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I've bought a AC/DC adapter which per its specifications should output 5VDC and deliver 3A. I was going to use this adapter to power my BeagleBone Black (BBB) with. However, when I'm measuring the output from the adapter while it's idle (not powering anything) with a good-quality multimeter, it reads 5.24V.

The BBB System Reference Manual (Rev. C) states the following:

The board requires a regulated 5VDC +/-.25V supply at 1A. A higher current rating may be needed if capes are plugged into the expansion headers. Using a higher current power supply will not damage the board.

As one can see, my adapter outputs a voltage that's pretty close to the upper limit of 5.25V that the manual says is the maximum supplied voltage for the BBB. Is it still safe to use it to power my BBB with?

InquisitiveMonkey
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  • The concern you have has to with voltage compliance and NOT the stated ripple, which is a different thing. Compliance has to do with regulation and voltage accuracy with load. Ripple is the AC component that rides on top of your DC. Although ripple will also be a concern at such high (close to margin) voltages. – placeholder Sep 12 '14 at 02:17
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    Have you got a 5V load, so 0.5A? I'd expect the power adapter voltage to drop a bit when it is driving something. – gbulmer Sep 12 '14 at 02:53
  • @gbulmer , forgive my ignorance, but I'm rather new to electronics. I figured that a 10 Ohm, high-power ceramic resistor would get the job done by simulating a load @ 0.5A - correct? `U/R*I -> I = U/R = 5/10 = 0.5A` Nonetheless, I hooked it up and measured the voltage to be 5.03V, so given that my calculations aren't too far from reality and that my test was 'OK', you were right about the voltage to drop. But what if the BBB would be connected to the adapter and fed with 5.23V while not being powered on - could that damage it? – InquisitiveMonkey Sep 12 '14 at 03:21
  • @placeholder , understood - removed the "ripple"-tag. Thanks!:) – InquisitiveMonkey Sep 12 '14 at 03:22
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    Yes, a 10Ω resistor is exactly right. I don't understand how the BeagleBone could be connected to the adapter while not being powered on (but that might be me misunderstanding something). However, you could simulate the BeagleBone being close to "not being powered" with a much bigger resistor, say 1000Ω, or even 10,000Ω. I'd still expect the voltage to be lower than open circuit. – gbulmer Sep 12 '14 at 04:49
  • @gbulmer , let's say that I would connect the adapter's DC plug into the BBB's power jack, which automatically would power up the BBB. If I then would proceed by powering down the BBB from within its OS or by pressing the power button, but leave the power jack connected - it would be powered off but still connected to the adapter until I either unplugged the DC plug and then re-plugged it, or until I pressed the power button. Thus, it would be fed with (in my case) 5.23V while not being powered on - risks? Will try with 1000Ω and 10,000Ω tomorrow and report back! – InquisitiveMonkey Sep 13 '14 at 02:13
  • I would recommend doing the experiment to gain confidence. I haven't looked hard enough at the power supply design for the BeagleBone Black. However, Table 5, section 6.2.2 of the Rev A6 manual says "Standby Power 7mW", so I would expect there to be a small flow of power through the BeagleBones on-board power supply, and hence pull the voltage lower than 5.23V, even when it has powered down. A 3.3kΩ is close enough to simulate that. A test with 1KΩ and 10kΩ will 'bracket' it and give a bit more confidence. – gbulmer Sep 13 '14 at 02:33

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To try to get closure on this, and for my own sanity, I read Section 5.4 "Power Management" of the the BeagleBone Black System Reference Manual (BBB_SRM), Rev-C.1.

It says BBB uses two devices to supply the board with power:
TPS65217C has a Vin (Max) 5.8V
LDO TLV70233 has a Vin (Max) 5.5V
These are operating voltages.

The BBB_SRM says the "external LDOTLV70233 provides the 3.3V rail for the rest of the board."

However, searching the BeagleBone Black Schematic (BBB_SCH), Rev C, does not find the TLV70233.

There is a TL5209 on the "BeagleBone Black Power Management" schematic. It is connected to an output of the TPS65217C. It is not connected to the input power socket, and hence isn't directly constraining the maximum DC voltage input. It provides 'VDD_3V3B', so I believe it is the LDO referred to in section 5.4 of the BBB_SRM.

The text of section 5.4 is the same as the Rev-A6 BBB_SRM, so I suspect the text of the BBB_SRM is out of synch with the BBB_SCH Rev-C.

Summary @Farham is correct.
The only active part connected to the DC socket is the TPS65217C, and it has a maximum operating voltage of 5.8V. So 5.24V should be fine.

I assume the BeagleBone Black engineers specify The DC supply should be well regulated and 5V +/-.25V to provide some headroom; the BBB power management subsystem will survive voltages more than 10% over nominal.

Edit: I have raised an issue about the inconsistency of LDO part number of section 5.4 of the BBB_SRM and the "BeagleBone Black Power Management" schematic BBB_SCH at BeagleBone Black github

gbulmer
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BBB has TPS65217 power management IC whose inputs are 20V tolerant. Max Vin is 5.8V so you are good.

Farhan
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  • Thanks, I looked it up in the BBB's reference manual and it's apparently the _TPS65217C_ Power Management IC (PMIC) by Texas Instruments that's the BeagleBone Black (Rev.C)'s main PMIC and its datasheet states a maximum V(in) rated 5.8 VDC. But then why is the BBB's reference manual stating a maximum V(in) rated 5.25 VDC? I believe there is a reason, or perhaps it's just a misprint in the reference manual? – InquisitiveMonkey Sep 12 '14 at 19:32