Heating problem: WeMos D1 Mini, sla-05vdc-sl-c Relay, HLK-PM01 Power Supply

Hi there,

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.


vickeyhort: I think under voltage is the reason that relays are getting hot.

No, undervolting makes a relay run cooler. You didn't say how big your circuit board is. That whole setup, with supply, and relays active, has to dissipate about 2.5watt. If you have that inside a small enclosure, it will get hot. Leo..

I am using 7 X 9 cm Fiber glass, double sided prototype board. Pictures are attached. Only the area of lower side of PCB where relays are installed, gets hot (Highlighted in red).

I have temporarily placed it in a 15 X 10 cm enclosure as I am still testing this board.

And Do you mean 2.5 watt are converted to heat?


vickeyhort: And Do you mean 2.5 watt are converted to heat?

Bigger relays = more holding power loss in the coil. 5.05V * 0.35A = 1.77Watt + Wemos + supply = close to 2.5watt. The relays should be able to handle that 55C though. Leo..

So, It means, it's ok to have this amount of heat. Or should I adopt some remedy in this regard?

Update: 1 Moreover, I have test one more standalone relay by connecting 5v supply to 5v and GND terminals and it also got enough hot. Whole relay becomes hot. Now I am sure that all those heat comes from both of relays and there is no problem with circuitry.

Update: 2 Datasheet also say's that Coil Temperature may rise up to 60 degree at it's maximum.