacket, I can't inspect the inner wires visually. I did put the volt-ohm meter across all pairs of the 3 wire cable, and did not detect any shorts.
 

Thread Starter

camner

Joined Jul 3, 2017
20
Odd. I was going to suggest a resistor across the power leads to the LED as a quick (but inelegant) fix. The idea is to pull down any ghost current. But the "equivalent" circuit implies there already is one.
I'm not sure what you mean by the "equivalent" circuit. If I were to put a resistor across the power leads, what wattage and ohmage would I use?

If your switch is capable of it, you could consider shorting the power leads of the "off" LED together. You obviously need to be very careful you don't create a short while power is applied.
I'm not sure how I would do that. This is a magnetic DPST switch mounted on the rail of a garage door, and is thrown by a magnet mounted to the garage door. When the door is closed, the magnet pulls the switch in one direction, and when the door is opened, the spring pulls the switch in the other direction.
 

Thread Starter

camner

Joined Jul 3, 2017
20
How is the 25 ft. of conductors physically laid out?
This may be more detailed than you want, but here goes...

The 18/3 thermostat has white, red, and green wires. The two LEDs each have a red and a black wire, and the switch has a white wire (to the common pole), and red and black wires to the other two poles.

Physically the LEDs are between the transformer and the switch.
  1. The white wire of the 18/3 cable is connected to one side of the transformer.
  2. The cable then runs to the LEDs.
  3. At the LEDs the green and red wires are cut. The side of the red and green wires toward the transformer are connected one each to the white wires of the LEDs.
  4. The other side of the cut ends of the green and red wires are attached to the red wires of the LEDs (separately, of course!).
  5. The cable then goes to the switch, where the white wire of the cable is attached to the white wire of the switch, and the red and green wires of the cable are attached to the red and black wires of the switch, respectively.
  6. Back at the transformer, the green and red wires are attached to the other side of the transformer.
That matches the diagram, no?
 

BR-549

Joined Sep 22, 2013
4,936
A thousand words can not describe a picture or a schematic. Carefully and clearly....please re-draw your circuit........showing where wires are together and where wires are apart. And please label the color of wires.

Are the 3 wires held together for the whole 25 ft. run?

Are the wires laid in parallel or are they twisted?

If you break the ghost circuit connection at the other end............does the ghosting stop?
 

Thread Starter

camner

Joined Jul 3, 2017
20
Wires are together the entire time, except where the green and red wires are broken at the LEDs, with each of the red and green wires connected to the two sides of their respective LEDs. Cable is 18/3 thermostat wire...laid parallel, not twisted.

2019-04-03_circuit2.png
 

BR-549

Joined Sep 22, 2013
4,936
So, when the circuit is in that condition, L3 is on and L4 is lit dimly?

How far apart is L3 and L4? Can you jump(tie the LEDs together), on the transformer side of LEDs?

Does the jumper stop the ghosting?
 
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Thread Starter

camner

Joined Jul 3, 2017
20
So, when the circuit is in that condition, L3 is on and L4 is lit dimly?
Yup!
How far apart is L3 and L4?
2 inches...they are mounted in a normal single gang switch plate I drilled out.
Can you jump(tie the LEDs together), on the transformer side of LEDs?

Does the jumper stop the ghosting?
I'm not quite sure I understand. Aren't the transformer sides of the two LEDs already tied together at the transformer? Are you suggesting tying them together at the LEDs and then running a single wire back to the transformer? That circuit would be equivalent, yes?
 

BR-549

Joined Sep 22, 2013
4,936
Well, for now, and just a quick test, jumper them like I said. Did the ghosting stop?

After I understand.....you'll understand. You are there, I am not, I can only go by what you say.

That's why I need to know if it stopped ghosting?
 

Thread Starter

camner

Joined Jul 3, 2017
20
My apologies for the slow reply...the notification email went to spam this time.

I did this project at my daughter's house, and I won't have access to it for a few weeks. The next time I do, I'll try your suggestion and report back.

The only other factor that occurred to me was that perhaps the switch somehow bleeds a bit? The switch is for a garage door (see https://www.123securityproducts.com/2317a-l.html).
 
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