MK 138 no output

The led lights and the relay clicks, but there is no 12VDC at the output. Can you help?

Check the diagram carefully. The output is a relay contact that is either open or closed, it is not a voltage that is switched between 0V or 12V…
Check this post: [url]Relay contacts and how to use them?]

Thanks for the help. I connected it correctly and its working fine now. I’m using it for a low cost incubator. I used 80K for resistor 5, 12M for resistor 7, and it’s holding the temperature to within .5 deg. C, which is quite good.

Neil

Hi Neil,

I was planning on using the MK 138 for a still air incubator that I have.

I asked the group about which thermistor would give me a temperature range that I can control between 99-103 degrees F (37-40 degrees C).

What is your temperature range? Did you change the thermistor etc…?

Thanks.
Ron.

I kept the same thermistor, but changed R5 to 80K to increase the max temp and R7 to 12M to reduce hysteresis. The incubator holds 37.5C +or- 0.3. The trimpot is at about 3/4 of its range. I got a mini fan from an old computer and put that in to distribute the heat more evenly.
Neil

The kit has been working fine for a couple of weeks, but now it’s buzzing and staying on.

12M is a bit on the high side… It might reduce reliability and it will render the circuit susceptible to cycling (= buzzing noise)…

Changed R7 back to 6M8, but still buzzing.

To increase the operating range of MK138 so it is suitable for use as an egg incubator thermostat (37.5º) with a hysteresis of approximately ±0.4º. Change the following components:-
Component Value Order Code
R4 4K7 1% 907-247
R5 3K9 5% 906-239
R6 2K7 5% 906-227
R7 10M 5% 906-610
RV1 1K 994-210