# Led Help!

Hi Guys, had an operation recently so I’m off work for a little while. Started a little project to keep me occupied until I can get back into it. But I’m stuck trying to figure something out.

I have these fans here in my apartment with a led light pcb. The company only offers a cool white light, which is think too harsh for my bedroom & lounge room.

So I thought maybe I could swap out the leds for a warmer white. Im having some trouble figuring out a how the circuit is wired without destroying the thing, I’m thinking it’s there’s 2 series circuits of 24 leds in parallel with each other. Was hoping someone more accustom to these can confirm this just from looking at the photos. So I can figure out which new leds to purchase to match the existing driver.

White LEDs usually have a forward voltage drop of 3.0 V to 3.6 V.
The power supply is a 0.43 A constant current supply with a 18 V to 36 V range.

Therefore the number of LEDs in series that can be driven by this supply is:
Maximum (36 V) = 10 to 12
Minimum (18 V) = 5 to 6

Assuming a smart designer, there will be from 6 to 10 LEDs in series. A really smart designer would make it 8 LEDs to be dead center in the range.

I’d use an ohm meter to figure out the exact LED PCB wiring configuration. It’s tedious and you’ll want to double check your results but I believe that is the best way to figure out how many are in series.

Assuming 8 LEDs in series, you’ll want LEDs that work well with 0.0717 A drive current (0.43 A / 6).

PaulHutch is right on what it ought to be, if they were a smart, careful, and cautious designer.

However, I’m pretty sure, based on the landscape of the copper/no copper regions (raised and not raised regions), that there is an inner and outer current path and that every two LEDs in the inner and outer rings are connected in parallel. Their anodes are tied together and their cathodes are tied together. Then the next two are again connected in parallel with anodes and cathodes tied together. This continues around the ring.

The net effect is 12 voltage nodes from +Vcc to -Vcc, leaving no more than 3V for each node. In each of the two strings, there are two parallel LEDs at each voltage node. So, each string takes 215mA and each LED takes about 107.5mA.

Are you planning on desoldering each LED and replacing with a warmer one? If so, I’d be very careful that you get each one done perfectly. Assuming my assessment of the schematic is right, if only one of any pair of LEDs is properly soldered down, that one in the pair will draw all of the current (215mA rather than 107.5mA), which could well burn it out. Also, if you happen to short out one of the LEDs, then the normal voltage drop across that whole string will be nearly 3V lower than the other string. That will cause almost all of the current to pass through only the string with the shorted LED, which could burn out all of the rest in that string.

Hi Paul, yes that was what I was the answer I was leaning towards, but it did not make sense compared to the layout of in the pcb.
Testing the components individually seems to be impossible as there is a clear insulating film over the whole board, minus the leds face as to not trap excess heat in I would assume.

Hey David, yes my best guess from looking at the pcb was that the raised sections was the conductive path, and then from there was where I was confused. Your explanation has helped a lot, although I still don’t understand it fully

I have found a local PCB manufacturer close to me as I thought it might be a difficult, finicky job better suited to a person with professional equipment. The manager said it would be no problem and quoted a pretty reasonable price to swap them out, but I would need to supply the leds or a part number for them.

The dimensions of the leds seem to be pretty stock standard.
I will have a look through the net to see if I can find any to match the electrical specs.
Thanks guys!

Standard sewing needles or pins used as DMM probes will usually allow you to poke through most insulating films to get a reading.
Since it’s low voltage in a dry location I wouldn’t worry about the left over prick holes, but if concerned you can cover the holes with a dot of clear/white conformal coating or electrical varnish.

That’s a sweet tip, thanks Paul. Should be able to figure out the Vf easily with this method.