I have built a ESP8266 WeMos D1 Mini WiFi control switch with two sla-05vdc-sl-c relays rated for 30A. It has built in Power supply i.e. HLK-PM01 3W (600mA) and DS18B20 temperature sensor to check system internal temperature. Switch is controlled via NodeRed and every thing is working fine. It also send temperature data to my server.
My problem is, I have noticed that after few hours, keeping this node online, temperature reading reaches to 55 degree Celsius when relays are kept ON and PCB also get’s enough hot. Major heat area is PCB underside of both relays. HLK-PM01 power supply also gets very slightly hot. However, everything is working such as temperature data is sent every minute, relays can be switched from ON to OFF and vice versa. Moreover, I have not plugged any load to these relays as I am just doing preliminary tests so far. I am just curious, if I have some problem with circuitry such as choosing wrong resistance value or power supply.
I have checked that voltage is 5.05 volts across the system, and system load is 350mA while keeping relays ON. I am using MMBT5551 transistor to control relays. Voltage across the positive and ground pins of relays is 4.5 volts. I think under voltage is the reason that relays are getting hot. However, After switching the relays OFF, temperature reading decrease to 38-40 degree Celsius. I have attached circuit diagram.