What size is the relay? 9V coil is popular for appliances. Relay coils make heat so 40°C is reasonable. Heat is also made by the current flow through the relay contacts. So they can dissipate 0.5-1W or more and they will warm up.
Some companies will use "peak and hold" where they pulse the relay at say 20kHz 50% duty cycle to keep the relay contacts in but the coil cooler.
More often the problem is cracked solder joints or bad relay contacts.
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9V relay temperature 40C
Hi, I'm seeing one 9V relay having temperature 40C on its 9V (primary) side. Does that seem right? I switched 2 relays on that board and the temperature was the same (on the same position), so I'm sure it's not a faulty relay. I measured voltage on that relay and it was 9.5V DC, so that looks good. I don't see any other relay on that board having that kind of temperature and they have 9V on them too.
I thought maybe the transistor that is switching this relay keeps switching on/off, so I also checked for AC and there was almost none: 0.3V using mV range. No steady frequency was shown, but some appeared every now and then.
Any input appreciated.
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