What happened to my optical splitter?

[sfbp]

A few months ago I paid rather too much for a device no one else could sell me but whoever it was on eBay, to take one optical signal and ACTIVELY (as opposed to the passive type of non-powered gadget) split the signal into 2 optical outputs.

Unfortunately, I managed to connect it to the wrong power supply (never mind why, but it never occurred to me that the difference between 5V and 6V would make a difference) and after working perfectly when it first arrived, is essentially useless. The LED you see in the pic comes on but rather dimly.

As there appear to be only two components, R1 and C1, could some electronics whiz please tell me likely values. You can see the circuit is blindingly simple. Or did I blow up the diode itself?

Your suggestions appreciated.

Thank you

 

[NGY]

I am not a whiz :-), but here are some assumptions  - based on what I see on the photo (would be good to see the other side of the panel too, to confirm/deny some of them):

- the sole resistor is the current limiter for the power LED, and if such, should have a value somewhere around 470 Ohm (300 ... 1K, or so), the writing on it would tell, but I cannot read it.

- the capacitor is filtering out high frequency noise, I guess it is somewhere around 1...100 nF, but pretty sure it is not damaged (neither did the resistor above)

- the output signal of the receiver is directly connected to the inputs of the two transmitters, and power is directly connected to the Vcc and GND terminals of the three devices.

- these opto devices have some built in electronics besides the emitting LED and receiving photodiode (-transistor). There are (older) 3.3V devices, and also 5V (or 3...5V) devices, this particular splitter uses 5V ones.

- datasheets of such 5V modules mostly show 6 Volts as absolute maximum rating for supply power.

- your (nominal) 6V power adapter is probably an unregulated one, with a bulky transformer inside, and a simple rectifier. As the exact output voltage of such adapters depends on the load (we discussed it in another thread), with this splitter draining maybe ~20...30 mAmps only, around 7...9 Volts could probably hit the device, that could blow one or more of the optical modules.

-  if just one module is blown, can internally short the power leads, resulting the dim power LED, and the two others not able to operate.

But these are only my assumptions, you may want to do some checks with a DMM.

I saw similar devices on ebay, for about 40 bucks or so. Individual modules cost around 8...10 dollars, maybe somewhat cheaper from Far-East suppliers. Or, maybe the best, they can be digged out from old junk audio-video devices, to repair yours. But once you do that, it would make sense to build a 7805 voltage regulator between the jack and the modules, the panel/box size allows it. 

[NGY]

2 hours ago, NGY said:

modules cost around 8...10 dollars, maybe somewhat cheaper from Far-East suppliers

Just checked on ebay: receiver, transmitter, $2,99 each + $4 shipping

[sfbp]

I have some transmitters (and at least one receiver or receiver/transmitter) almost identical to the ones you show. The problem with all is the largeish lug on the top of them which won't fit nicely inside the case.

Probably I used 9V by mistake, then, I really don't remember. The damage was done before I realised.

I was quite ticked when I realised that swapping the powersupplies (I purchased a couple of similar things and they arrived that day) didn't work. Yes, I know it was my own silly fault. I measure 9 ohms 500 ohms across R1, but of course that's without removing it hahaha.I cannot read the writing either but it sounds like if blown I need to remove it and substitute a "normal" resistor between (?) the plus of the diode and ground, correct? Or is it still one or more of the modules got blown?

Update: I was using the MM before my first coffee. Sorry. So your 470 is right on the money.

And there are no non-infinite resistances between the pins of any of the TosLink devices. Or C1.

 

[NGY]

1 hour ago, sfbp said:

I have some transmitters (and at least one receiver or receiver/transmitter) almost identical to the ones you show. The problem with all is the largeish lug on the top of them which won't fit nicely inside the case.

...

I measure 9 ohms across R1, but of course that's without removing it hahaha.I cannot read the writing either but it sounds like if blown I need to remove it and substitute a "normal" resistor between (?) the plus of the diode and ground, correct?

...

Or is it still one or more of the modules got blown?

You can simply cut off and file flat those lugs, they only there to hold a screw.

I would not expect R1 to be blown. Red leds operate at 1,9...2,1 Volts, and, must have a current drive somewhere between 5...20 milliAmps. I took the resistor value above from these figures (Ohms law). Worst case (at highest LED illumination/current, that is not probable) it would be 150 Ohm, and when the resistor/LED path gets 9 Volts, the current running thru the resistor is still only ~47 mA, and the wattage load on the resistor is 0,33 W - even if it was only a 0,25 W resistor, still would not burn. And that the LED is still lit, shows the resistor is still conducting.

The 9 Ohm you measured worries me more. It would be a better way to first desolder the anode of the LED (the one that connects to the resistor - the resistor's other lead connects to V+, while the cathode of the LED is connected to GND, the large surface on the back of the board), or just remove the whole LED. If it is still 9 Ohms on the resistor, then yes, the resistor got a damage, but the LED did too, because the lower the resistor value gets, the brigher the LED is supposed be, and a 9 Ohms resistor would allow ~300 milliAmps on the LED (at 9V Vcc), and these simple ones cannot bear that for long. But I would expect more than 9 ohms then.

Anyhow, once you desoldered the LED, connect your 5V adapter, and check the voltage on the jack's terminals on the board. You should see very closely the same voltage when you just measure the adapter itself, unplugged from the board. If you are able to measure mA, then connect your DMM in series of the splitter and the power adapter, to see, how much current it draws, and if everything is in order, I would expect around 50 mA at max. But if you see several hundreds mA, then one or more of the modules are blown. Maybe not all, by trial and error you can desolder one, repeat the mA measure, until it shows only a couple of ten mAmps.

I know I am still assuming a lot. But this is the case when someone does such troubleshooting remotely :-) .

[sfbp]

I'm so sorry to have you spinning your wheels over my error, look at my update.

[NGY]

No worries. I was doing a couple of things at the same time, this window was open for quite a while, as I could only get to it for short times, and of course the page was not refreshed.

I would now check the current the device draws, as above.

[sfbp]

16.5 mA provided I read the DVM correctly. LED is lit.

[NGY]

And this is the total current the box draws, is it? It does not look bad, but is a somewhat low value.

This shows, that even if a module is blown, it is not a short circuit inside, it just smoked the silicon away. But as your LED draws about 6 mA, the rest is probably drawn by only one single module. 

Now, if you replace just the receiver, giving a chance that not both of the transmitters were blown. Then (1) if the current increases, the receiver was also burnt, and (2) you should get an output signal at least on one of transmitters. Should you not, then they are probably both gone.

[sfbp]

I agree. I just rechecked and with a proper input connected there is no sign of life from EITHER transmitter. So that would suggest the least number of moves would be to start by replacing the receiver. Now the bad news, I only have transmitters. So I will have to wait until I can get a receiver. Not to act like a spoiled kid, really I would like to get the ones with no lug on top.

Thanks. This was exactly the sort of handholding I needed.