LEDs with negative and positive voltage

[pugabral]

I have a model railroad, to switch turnouts with rotary switch you need a dual power supply. I have two 12 VDC @ 1A power supplies wired with the negative and positive sides of each power supply connected as a common ground. The turnout motors are wired with the common ground to one terminal, and the switchable wire connected to the other terminal. When I touch the positive wire to the switchable circuit the motor moves in one direction, and if I touch the negative wire to the switchable lead the motor moves in the other direction. This is correct.
My problem is that I want a colored led to illuminate as I select the direction. I have two LEDs wired in reverse polarity in the switchable wire. When I switch the positive wire one LED lights, when I switch to the negative side the other LED blinks? I need both lights to be steady on and not flash??

 

[alec_t]

Welcome to ETO!
Are the two LEDs identical, or have you got hold of one flashing LED by mistake?
Is the turnout coil the only load that the LEDs see?
How much current does the turnout draw?
What is the maximum allowable current for the LEDs?

 

[gophert]

You should put three LEDs in series for each color plus a current limiting resistor. LEDs are usually limited to 5v reverse voltage so 3 in series will cover your 12v supply.

Also, I agree with alec_t that one of your LEDs has an internal flasher, or your supply is cycling in one direction.

 

[Pommie]

It sounds like the turnouts turn off the current once they've moved. Try connecting your LEDs in parallel with the turnout but make sure to include a current limiting resistor.

 

[pugabral]

OK, the turnouts are motors, and the initial pulse of current starts them moving on one direction and then they stop, but there is still power on them so the LED stays lit.
The LEDs work common to positive correctly, but when connected common to negative they blink. All of them do the same thing? If you reverse the leads in either combination they do not light (as they should not)
The current draw is very low and 1 amp will power all 30 turnouts and respective LEDs
I need 1 LED per turnout leg so stringing them together does no good.
The LEDs only flash when hooked up common to negative, common to positive they are steady.
The power supply appears steady on a meter, needle reads approximately 12 VDC (both neg and pos)
The way the wiring diagram for the turnout motor describes, the two LEDs are wired as a pair in reverse with the lead going from power through the LEDs then to the motor, Depending on the polarity one of the LEDs light. When positive LED steady, when negative the other LED blinks.

 

[Pommie]

Is there a schematic anywhere for this?

Mike.

 

[KeepItSimpleStupid]

You might have two grounded power supplies which are causing some grief. i.e. The negative lead is connected to earth ground.

Put a single bi-color LED should work with what you have. e.g. Red/green 2 lead.

i think you can get away with two circuits consisting of a diode + resistor + LED across your turnstyle terminals. One LED would respond to positive voltages and the other negative.

Now, you could use one of these https://www.banggood.com/5Pcs-L298N...oard-p-948150.html?rmmds=buy&cur_warehouse=CN which would control two of your switches and a single power supply.

 

[alec_t]

The LEDs only flash when hooked up common to negative

I'm not sure how to interpret that, but if a LED is connected with its cathode more positive than its anode by more than about 5V it will break down. Perhaps that is what causes the flashing?

 

[Diver300]

I'm not sure how to interpret that, but if a LED is connected with its cathode more positive than its anode by more than about 5V it will break down. Perhaps that is what causes the flashing?

While 5 V is a safe reverse voltage for many LEDs, a lot of LEDs don't actually break down unit the voltage is 40 V or so.

I think that we are struggling without a circuit.

 

[gophert]

Could the pulse could be from the inductive kick of the motor changing directions.

 

[pugabral]

Here is the schematic I used. It shows one LED for each route off the positive lead. Instead of the 9 VDC power supplies I used 12 VDC due to the number of turnout motors I am switching.
I have a voltage meter I put in line for test purposes. It shows the higher voltage drop when the motors are running, and the more motors running at once increases this, but on either positive or negative terminals there is no wavering.
If I take a LED with a resister in circuit and apply it to the terminals positive and common it is steady on in one direction, but no light if I reverse the terminals, which is the way a diode should work. If I use the same LED/resister and touch it to the common and negative terminals it flashes, reverse the terminals no light.
My goal is a red LED on the leg of the turnout that takes the train in a different route, and a green LED when the turnout goes straight. I thought about red/green bi polar and decided not to use that. Like all hobbyist I have a control panel already configured and (can not undrill the holes) did not pretest the circuit as I now believe i should have. Obviously there is no simple way to fix this without going into small logic circuits on each negative LED lead.
By the way, I also have 5 turnouts being switched with a double pole double throw switches with a reversed pair of LEDs on one of the legs going to the motor. They work fine

 

[Diver300]

The circuit is the right way up here:- http://www.circuitous.ca/StallRoute6T.GIF

I can't see any reason why the LEDs should flash. There shouldn't be any problem putting other LEDs in the circuit. Can you draw on the circuit where you have tried them?

 

[pugabral]

Actually what I did is, on the lead going to the motor, I put a pair of LEDs wired in opposite polarity (each has a resister wired in). This switches the motor well, and in each direction the appropriate LED lights, the only problem is when the negative lead is selected the other LED blinks.

 

[Visitor]

Shouldn't the LEDs be across the motor (i.e., parallel with the motor) rather than in series with the motor?

 

[unclejed613]

LEDs are usually limited to 5v reverse voltage so 3 in series will cover your 12v supply.

when AC or negative voltage is expected, i usually put a reverse diode across the LED. to protect it from excessive reverse voltage. it's a cheap and simple solution. make sure the diode can handle twice (for red LEDs) to 6 times (for white or blue LEDs) the operating current of the LED because it's forward voltage is that much smaller than the LED's forward voltage, which will make the current through the diode higher

 

[pugabral]

I am not an electronics guy (which should be obvious) and am following a schematic I found in Model Railroader shown in my earlier posts. I am not using AC; I am using two 12 VDC @ 1 A power supplies wired with the positive of one power supply and the negative of the other connected as a common. That leaves one lead at 12 VDC positive and the other 12 VDC negative.

 

[dougy83]

From what you've said, one of your power supplies is faulty, and putting out a "flashing" voltage; the simple solution is to replace the "negative" power supply.

If you want to test this, swap the supplies, and you should see that the other LED now flashes

 

[gophert]

Ah, most power supplies go into safe mode if they are short circuited or drawing too much current. Safe mode is usually a 1-second down time followed by a slow chirp to test if it is ok to recover.. Check voltage across 0-12VDC on each supply one you have everything connected. Your power supplies may have common grounds and, therefore the common node is really a short.

 

[Diver300]

Shouldn't the LEDs be across the motor (i.e., parallel with the motor) rather than in series with the motor?

Yes, but there should be a resistor in series with the LED.

 

[pugabral]

Did that, no change and the meter reads steady voltage. Swapped LEDs, power supplies and tried battery voltage same result.