Power supply question on Raspberry Pi

Oct 23, 2026 Last reply: 6 hours ago 2 Replies

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bp 13 hours ago

I've got a pair of Pi2Bs using externally-powered USB mechanical hard disks.

Both are self-hosting FreeBSD-current with varying levels of success and one variation might be the power supplies used with the powered hubs to run the disks.

I've placed a readme and an image of the power supply voltage trace taken directly from the powered USB hub used to connect the disks to the Pis at

formatting link

The image labeled "badpower.png" is from the machine with disk corruption problems, that labeled "goodpower.png" is from a second Pi2B that has so far exhibited no trouble of that sort.

Never having made such a measurement before I don't have a very clear notion of that's acceptable or normal. Neither trace is magnificent, in fact they're both rather noisy, with the troublesome machine slightly worse.

If anybody's made a similar measurement and can offer an opinion as to what's normal or at least tolerable I'd be grateful for a reply.

Thanks for reading,

bob prohaska

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Theo 10 hours ago

The spec is I think 5V±5%. So that's 4.75V to 5.25V. 'goodpower' has spikes of 200mV and 'badpower' of 50mV, so goodpower is actually worse. But they're both AC coupled waveforms so there's no clue what the DC voltage is. eg badpower could be 4.78V -50mV and goodpower 5.10V -200mV and the first would be out of spec and the second not. Can you measure the voltage with a meter?

Theo

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