Hi,
I’ve recently put together the Thermostat Kit. I’m getting relay chatter around the switching point, Is this due to low hysteresis. If so I need to reduce the size of R7. Any suggestions as to a resistance to start with?
Many Thanks
Andy Baillie
I have never had relay chatter, but I have experimented with the hysteresis. I have used resistance values from 10M down to .5M. If you have already soldered in the stock (6.8M?) resistor, you might just try using clip leads to put another resistor (say another 6.8M) in parallel with the existing resistor to see if that stops your relay chatter. If not, try a 1M in parallel with the existing resistor. My guess is that the relay chatter is not due to the hysteresis, but I don’t have an alternate theory. Velleman support probably has a better suggestion.
Thanks for the tip I’ll give that a try today.
Cheers
Andy
Hi
I bought a 5M5 pot from Maplins today and replaced the 6M8 resistor. The relay chatter has stopped, although I’m not convinced I’ve solved the problem. I’m getting alot of noise from the circuit still around the switching point. It’s as if the chatter is still there but it’s now so fast that it’s just a buzzing noise. I’m still trying to fine tune the pot, but I don’t think its going to get me anywhere.
Any input from Velleman Support would be much appreciated as I can’t seem to get hold of you any other way!
Judging from other posts, other users of this kit do not seem to be suffering the same problem.
Thanks
Andy
Hi,
Just wondering are you running AC through the relay?
Hi
No, I’m running 3 12v DC fans in parallel of a 12v transformer
Hi again,
Just a guess but did you check D1 to see if it was open or the solder connections are good?
Hope this helps.
Hi
No I didn’t, but you can be sure I will now! Just out of interest, how did you come up with D1?
Thanks all the same
Andy
Hello again,
Looking at the schematic D1 goes over the coil of the relay.
Looking at most of the Velleman kits with relays and other kits and schematics they almost always have a diode over the coils.
After searching on the internet this appears to be one of the ways to stop the chatter.
Although this may not be correct if not I hope someone corrects me.
When checking it it would be a good idea to lift one leg off of the board.
Not doing that will give you false readings as you will be reading the coil of the relay.
I hope this may help.
Hi
I’ve checked D1 this morning, infact I’ve checked all of the components and solder joints. All are fine. Thank you for your idea though, but unfortunately problem not solved.
Cheers
Andy 
[quote=“Airbus_Driver”]Hi
I’ve checked D1 this morning, infact I’ve checked all of the components and solder joints. All are fine. Thank you for your idea though, but unfortunately problem not solved.
Cheers
Andy [/quote]
Andy,
It may be that Velleman support hasn’t responded to your question because I jumped in so quickly and offered an opinion. They usually are very responsive, but perhaps they are not reading this thread any further. At any rate, maybe you want to start a new thread with a restatement of the problem and a summary of what you have tried thus far. Maybe that will get their attention, especially if you you specifically ask for their input in the post title.
That being said, there are a few things that come to my mind that may be causing your problem. 1. If your power supply voltage was very low or poorly filtered, that might cause problems. 2. If the zener diode was bad or in backwards, the comparator circuit wouldn’t work properly. 3. If the thermistor was bad or the wiring connecting it to SK1 was intermittent, the circuit could be unstable. 4. T1 might be defective. (LD1 should turn on fully and light brightly when the relay is energized.) 5. The relay itself could be defective. 6. The LM 324 could be defective. (If you had another, it would be easy enough to pop a new one in the socket and check that.) 7. C3 might be in backwards.
I am no expert, but I have put together two of these kits and have built from scratch two other thermostat circuits; I have had good luck with them all. If you wanted to post some closeup pics of your build, I (or someone else here) might be able to spot an assembly problem. I also have some spare parts that I could give you, although I presume you are in Europe and I am in the US, so it might not be cost effective.
Good luck,
Charles
Charles
Many thanks for your input. I’ll start a new post and see if I can get Velleman Support’s involvement.
I’ve check over the circuit as best I can, and everything seems as it should be.
Thanks again
Andy
[quote=“Airbus_Driver”]Hi,
I’ve recently put together the Thermostat Kit. I’m getting relay chatter around the switching point, Is this due to low hysteresis. If so I need to reduce the size of R7. Any suggestions as to a resistance to start with?
Many Thanks
Andy Baillie[/quote]
Hi Th only time I got relay chatter was when the power source I was using ie a 12v battery dropped in voltage due to load!!
Regards Windy
Hi Windy
Thanks for that, I was going down that train of thought. I was was going to stick a big cap in to see if it would help, but I haven’t got round to it yet.
Thanks
Andy
Sorry for the late reply, it’s also holliday periode here…
You got allready a lot of tips, you can always send some hi-res pictures.
Maybe decoupling the op-amp input connected to the pot wiper will help, using say 0.1uF. I posted the text below in another thread a while ago which is not quite what you are experiencing but perhaps the sensitive nature of the circuit at that point is causing your problem as well? Anyway hope it helps.
“I have 2 of the MK138’s running 2 modified fish tank heaters (2 different tanks of liquid for PCB production). The 2 MK138’s are in the same box and I have mounted the control pots on the box panel. This means that there are cables about 100-150 mm long between the pots and their respective boards. I found that there was an interaction between the 2 boards when each was at around the switching point, which resulted in one board turning off the other’s relay as it switched on. This was without any output connected to the relays and also with independent power supplies. The problem appeared to be caused by the very sensitive nature of the high impedance input to the op-amp connected to the pot wiper. I soldered a 0.33uf ceramic capacitor between the wiper connection at the PCB end of the wire and ground which cured the problem. Actually, I think around 0.1uf would do but I had the 0.33’s handy!”
found problem burned out board…will start new thread.
best option is changing the relay to a solid state ssr relay, velleman VR3SS1A etc, i dont think the mk138 thermostat onboard relay is really mad enough for a incubator,
i did try reducing R7 to 3.3m resistor and soldering a 100mf cap between ground and the connection of R3 & T1, but still getting a little chatting, but i did hatch 6 out of 6 californian quail eggs, temp 99.5f and humidity 60%-65% pretty good even with a chatting relay lol