Blew a fuse in my PC Engine... but how?

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Sumez
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Blew a fuse in my PC Engine... but how?

Post by Sumez »

I hope some of the hardware geeks in here are able to give a clear answer here...

I just hooked my PC Engine (CoreGrafX model, in an interface unit with the CD-ROM drive) up to my capture setup, which I have done a few times before.
It's running through a simple sync stripper circuit using a LM1881 chip which has its own external 5V power supply. I just measured, and it seems this 5V line is also connected to pin 8 on the SCART input, which is definitely a mistake in hindsight (I probably thought I could draw 5V from that and skip the external PSU).

While recording I accidentally knocked out the power supply for the sync stripper, but the capture kept on going for a few seconds before the console just shut off.

Opening it, I can see the fuse at the power input is blown. I can replace it easy enough, but I want to prevent this problem in the future, so I'd like to know what caused it. I assume it can be done simply by cutting the connection to pin 8, but I'm kind of confused why it would blow after *removing* the external power source?

Can anyone "Explain Like I'm Five"?
skum
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Re: Blew a fuse in my PC Engine... but how?

Post by skum »

I doubt that the fuse was blown due to it feeding the LM1881 (it draws max. 10mA according to the datasheet), unless the system was otherwise stressed to the max. What, if any, other components were on the 5V rail? It would seem you've had two 5V rails coupled in parallel, so when the one rail was removed, the other "took over" so to say.
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Sumez
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Re: Blew a fuse in my PC Engine... but how?

Post by Sumez »

No other components. It's run through the LM1881, and the output is connected to a capture device via a standard VGA plug (D-sub) which doesn't use any 5v lines - just RGB, sync, and ground.
I wasn't even splitting the signal at the time, just feeding it directly to the capture device.

The fuse was 1A, and I do know the CD drive eats a bunch of power, because my 650mA PSU wasn't enough to run CD-ROM games on my PCE, even if the base unit will run HuCards fine. But yeah, I agree, it sounds unlikely that 10mA should be enough to tip the scales.
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