My OSSC is being powered by 5V coming from HDMI

[Tony359]

Hello there – I’ve got a new HDMI capture device today, it’s one of those cheap ones based on the MS2130 (works really well though) and I’ve discovered the OSSC is being powered by the HDMI cable! If I unplug the 5V PSU, the OSSC stays on.

I’d imagine a CAPTURE DEVICE shouldn’t have 5V, 5V should be supplied by sources, not receivers?

But it is what it is – can I damage the OSSC this way? Is this somehow expected?

Thanks!

[BuckoA51]

🤣 HDMI truly is the successor to SCART isn’t it.

There shouldn’t be any damage (though Markus would be more qualified to answer that) but sync devices should only be outputting a few mA, I’m surprised there’s enough to actually power the device, that seems like a badly designed sync.

 

[Tony359]

When I flipped the switch and nothing happened, I was puzzled, I blamed the switch and unplugged the PSU and after a few seconds the screen started dimming.
I’d imagine maybe there is some protection in the dongle and power was cut because the OSSC was using too much power?

I wonder if adding a diode would be a viable option. Mostly because I tend to power-cycle the OSSC to put the test pattern back on screen so I know my chain is working 🙂

 

• This reply was modified 1 month, 2 weeks ago by Tony359.

[Tony359]

Can I bump this thread please, the OSSC is 100% being powered by my HDMI capture device! 😀 Is it ok to fit a small diode on the 5V line of the output?

Thanks!

[marqs]

The polyfuse should trip easily if powered only via HDMI. You could add a diode but a better solution would be to just disconnect the pin on the capture device side as it clearly doesn’t need it

[Tony359]

Thanks @marqs

I’ve unintentionally left the OSSC powered via HDMI for a little while the other day and it stayed powered. The polyfuse you mentioned is F2?

The capture device is a dongle, I’d rather not try to disassemble it. What would be the downside of adding a diode – or removing F2 entirely? Isn’t that 5V used as “hot-plug” signal to tell the receiver there is something plugged in?

[marqs]

Sure, you could add a diode to work around the issue if you’re familiar with SMD soldering. My point mainly was that it’d always better to fix the root cause, but it’s understandable if you want to avoid opening a plastic dongle that doesn’t have screws.

[Tony359]

Ok so with “disconnect the pin on the capture device” you meant opening the dongle – it’s actually a very good dongle I found online for not much and I’d rather leave it alone. 🙂

I think a diode would be best – what is the purpose of that 5V coming from OSSC? Is that to signal the receiver that an HDMI cable has been plugged?

Thanks!

[arebokert]

I have the same issue, also with a MS2130 based capture card. I’ve been searching around about 5V backfeed and there seems to be a lot of different opinions on how bad it actually is in general to the source device. Seems to depend a lot on the circuit that is getting backfeed in to, naturally. As Bucko said, maybe Markus could chime in here? I am personally using the capture card specifically for the OSSC so that is pretty much the only device I care about.

Let’s say I were to modify the capture card, would perhaps cutting the 5V trace entirely work? Or could that destroy the handshake process? Trying to fit a diode in the very small housing of the capture card seems a lot harder than just cutting the pin or trace.

EDIT: Should I take it as long as the fuse does not blow, the OSSC is virtually unharmed by the backfeed?

• This reply was modified 2 weeks, 6 days ago by arebokert.

[marqs]

The DDC5V line is supposed to power circuitry in receiver which provides EDID so that transmitter can read it even if receiver is otherwise powered off. Neither OSSC or OSSC Pro relies on this as they don’t generally use the EDID. However, hot-plug assertion mechanism may also be reliant on the DDC5V in some receivers, so removing the fuse altogether is likely to result in total incompatibilty with some other displays. If modifying the receiver is not an option, then next best thing is to replace the SMD fuse with same-sized SMD diode, or preferably putting it in series with it. Leaving it as-is should be also relatively safe if 5V on receiver is well regulated.

[Tony359]

Thanks @marqs

@arebokert

I’ve now modded my OSSC, I’ve stuck together the fuse and a shottky diode I had in my stash. It works fine – and 5V output is still 5V which is weird as the diode has a 0.15V voltage drop, I thought I read 5.0 before the mod so I am puzzled I now read 5.0 after the mod 😀

In any case I also tested WITHOUT F2 (empty pads) and the dongle I am using (branded Hagibis if I remember right) had no issues displaying an image. So I’d imagine that removing the fuse altogether might work if nothing else needs to be used as a display.

I’ll publish a short video where I show the mod, with the admin permission I’ll post the link here.

[Author]

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