Anyone ever try to convert Fanuc 6M indicator lights to LEDs?

CevinMoses · Apr 19, 2018

I have 4 Amada turret presses with Fanuc 6M controls on them. Each one take several 388 mini lamps to light up the buttons to indicate that they are turned on, but the lamps are a pain to replace when they burn out. I am trying to replace them with LED lamps I found through McMaster-Carr, but the lamps don't light up because the polarity of the lighted switch is opposite. With DC power, LED's need the polarity a certain way. I reversed the wiring on all of the buttons (10 I think), but now all of the button lights are on all the time. The illumination circuit of the buttons have 10VDC to ground when not pushed in, and 24VDC when pushed. Obviously I'm getting voltage from somewhere, but without a wiring diagram for this panel, it's a lot of guess work. Just wondering if anyone had tried this before, and if they were able to make it work.

ewlsey · Apr 19, 2018

You are likely just seeing some ghost voltage (induced voltage). Check the voltage at the light bulb when the bulb is plugged in. You should see it more like 24 and 0.

You might be able to get it to work by wiring in a pull down resistor in parallel with the LED. That would provide enough load to short the induced voltage with no affect on the LED. It might take some trial and error to size it.

NAST555 · Apr 19, 2018

From my bit of messing around with electronics you might have to wire a resistor on the positive side of the LED as ewlsey said.

vmipacman · Apr 19, 2018

what McMaster pn are you using? presumably it already has a resistor wired in for the 24V supply. It would burn up if it did not.

Are you actually measuring your voltages to ground, or are you just putting the multimeter on the + and - in the bulb socket?

Try putting the original bulbs back in. Do they work exactly as before or are then now changing from dim to bright? If it was me and I found this to be true I would probably just put things back and run on the incandescents. If the originals now change from dim to bright then you caused a change with your polarity reversal. Was there only one wire to each side of the lamp socket? could be that a second wire is daisy chained off and you missed swapping that one.

As said I guess it could be an induced voltage, but in that case it would show as a voltage on your multimeter, but not carry any current (or not enough the turn on even a small bulb). it would drop to 0 once a load was applied. I think the circuit in the LED would simulate enough of a load as to not light off an "induced voltage". My guess is that the incandescents will now light dimly and either they have always been that way and you never noticed it or you missed something with the polarity swap.

CevinMoses · Apr 19, 2018

Thanks for the suggestion. I'm assuming I would have wire in a resistor with each individual lamp (10 per machine). How big of a resistor should I try? The 24VDC lamps are rated at .01A.

CevinMoses · Apr 19, 2018

vmipacman said:
what McMaster pn are you using? presumably it already has a resistor wired in for the 24V supply. It would burn up if it did not.

Are you actually measuring your voltages to ground, or are you just putting the multimeter on the + and - in the bulb socket?

Try putting the original bulbs back in. Do they work exactly as before or are then now changing from dim to bright? If it was me and I found this to be true I would probably just put things back and run on the incandescents. If the originals now change from dim to bright then you caused a change with your polarity reversal. Was there only one wire to each side of the lamp socket? could be that a second wire is daisy chained off and you missed swapping that one.

As said I guess it could be an induced voltage, but in that case it would show as a voltage on your multimeter, but not carry any current (or not enough the turn on even a small bulb). it would drop to 0 once a load was applied. I think the circuit in the LED would simulate enough of a load as to not light off an "induced voltage". My guess is that the incandescents will now light dimly and either they have always been that way and you never noticed it or you missed something with the polarity swap.

Thanks for the questions.
McMaster-Carr p/n: 6570T972
I'm measuring between the positive side of the bulb and ground.
I had left one original bulb in when I first did it, and it continued to function normally. I removed it and put in and LED, and then it was illuminated all the time.
All sockets share a common wire daisy chained between them and an individually numbered wire to the other side. It's that individually numbered wire that I am seeing voltage to ground on. Each one of them shows voltage potential.
I checked one of the other machines that I haven't modified at all, and they both show 2.2 volts to ground on those numbered wires.
Possible that I missed something on the polarity swap, but odd that I'm seeing it on different numbered wires that should not be common. I will try removing all of the bulbs and checking the voltages, as well as tracing back the wires to see if I missed something.

Scottl · Apr 19, 2018

I think the reason all the lights come on when you reversed the wires might be that there was a common feed and the lights illuminated when the other side was pulled "low". This might be a lamp driver, as usually on control circuits the button is an input and the lamp is driven by an output confirming the action occurred. There may be a secondary path that does not provide enough current to light the incandescents. Perhaps there is a warming current to reduce inrush when the bulb is illuminated.

I would suggest trying to use the base of one of the burnt out bulbs as a connector to reverse wire one of the LED lamps. If it works normally you need to obtain reverse wired equivalents of the ones you bought. Usually there is a manufacturer and model number somewhere and the correct one would be a slight variant of the part number. If there is no such information ask McMaster to supply technical info. It might then require a call to the manufacturer but then you could buy the correct LEDs or at least have enough info to identify ones from another source.

EDIT: I just took a look at the McMaster part and that is an ac/dc 24-28v lamp. I'm very puzzled that they don't work without changing any wires.

andrewmawson · Apr 19, 2018

I have replaced illuminated switches on my Fanuc Tapecut Model M which has a Fanuc 6 M control.

The problem that I had wasn't polarity, it was that the LEDS were always ON - which I traced to the fact that Fanuc keep a small current flowing through incandescent bulbs to keep the filaments warm but not glowing, as this increases their reliability vastly.

It is just a resistor to the supply for each LED, which I removed on the relevant buttons (not all are LED yet)

Scottl · Apr 19, 2018

andrewmawson said:
I have replaced illuminated switches on my Fanuc Tapecut Model M which has a Fanuc 6 M control.

The problem that I had wasn't polarity, it was that the LEDS were always ON - which I traced to the fact that Fanuc keep a small current flowing through incandescent bulbs to keep the filaments warm but not glowing, as this increases their reliability vastly.

It is just a resistor to the supply for each LED, which I removed on the relevant buttons (not all are LED yet)

That's exactly what I suspected based on the voltages he measured. With the switch cap off in a pitch black room you might see just a tiny bit of orange at the incandescent bulb filaments. Without the heater you'd be replacing them all the time.

CevinMoses · Apr 19, 2018

andrewmawson said:
I have replaced illuminated switches on my Fanuc Tapecut Model M which has a Fanuc 6 M control.

The problem that I had wasn't polarity, it was that the LEDS were always ON - which I traced to the fact that Fanuc keep a small current flowing through incandescent bulbs to keep the filaments warm but not glowing, as this increases their reliability vastly.

It is just a resistor to the supply for each LED, which I removed on the relevant buttons (not all are LED yet)

That is fascinating! Thanks for the insight. So you removed a resistor that's wired into the supply for each lighted button? There is a board that looks like a rack of resistors that I had to unscrew and move out of the way in order to reach the switches. I haven't traced the wiring to see if they are in the same circuits, but their proximity to the switches would make sense then. I'm not sure how that circuit to keep the bulbs warm would work, then. Are these the resistors you removed?

andrewmawson · Apr 19, 2018

I suspect so, it was a long time ago.

Back in th 1940's Tommy Flowers knew that keeping thermionic valves warm enhanced their reliability, and he used this knowledge to build the first computer, the one that decoded Enigma and its successors. His bosses in the GPO didn't believe him and he used his own money to build the prototype. Despite others claiming to Be the first computer makers, Poor old Tommy was actually the first, but it was keep secret for years.

We ALL owe him a big debit of gratitude

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