Power connection question

Discuss KerberosSDR - 4x Coherent RTL-SDR
rtlsdrblog
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Re: Power connection question

Post by rtlsdrblog » Sat May 25, 2019 5:48 am

Stromeko wrote:
Fri May 24, 2019 6:36 pm
USB-A doesn't (officially) allow for 3A, so most PSU only supply a maximum of 2.4A per port (I have a bunch of those). The obvious non-USB option would have been a barrel plug, but I recognize that these may be more difficult to fit onto the board unless you're going to solder them separately.

Again, finding a supply that just puts out 5V/3A isn't a problem, and a passive adapter to USB-C should then just work. Beware of active adaptors, though; those have switches integrated into the plug that would again remove VBUS if the USB-C end is not properly terminated. I was specifically hoping to use a supply that has a USB-C 3A and three or four USB-A 2.4A outlets, but since that is unlikely to work with the Kerberos I'll have to switch to using multiple PSU for now.
A 2.4A supply is sufficient for the Kerberos power supply, it only really needs a little more than 1A. But for the last few years i've only come across 5V/3A USB-A supplies, never seen a lower one.

The 3A power concern is mostly for the Pi3, which certainly needs the full 3A. 2.4A won't work for the Pi3.

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kb3cs
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Re: Power connection question

Post by kb3cs » Sun May 26, 2019 1:04 pm

interesting, as i have a Pi3B+ and have yet to observe it pull more than 2A with any of the workloads i have presented it so far.

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Re: Power connection question

Post by rtlsdrblog » Sun May 26, 2019 10:06 pm

kb3cs wrote:
Sun May 26, 2019 1:04 pm
interesting, as i have a Pi3B+ and have yet to observe it pull more than 2A with any of the workloads i have presented it so far.
If the Pi3 doesn't get enough power, but still enough to run, it will simply throttle down the CPU. That makes the update rate of the Kerberos software very slow.

If I use even a 2.4A supply with the Pi3, Raspbian displays the yellow lightning symbol on the top right, and it will never throttle up.

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kb3cs
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Re: Power connection question

Post by kb3cs » Sun May 26, 2019 11:33 pm

have not observed any throttling. i will then propose the current meter i have is smoothing the readings. power source is a RAVPower 4-USB supply and has proved to be an improvement over the 'official' Raspi power supply. also using a short USB cable to the Raspi.

Stromeko
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Re: Power connection question

Post by Stromeko » Sat Jun 01, 2019 3:54 pm

The amperage on the PSU is only a tangential indicator of how good the regulation is under higher load. The problem really is that the Pi needs a stable voltage and the resistance in the supply will create a large enough voltage drop to make it unhappy if the unloaded voltage (at the PSU end) starts out below 5.1V. On the other hand the voltage shouldn't go above 5.25V really, so that's not an easy target. Just measure the output voltage of the PSU under no load and at around 1A to figure out the load characteristic, if you can also measure the voltage at the PSU and and the Pi you'll also be able to determine the resistance from the cable/connectors. You'll want to get that to around 100mOhm, but you'll find that it is often three or four times that value. Do not connect/disconnect the microUSB end under load if you can help it, these miniature connectors degrade quickly under that stress.

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Re: Power connection question

Post by Stromeko » Sat Jun 01, 2019 4:01 pm

Is there a documentation of the GPIO connector on the board and can the Kerberos be powered through that?

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Re: Power connection question

Post by Stromeko » Wed Jun 12, 2019 6:32 pm

First verified working option for powering the KerberosSDR via USB-C: a PSU meant for the rasPi 3B+ (3A fine-tuned to 5.1V output) and a passive adapter from microUSB (2.0) receptable to USB-C plug. Even though the adapter is really tiny, mechanically a short cable would work better, but I didn't find one that was short enough (less than 30cm/1ft).

With external power the upstream port provides VBus into the host. That isn't actually spec conforming although quite a number of USB hubs do it, so it usually doesn't lead to destroyed ports. Another thing that would be nice to correct in a future version of the board (requires switches though).

Stromeko
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Re: Power connection question

Post by Stromeko » Sun Jun 16, 2019 11:07 am

The USB-C PSU (captive cable) showed up yesterday in the mail and I'm happy to report that it also works with the KerberosSDR. I specifically selected one that was having variants with different connectors (barrel and micro-USB) available on the assumption that it would be a "dumb" PSU and not pay attention to the USB-C sense lines. Also, it was speced for 5.25V output, so even with the IR drop at high currents it will still keep the SDR voltage above 5V.

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