Raspberry and 7" touch screen power problems

#1

i have just purchased a dual micro usb power cable (cab0222) with one micro usb socket on one end and two micro usb plugs on the other, and connected it up . one end (single socket) to the mains power supply/adapter that came with the raspberry starter kit the other two, one to the micro usb port on the raspberry pi 3 the other into the micro port of the 7" touch screen. i get the green light on the raspberry but also the low power light (orange flashing) and the low power symbol on the screen during boot up, can anyone tell me why, or how to fix this
what is the point of the cable (splitting) if it can not supply enough power to run both the rasberry and the touch scree

#2

I’ve used the split cable for a long time now, and can confirm it works. What power supply are you using?

#3

Hi Phil
thanks for your reply. I am using the "offical uk mains adapter’ that came with the raspberry Pi 3 starter kit i purchased. from google research and reading it seems the raspberry pi 3 is a little more power hungry than previous models and if the supply drops below 4.8 the orange ‘low voltage’ light on the raspberry pi 3 will flash, i also get a yellow lighting symbol flashing during boot up, again i am using raspian os as recommended.

#4

You can check the power requirements of the various Pi models here, https://www.raspberrypi.org/help/faqs/#topPower
The 3B is ~ 400ma idle. The second highest power consumption from that list.

I have the 7 inch Pi foundation touch screen mated to a Pi 3b. I’m using a 5V 10A power supply though. Overkill I know but it’s my breadboard rig so I wanted lots of overhead. I have an almost identical 5V 4A power supply that also works with this setup. It was bought for another PI project. Both use barrel connectors instead of micro USB, .https://www.adafruit.com/product/658 I use a panel mount barrel connector with solder terminals. Then made up my own power cables. Essentially a why cable similar to what your using.

Anyway, long story short, your likely getting the lightning bolt because the current draw is causing a voltage drop over your power cable. The wires have some resistance, small but there non the less. The thinner the wires the more resistance they present. The voltage dropped over the wires subtracts from what the power supply puts out. If the voltage measured at the Pi’s input drops below 4.65 volts you get the low voltage warning.

The other thing that can happen, is under heavy load, and high current draw, the power supplies output voltage drops. Its a safety feature to protect the power supply from overload. Lowering the voltage lowers the current supplied. Its ohms law.

To get around this some PI power supplies supply 5.1 or 5.2 volts. That way you still get ~5Volts at the Pi even under heavy load. And use heavy gauge wires with less resistance so less voltage is dropped over the power cord.

#5

I’m using a Pi3b and a 7" touchscreen with a 5V 3A supply and it’s fine. I’m not using a usb connector though, connecting to the 5V and 0V on the io connector, which means I’m not dependent on a usb rated (maybe only half amp) lead. Hope that’s helpful. Dave