MK138 : Stop relay chatter

Hi there,

I’m successfully using the MK138 as a thermostat in an incubator after changing appropriate resistors etc to increase temp range and modify hysterisys.

However, one problem that im having is the amount of relay chatter when the lamps that im using are coming on. The relay makes a sort of slow buzzing noise. Im using two recessed 12v spotlamps, one @ 10W and one @ 20W. On a suggestion from another forum, i have tried putting a 100uF capacitor across the connection from r3 and the ground of T1, and this may have made a small difference (although its hard to tell).

Is there anything else i can do to remove this chatter?



You could reduce R7 to 4M7 or even less. Please experiment.

If you are running it off of batteries you might want to make sure they are putting out enough voltage to run the MK138.

With regards to running it off batteries, im am not. Im running it off a 12v/3A power supply.

I have tried reducing R7 to both 4M7 and 3M3 and the relay chatter remains evident unfortunately. I have also tried reducing the power draw from the bulbs by removing the 10W light bulb (therefore only running 20W) but there is still chatter, although it is not as noticeable.

Would it be worth running the bulbs on a separate power supply to that of the kit, even though 30w should be within the boundries of what is reasonable with the MK138?


No. In this case, a solid state relay would be a better solution, if hysteresis is really small.