I have a PI 4, WHEN using a 3.5A USB power measured 5.1v supply, the red LED is constantly on. When using a power supply which was rescued from a DVD player, checked supply 5.1v I get a flashing red LED (once every 2/3 seconds). Is there any difference as to how the voltage is connected ie USB-C connector or via GPIO pins 2 and 6 ? what does a red flashing LED every 2/3 seconds mean? All measurements taken from GPIO pin 2 an 6 using a volt meter. I have done several projects and using either way to power the unit up it does not seem to bother the PI 4 Projects here ANy feedback welcome
A red flashing LED is an insufficient power supply.
Despite the many posts (on this site and others) if the red LED flashes (except on reboot) the Voltage is too low i.e. BELOW 4.63±0.07V - no amount of speculation about this will change the fact.
The voltage monitor chip responds to transient voltage, and the kernel stretches this to 3 sec so the flashing is more obvious.
Multimeters are designed to measure AVERAGE Voltage and will not show show transients.
Even if the LED is lit the voltage may still be below the USB minimum (4.75V), although the Pi itself may still work.
This MAY be due to a poorly designed Power Supply which cannot supply the required current while maintaining the required voltage. (Although all Pi - including the Pi4 will not require more than 1A itself.)
No matter how good your Power Supply if you use poor quality cables to connect to the Pi you will have problems. Many (the majority?) of μUSB cables are designed to carry data, and have very thin wiring. This makes the cables thin, light and inexpensive but they are unsuitable for power.
As an Electrical Engineer I have designed many computer systems and power supplies. Expecting a 5V supply to power a device which may require 3A over a 1m cable without ridiculously large cables is impossible (just look at the size of multiple wires on an ATX PS to supply 5V over a much shorter distance), so the official Pi supplies are designed to provide 5.1V and have decently sized cables to minimise the inevitable voltage drop.
See Raspberry Pi Power Limitations for further detail.