low voltage transformer leakage

[benfieldcodeman]

I am looking for some help with a designed piece of equipment.

I have a 120v fed equipment with a 24v transformer inside the equipment that is used for LED indicator lights within the equipment. When 120v power is shut down ; the 24v transformer is leaking voltage to the LED indicator lights. Which in turn is keeping the LED indicator lights lit after 120v power is removed.

I think I need a in-line resistor on the 24v side of the transformer but I am not sure :

1. IF that is the proper solution to this problem
2. How to calculate the proper size resistor , if a resistor is needed
3. Where to place the resistor in the circuit to stop this leakage from occurring

Thank you everyone for your help and thoughts on assisting me in solving this problem.

Thank you,

Ryan D. Price

 

[wdemos]

Have Meter. Will travel.

 

[hbiss]

I have a 120v fed equipment with a 24v transformer inside the equipment that is used for LED indicator lights within the equipment. When 120v power is shut down ; the 24v transformer is leaking voltage...

Leaking voltage from where??? If the 120v primary is not supplied with voltage there can't be anything on the secondary so you need to look and find out where it's coming from. What switches the primary, a solid state device or a real set of contacts? Is there any way for a backfeed to occur into the 24v circuitry? A resistor to load things down would just be a band-aid.

-Hal

 

[benfieldcodeman]

Thanks Hal,

I am not sure where the back feed is coming from. The primary for the transformer is a switch type of power source. When I looked at it . Its a solid state relay that on the primary switch side. When the relay closes the transformer is engerized and the indicator circuit board is engerized.

Thank you for the thoughts. I am still just confused on the remove 120v power feed and still have the indicator lights lit. I mean I totally disconnect power at the M.C.B. ; NO POWER at all. Those lights are still lit and NEVER go off. Confusing. Kenetic energy is somewhere and is stored somewhere. I just don't know why or where.

 

[nhfire77]

There is a capacitor somewhere?
.

There is no kinetic energy stored there. They may just appear to be glowing.

 

[LarryFine]

There is a capacitor somewhere?

That was my first thought, too; a charged filter cap.

I'd try a high-value discharging resistor across it.

 

[LARMguy]

That was my first thought, too; a charged filter cap.

I'd try a high-value discharging resistor across it.

Sounds like a power supply in between the xformer and the LED's. That is where you will find the cap.

 

[SG-1]

What is the voltage input/output of the 24 volt transformer when the MCB is open ? What are you measuring with ?
The MCB could be the leak.

How far between the MCB and the equipment ?

Are the LEDs at full brightness or just barely on ?

 

[dbuckley]

... the 24v transformer is leaking voltage to the LED indicator lights. Which in turn is keeping the LED indicator lights lit after 120v power is removed.

I don't like that "keeping" word.

Keeping the indicators lit for a few seconds? Half a minute? All day?

A cap in the PSU may keep the supply up for a short while, but if the LEDs stay on all day then thats not a cap in a PSU; there is real power being supplied.

 

[LarryFine]

One more note: I've noticed that some LED's appear to be on when room light is hitting them. Try cupping the light with a hand, or turn off the room lights, just to be sure.

 

[hbiss]

I don't think anybody directly said it but LEDs are DC devices so they won't be connected to the 24 volts AC directly. Normally there would be a DC rectifier with possibly a filter capacitor to make nice clean DC. If that capacitor is large enough it could hold enough to power the LEDs for awhile after power is removed. But eventually it's going to run down as has been said, if that's your problem.

-Hal

 

[SG-1]

Here are some LED indicator lights that operate directly from AC voltage.

http://www.automationdirect.com/adc...E22_Series)/Non-Metallic_Monoblock_Indicators


I suspect the rectifier is built into the bulb. The LED bulbs are interchangable between AC & DC circuits. They look like an LED built into a bayonet mount style bulb.

The AC indicator lights are very susceptible to capactive coupling. Only takes about 2 volts to make them glow.