# 5 V from power bank not constant when supply circuit

Now I'm working on my project which contains Arduino, LCD, electromagnetic lock, 6 channel 5v relay and 500V isolated converter. My project was designed to power by using a power bank. Now I use eloop E14 20000mAh power bank which can provide 5V 1A and 5V 2.1A so, I use 5V 1 A to powering the Arduino via USB and 5V 2.1A power the circuit. In the circuit part firstly the LCD consumes current about 200-300 mA and the voltage must maintain around 4.75 to 5.25 V. Next the electromagnetic lock consumes 12V 200mA so, I increase 5V to 12V by using a 5-12 converter that can withstand around 1-2 A and the 500V converter that uses the input of 12 V then boost to 500V and this converter consumes 8mA. And the last one 6 channel 5 v relay which consumes around 70mA when power 1 channel and I only power 1 channel in a time The question is when I started to supply my circuit the voltage will drop to less than 4.75V so that the LCD will shut down. Can anyone tell me why is this happen?. I have calculated the total current of 5V 2.1A port which is about 508mA ( less than 2.1A). According to my understanding, I reckon that the 5V will start to drop when we supply the circuit that exceeds 2.1A, so why is this happen. Lastly, I power my circuit via USB-micro USB. Any assistance will be appreciated. Thanks

• It sounds like you've answered your own question. You've overloaded the supply and so the voltage is drooping. What is the question you're trying to ask. PS. You must include a complete schematic. – scorpdaddy May 20 at 2:27
• Resistance can (by which I mean, does) also drop voltage... wires, cables, connectors all contribute some. Then remember V = I * R – vicatcu May 20 at 2:37
• Thank you very much for your guidance but I don't understand why you said that I have overloaded the supply( the total current is around 500-600mA but the supply is 5V 2.1A) – Panupong Hirunrote May 20 at 2:49