LED Bulb Replacement In A Sign

The system I worked on (circa 1979), used Triacs to switch the current in each light bulb.

It could be leakage current of the triac that allows the LED to flicker. Have you checked this out with an oscilloscope?

I don't think a MOV would work across the LED, as this would short the bulb out once the switch was turned on.

I'm guessing your wanting some simple fix that does not involve a 100% redesign.

It could be a dv/dt transients turning on the triac for a half cycle. Check the specifications of your switch device, for leakage and if inductive noise dv/dt could cause inadvertent turn on.

But I'm guessing the switch is a Triac.

by the time I got to the sign , it was late in the day and didn't have the opportunity to go through the whole controller... but I suspect your right.and no I didn't have time to scope it, but w/a fluke DMM you can see the residual voltage (rise 120v off 6-8v to neut@ bulb, 12-14 to ground) The customer is hesitant so far about a major amount of work,but is open to options for extending the life of this unit. apparently the controller is no longer made/serviced by orig mfg...they've been bought out and gone. I suppose I could put a small relay set in between the controller and bulbs to mitigate the residual voltage effect but I was looking for something a little easier to implement from 25ft up on a lift...

You didn't indicate if these were Triac switches.

Thinking about this after I posted a reply, is it possible there is cross talk noise (that has always been there from day one), of the various gate control signals that is triggering the triacs randomly on?. Do the gate circuits have Diacs in series to prevent false triggering?

How are you measuring the neutral residual voltage, i.e. between what ground and the neutral is the meter reading? If it is the controller ground and neutral, that could cause enough voltage between the triac terminal and it's gate to trigger it for a half cycle.

Are these replacement LED's configured in a medium base(or other size),screw in replacement for the incandescent bulbs?

Seems that would be the simplest method.

Check voltage between neutral and ground.

Should be zero volts.

Is the sign chassis bonded and grounded properly?

1) Try a mains filter, some IEC sockets for example have them built in.....easy to do. I recently repaired a Knitting machine with such a unit. Cheap to buy at around $5 for a 7 amp version. They go to 15 amps I believe....

2) I would re-bond earth and neutral (illegally probably).

3) try ferrite rings on signal leads.....

4) use a 1:1 transformer to supply the mains to the device, but isolated from the mains......!that should get rid of transients...

As the bulbs are now LEDs and they are not all on together, to be safe, a transformer that can handle 130 x 4 = 520 Watts at 110VAC = around 5 amps.....even with the controller....

Boy was this sign wasting power before!!!

I am no electrical genius, but the resister is what I was thinking. Only I was thinking about a shinny gold one with stripes on it.

By 'slightly flicker', I presume you mean that those lamps that are supposedly in the 'off' condition still glow slightly.

Take a look at the thread "LED Light Bulb" of Jan & Feb 2013, and especially my post #61 of 2/7/13.

I haven't yet tried it, but since the current is only a few microAmperes, I suspect that a resistor of something like a MegOhm soldered across the base of each lamp would extinguish the light.

Might be it can be done to change the drive from "High" side to "Low" side switching.

Connect all LED anodes to mains voltage. As the power is now only around 600W, use a 2KW solid state relay as a switch between all cathodes and ground. Even if the SSR has a snubber circuit - which causes some leakage - this will then be distributed over all 130 LEDs and for sure less visible.

The controller can then be replaced by a simple digital ON /OFF timer with sufficient output to drive the LED in the SSR via a simple resistor.

You must have an old AC operated incandescent devices that were now replaced with LEDs... As LEDs are normally a DC operated devices, when operated through AC has a tendency to flicker @ 60 hertz..

Be sure your power supply has been properly converted to a filtered DC !

This is not much of a problem, and can be fixed in various ways, as some have outlined, but when working with industry, the cleanest way we found was to insert an interposing relay into the circuit. the input to the relay is the output normally going to the lamps and the output of the relay is the output to the lamps. As the voltage drops almost to zero the relay will drop out and isolate the lamps to truly turn off and the problem will go away. What I have found lately is that many of the 120 volt led bulbs are not 120 volt at all but have an internal circuit that regulate current and will work anywhere from a 9 volt input to 265 volts AC or DC. These are made in china and are not dimmable. when the voltage is below 9 volts the circuit stores energy and discharges it to the LED when enough energy is stored, and then the bulb "flickers" the rate depending on the amound of energy available in the "off" condition. The relay solves the problem.

It would take major effort and space and cost to add 130 relays!

In case you did not notice, this is a time/temperature display. If the lamps are arranged and connected as 4 7-segment displays with a colon, he might be able to get by with only 29 relays, but even that is a pretty major addition.

There is probably a major terminal block joining the controller to the lamps. It should be cheap and fairly simple to add a high value resistor across the terminals for each lamp or group of lamps.

"If the lamps are arranged and connected as 4 7-segment displays with a colon"

Since the OP did not specify how the 130 lamps were configured, all we can do is just speculate! It is also very possible that every bulb is configured to represent a degree in temperature? Similar to the tall "Thermometer" that can be seen along the I-15 between LA and Las Vegas. (another guess /speculation)

Instead of the troublesome electro-mechanical relays being switched, a more stable, faster responding electronic device can be used instead. Devices such as; SCR, Triac, or Solid State relays, all of which can be contained in one small package together with the sensors.

Sorry,out of town working, haven't had time to check on this.... I was originally thinking of relays, precisely for that issue..cant flicker if there is no voltage. these are all led replacement bulbs and they keep trying to drive even as the voltage falls away. So it seems the simplest but time consuming addition would be relays, although I will check the burden resistor idea just to see how it works. This customer (and another one with a slightly larger sign) are facing ultimatums as I suspect many are of "If you can fix it, you can keep it" due to the many changes in local sign codes involving sign size, location etc. Thanks to all for the suggestions!!

Try an isolation transformer, far less work.....customer pays.....sell it on safety as well!!!

Transformer can be located elsewhere.....need not touch the sign at all......ground one side of the secondary, add an RCD (or whatever they are called in English) for safety AFTER the transformer.

I don't know how many "outputs" you have, but I have always found relays expensive - $20 to $100 each (more with base and mounting - and where to mount them. The resistors are about $0.10 each, and usually easy to mount. They are also limited in the number of cycles, and if the sign is flashing once per second you will over 2 million operations in a month, and that is around the life expectancy of many relays.

If you can identify the output circuit, you may try eliminating the "leaking" components, but that is getting closer to "brain" surgery - you still want to ensure adequate circuit protection with the new lamp load you have switched to.

What type of base does the old incandescent bulb have?

Is it a screw-in type bulb?

What type LED replacement are you using?

More information is needed to give you a valid solution to your problem.

Surely you are not the first one to encounter this problem,so the answer is out there somewhere,maybe here if enough information is provided.

Current setup is Med base screw in existing bulbs were 30w 120v incandescent... replacements are 4 w LED there are individual outputs for each bulbs w/ common neutral Sign appears to be grounded by virtue of existing ground wire up on sign, but have not had opportunity to open base of sign to verify prescence of actual ground

130 relays? that would be$%^ expensive and time consuming to wire up.

Try the burden networks first.

I still say the isolation transformer, just for the sign itself, properly grounded on one side of the secondary and tested in a small way first, is probably the simplest method to fix the problem......can be implemented well away from the sign, just where the mains comes from....Easy Peasy.....

Customer pays and a transformer capable of supplying around 5 amps at mains voltage should handle it easily....say 600 watt or so.....ebay maybe?

It won't fix the problem.

Now explain why!!

Thanks in advance.

The output stage is typically a triac with an RC snubber circuit around it. The other alternative is a diode bridge with single transistor across two arms, still acting as a switch. In my experience the RC snubber circuit "leaks" enough current to illuminate an LED. Although the current is minimal, the only path is through the LED, hence light.

please excuse the "keyboard circuit"

---(R)---|c|--->|--

series RC in series with LED - triac in parallel with RC not shown since it is "off"

now if we add a burden such as :

---(R)---|c|------>|-----

..................|................|

...................|---(RL)----|

The (RL) now shunts the leakage current past the LED.

an RC burden also works.

Isolating the power system does not change the characteristics of the output circuit.

Code requirements in Canada and USA strongly urge you to hard ground these systems and not "float" them.

If thats the circuit, you could be right!

Are you replacing them with a med base screw in LED substitute?

If so, what brand and part number.

Some types are dimmable, some are not.

It is possible that you have a dimmable type,which will respond at low voltages.

If so, change to a non dimmable type bulb(Cheaper,too.)

Let me know.

A second thought:

With the present theft of copper from any source, including power poles,the loss of the ground wire between the sign and ground is a distinct possibility.They have even taken the ground wire from power poles ,tied them to trucks,and pulled them loose as far up the pole as possible.

Perform a good visual of the entire grounding system just for good measure,all the way out to your service pole.

yes they are med base LED substitute... as I am helping on this I will have to check to see if they are dimmable. (currently in Iowa and don't seem to be able to find the bulb number in my case...) I had also thought about the dimmable aspect but considered if I could reliably "shut off" ie zero power that characteristic would be a mute point

You are right about that.

Zero volts=Off,no matter what.

As mentioned elsewhere,the residual voltage may be a characteristic of the control module that will not be economically remedied.

I would still double check my ground wiring all the way to the service pole that provides power the sign.

customer understands that "economically" is kind of a misnomer... but as compared to removing /all the hassle and expense of the new sign, if they even let them replace it at all, ie permits ,studies and village(idiots) goverment whims, most anything short of putting a little guy w/ a bunch of pushbuttons and cords inside the sign may be cheaper....as long as it works

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At one time our Traffic Engineering dept. was using a thin resister disc under the light bulbs in the traffic signal device. It was explained to me that the added resistance would lower the voltage to the bulbs, and extend their life. Those bulbs were 87 watt I think. Would they have any value on the 4 watt LEDs?

Were those disks really resistors, or as I suspect, diodes?

I just don't know. I was fixing the trucks that were used to repair traffic lights. The driver gave me a few, and told me to use them under any light bulbs that I wanted to last longer. As I recall, the bulb seemed to reach full brightness slower with them in place. I may still have one, but where is hard to say. I do have the # of the man that gave them to me though. I recall them as being red rubber, with a dark grey center. Is it worth a call to him?

Not really, although according to your description of "the bulb seemed to reach full brightness slower with them in place." is interesting. That makes them sound like some form of soft starter. Since it is well known that incandescent bulbs almost always burn out at turn-on, a device that lowered the initial current would definitely make the bulbs last longer.

The disks I remember were indeed (I was led to believe) diodes, which of course only allow the current to flow half the time, so the bulbs would never reach the normal full brightness. Reducing the average current to half would lower the temperature of the filament, thereby reducing the sublimation rate of the filament. I think I still have one somewhere. When I get home next week, I'll see if I can find it and check it out.

A simple resistor would also prevent the filament from reaching full brightness, but a temperature-dependent resistor whose resistance drops with increasing temperature (a thermistor) could prevent the initial surge with high initial resistance, but then drop to a lower resistance when it heats up, and only reduce the brightness a little.

They used a NTC thermistor,(A Negative Temperature Coefficient ). This limited the current(High Resistance) when cold,and decreased resistance when hot.

Very simple.

Bulbs were marginally dimmer, but not noticeably so,because there was always a certain amount of resistance between the lamp and the line.

And besides, if that is the plan, they can always use a slightly brighter bulb when installing the "disc".