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How to Calculate Resistance of a Pi
Posted: Tue Oct 01, 2019 7:19 pm
by pixelpop
I've been struggling to get a Pi 3 A+ with a 7" LCD attached to boot. I either get under-voltage warnings are it goes into a boot loop. So I got a Charger Doctor and began testing. As long as I kept the input voltage at around 5.25-5.3V I did not have any issues booting the Pi. And even more interesting, I never saw more than 0.8A being pulled by the Pi/LCD combo. Using a one of those online calculators, this means that the Pi/LCD is presenting only about 6.5 ohms. Does this make sense?
Also, does this mean that if the PowerBoost 1000C I bought actually output 5.2V as advertised (mine is rock-solid at 5.07V), I would be very close to staying under the 1A output limit on this boost/charger?
Re: How to Calculate Resistance of a Pi
Posted: Tue Oct 01, 2019 7:30 pm
by gordon77
The pi will take higher current during booting. You probably aren't measuring these peaks in current. I would suggest your psu and/or usb cable aren't sufficient.
Re: How to Calculate Resistance of a Pi
Posted: Tue Oct 01, 2019 7:39 pm
by pixelpop
So does that mean that (forgive me, I'm a programmer, not an EE) an input voltage of 6-7V (not that I would do that) would absorb these spikes and keep the current draw to something less than 1A?
Re: How to Calculate Resistance of a Pi
Posted: Tue Oct 01, 2019 7:42 pm
by W. H. Heydt
pixelpop wrote: ↑Tue Oct 01, 2019 7:19 pm
Using a one of those online calculators, this means that the Pi/LCD is presenting only about 6.5 ohms. Does this make sense?
NO. It doesn't make sense. Even if you were trying to compute impedance, rather than just resistance, it wouldn't make sense. The Pi is an active device and the current it draws will vary by a considerable margin, depending on what it is doing at any given moment. It can't be modeled as a resistor.
Re: How to Calculate Resistance of a Pi
Posted: Tue Oct 01, 2019 7:43 pm
by davidcoton
Your calculation of the effective resistance of the Pi and LCD looks right, but by itself it doesn't tell you much.
It will vary depending on how hard the Pi is working, and you probably won't see the peaks of the load on the Charge Doctor. Therefore, you do not know the minimum effective resistance when the Pi is drawing peak current during boot or while doing lots of work.
It does give you an idea of why you need thick, short wires between the PowerBoost and the Pi -- you can't afford to lose even 0.5V, so the resistance of the supply leads (including the Charge Doctor) must be less than 0.1 times the minimum effective load resistance. Use 18AWG/SWG wires, as short as possible.
It is also possible that the PowerBoost cannot maintain its output voltage during the current peaks. These probably exceed its 1A rating, so voltage droop is to be expected. This is what will give you under-voltage and eventually a boot loop.
Note: more posts while I was writing. Just increasing the voltage will not absorb the spikes, and the off-load voltage becomes dangerously high for the Pi and connected USB/HDMI equipment. You need to reduce the causes of the voltage loss, which are resistance in the supply wires and an inadequate regulator.
Re: How to Calculate Resistance of a Pi
Posted: Tue Oct 01, 2019 7:57 pm
by gordon77
pixelpop wrote: ↑Tue Oct 01, 2019 7:39 pm
So does that mean that (forgive me, I'm a programmer, not an EE) an input voltage of 6-7V (not that I would do that) would absorb these spikes and keep the current draw to something less than 1A?
As others have said, no. You can't exceed 5.25v, or you will destroy your Pi!. You need a psu with more current capability.
Re: How to Calculate Resistance of a Pi
Posted: Tue Oct 01, 2019 8:07 pm
by pixelpop
davidcoton...thanks for taking the time to provide some in-depth info. I feel like I've learned something today.
I had read elsewhere to keep the wiring as short as possible (mine is abut 4" from the Pi to the PowerBoost). I also had read that USB connectors are a big source of voltage/amperage drop. Now I understand the "why".
What bugs me is that, based on what you have said, my PowerBoost is "supposed" to output 5.2V and mine only gives 5.07V, so that 0.13V is really critical when you have anything attached to the Pi. I can use a boost or buck to get the voltage I need, but I've been struggling to find a way to incorporate a battery UPS. I know that upping the voltage will decrease the battery life, but everything is a compromise.
Re: How to Calculate Resistance of a Pi
Posted: Tue Oct 01, 2019 8:12 pm
by pixelpop
gordon77 wrote: ↑Tue Oct 01, 2019 7:57 pm
As others have said, no. You can't exceed 5.25v, or you will destroy your Pi!. You need a psu with more current capability.
As long as we are on the subject, can I offer a small bitch about Adafruit? Their PowerBoost 1000C has an output current limit of 1A. But the latest Pi (the 4 B) recommends a 5V 3A psu. Even the Pi 3 B+ says you should use a 5V 2.5A psu. So what's the point of offering anything these days that is limited to a 1A output?
It's also interesting that they show a Pi (presumably a 3 B or B+) running a 5" LCD with a HAT and a keyboard/mouse dongle. With a maximum of 1A on the output, it doesn't seem like this should fly.
Re: How to Calculate Resistance of a Pi
Posted: Tue Oct 01, 2019 8:20 pm
by W. H. Heydt
pixelpop wrote: ↑Tue Oct 01, 2019 8:12 pm
gordon77 wrote: ↑Tue Oct 01, 2019 7:57 pm
As long as we are on the subject, can I offer a small bitch about Adafruit? Their PowerBoost 1000C has an output current limit of 1A. But the latest Pi (the 4 B) recommends a 5V 3A psu. Even the Pi 3 B+ says you should use a 5V 2.5A psu. So what's the point of offering anything these days that is limited to a 1A output?
It would be fine for a Pi0, Pi0W, A, B, A+ or B+.
Re: How to Calculate Resistance of a Pi
Posted: Tue Oct 01, 2019 8:27 pm
by pixelpop
W. H. Heydt wrote: ↑Tue Oct 01, 2019 8:20 pm
It would be fine for a Pi0, Pi0W, A, B, A+ or B+.
Sorry....I was editing (ranting) my last comment while you were responding to it.
"It's also interesting that they show a Pi (presumably a 3 B or B+) running a 5" LCD with a HAT and a keyboard/mouse dongle. With a maximum of 1A on the output, it doesn't seem like this should fly."
Re: How to Calculate Resistance of a Pi
Posted: Tue Oct 01, 2019 8:29 pm
by davidcoton
pixelpop wrote: ↑Tue Oct 01, 2019 8:12 pm
can I offer a small bitch about Adafruit? Their PowerBoost 1000C has an output current limit of 1A. But the latest Pi (the 4 B) recommends a 5V 3A psu. Even the Pi 3 B+ says you should use a 5V 2.5A psu. So what's the point of offering anything these days that is limited to a 1A output?
It's also interesting that they show a Pi (presumably a 3 B or B+) running a 5" LCD with a HAT and a keyboard/mouse dongle. With a maximum of 1A on the output, it doesn't seem like this should fly.
Have you tried pointing out the problem to Adafruit? I hope they would be eager to correct any misleading information.
Re: How to Calculate Resistance of a Pi
Posted: Tue Oct 01, 2019 8:35 pm
by trejan
pixelpop wrote: ↑Tue Oct 01, 2019 8:12 pm
It's also interesting that they show a Pi (presumably a 3 B or B+) running a 5" LCD with a HAT and a keyboard/mouse dongle.
The Pi in the photo is an old B+. The Powerboost 1000C was designed back in 2015. The recommended PSU for a B+ is 1.8A but 1.2A of that is for USB peripherals. I don't know how much of the 1.8A is reserved for the camera or a HAT. The "1000mA+ from a 3.7V LiPoly/LiIon battery" rating of the Powerboost 1000C is should be okay to power the Pi + display but no high power USB peripherals.
[edit]1.2A of USB peripherals[/edit]
Re: How to Calculate Resistance of a Pi
Posted: Tue Oct 01, 2019 8:37 pm
by pixelpop
I think I might just do that!
Re: How to Calculate Resistance of a Pi
Posted: Tue Oct 01, 2019 8:43 pm
by pixelpop
trejan wrote: ↑Tue Oct 01, 2019 8:35 pm
pixelpop wrote: ↑Tue Oct 01, 2019 8:12 pm
It's also interesting that they show a Pi (presumably a 3 B or B+) running a 5" LCD with a HAT and a keyboard/mouse dongle.
The Pi in the photo is an old B+. The Powerboost 1000C was designed back in 2015. The recommended PSU for a B+ is 1.8A but 500mA of that is for USB peripherals. I don't know how much of the 1.8A is reserved for the camera or a HAT. The "1000mA+ from a 3.7V LiPoly/LiIon battery" rating of the Powerboost 1000C is probably just about okay to power everything but increasing the current draw will likely cause it to brownout and crash.
You must mean a Pi 1 B+ because the recommended psu for the 3 B+ is much larger:
"A high-quality 2.5A micro USB power supply, such as the official Raspberry Pi Universal Power Supply"
Re: How to Calculate Resistance of a Pi
Posted: Tue Oct 01, 2019 8:47 pm
by trejan
pixelpop wrote: ↑Tue Oct 01, 2019 8:43 pm
You must mean a Pi 1 B+ because the recommended psu for the 3 B+ is much larger:
"A high-quality 2.5A micro USB power supply, such as the official Raspberry Pi Universal Power Supply"
Yes. It was called just B+. They didn't put a version number.
I misread the specs. The USB peripherals can draw a maximum of 1.2A so this leaves 600mA for the Pi B+/2B itself + camera + HATs. The typical active current for a B+ is 330mA and for a 2B it is 350mA. See
https://www.raspberrypi.org/documentati ... /#pi-power
The Powerboost 1000C should be fine to power a B+/2B with a display but no extra peripherals as shown in their photo.
Re: How to Calculate Resistance of a Pi
Posted: Tue Oct 01, 2019 8:54 pm
by pixelpop
trejan wrote: ↑Tue Oct 01, 2019 8:47 pm
pixelpop wrote: ↑Tue Oct 01, 2019 8:43 pm
You must mean a Pi 1 B+ because the recommended psu for the 3 B+ is much larger:
"A high-quality 2.5A micro USB power supply, such as the official Raspberry Pi Universal Power Supply"
Yes. It was called just B+. They didn't put a version number.
I misread the specs. The USB peripherals can draw a maximum of 1.2A so this leaves 600mA for the Pi B+/2B itself + camera + HATs. The typical active current for a B+ is 330mA and for a 2B it is 350mA. See
https://www.raspberrypi.org/documentati ... /#pi-power
The Powerboost 1000C should be fine to power a B+/2B with a display but no extra peripherals as shown in their photo.
I just sent Adafruit a message requesting that they update the PowerBoost to support today's popular models (3 B+ and 4 B). I also told them the photo on the product page was misleading, at best. It will be interesting to see if/how they respond.
Re: How to Calculate Resistance of a Pi
Posted: Tue Oct 01, 2019 9:00 pm
by trejan
pixelpop wrote: ↑Tue Oct 01, 2019 8:54 pm
I just sent Adafruit a message requesting that they update the PowerBoost to support today's popular models (3 B+ and 4 B).
It would be a major redesign since the DC to DC converter used can't supply that much current. I suspect it would be more economical to buy a USB power pack of sufficient capacity + current and repurpose it than to DIY your own one.
pixelpop wrote: ↑Tue Oct 01, 2019 8:54 pm
I also told them the photo on the product page was misleading, at best. It will be interesting to see if/how they respond.
I don't have any problem with the photo but they should state more clearly that it is only usable with older Pi boards that didn't draw as much current. I expect if you do get a resposne then it'll be to post on their forum. They seem to do all of their customer support on there.
Re: How to Calculate Resistance of a Pi
Posted: Tue Oct 01, 2019 9:05 pm
by gordon77
pixelpop wrote: ↑Tue Oct 01, 2019 8:54 pm
I just sent Adafruit a message requesting that they update the PowerBoost to support today's popular models (3 B+ and 4 B). I also told them the photo on the product page was misleading, at best. It will be interesting to see if/how they respond.
Worth a read...
https://forums.adafruit.com/viewtopic.php?f=8&t=114810
Re: How to Calculate Resistance of a Pi
Posted: Tue Oct 01, 2019 9:31 pm
by pixelpop
I don't have any problem with the photo but they should state more clearly that it is only usable with older Pi boards that didn't draw as much current. I expect if you do get a resposne then it'll be to post on their forum. They seem to do all of their customer support on there.
Yeah, I didn't mean that it was "false" advertising, just misleading. And I did just get a response from them:
Hello,
Thanks for the product suggestion - we will share this with our teams.
Thank you!!
Adafruit Support, Ben
I doubt we'll see anything from this in my lifetime (but then, I am pretty old
).
Re: How to Calculate Resistance of a Pi
Posted: Tue Oct 01, 2019 10:17 pm
by Imperf3kt
The Powerboost was designed for the Arduino.
That it works with a Pi is a mere bonus. It isn't their target market. I highly doubt it will be updated