How to adjust this DC incubator circuit for optimum temperature stability?

[gozitano]

Hi everyone,
I have been using the following 12v incubator circuit and I am finding difficulty keeping a stable temperature. I think it relates to the proper adjustments of the three trimpots used,namely VR1,VR2 and VR3.
Can anyone help with how to adjust same for optimum temperature stability?

https://www.craig.copperleife.com/tech/thermo/741thermo.gif

Many thanks.
Clem.

 

[KJ6EAD]

The instructions and notes are here: Craig's Thermostat Circuits. Can you be specific about what part of the process is failing you?

 

[gozitano]

Hi KJ6EAD,
Thank you for your interest.
I have used Craig's (LM741) circuit with rather poor results,namely three hatchlings out of 24 eggs. I blamed the circuit because on a number of occasions I had to keep resetting the temperature since it would spike from 38.3 to 41 plus.there are three trimpots in the circuit. VR1 and VR3 adjust quite readily but there is no response from VR2.VR2 is the hysteresis. Is there any set way how to adjust the trimpots to come up with a stable temperature.?

 

[KJ6EAD]

My first guess would be that you have the circuit set up for cooling instead of heating. My second guess would be that the hysteresis feedback is going to the wrong input. Make sure the thermistor is connected to pin 2, VR1 and VR3 are connected to pin 3.

This kind of wiring error is always more likely when working with a schematic that's drawn differently than desired. It's worth redrawing the schematic to avoid the problem.

 

[keith1200rs]

It is a crude control circuit. The hysteresis could potentially make the control worse. Positioning of the thermistor relative to the heater is probably critical.

Keith

 

[gozitano]

Hi Keith,
Thanks for your input. I will position the thermostat a distance away from the heat source,since both the lm741 and the transistor are heat sensitive in the hope of getting some temperature stability.the hysteresis is still a ?

---------- Post added at 15:57 ---------- Previous post was at 15:39 ----------

Hi,
Thanks KJ6EAD,
I did follow the instructions that came with the circuit. I will check it all again to see if I have made any errors.
Many thanks.
Clem.

 

[keith1200rs]

Some hysteresis is necessary otherwise the relay will 'chatter' round the switching point. With those values I wonder if there is too much hysteresis. You could try removing the potentiometer and see what happens.

Keith

 

[gozitano]

Hi Keith,
I removed the potentiometer as suggested and chatter developed at the switching point.It's evident that some hysteresis is necessary.I replaced the pot. and we're back where we started,namely that switching takes too long to happen causing the temperature to drop by 1.5 degrees. It would be great if it could be brought down to about .5 of 1 degree.

 

[KJ6EAD]

I there any chance that the thermistor is a PTC type instead of the required NTC?

 

[keith1200rs]

If the thermistor was the wrong type then the heater would be permanently on or off.

It sounds like you really jus need more hysteresis. Unfortunately, with such a simple circuit that is not easy. The hysteresis is determined by the feedback resistor and R1//R5 in parallel (which is variable). You already have the largest pot for VR2. You could remove VR2 and use a fixed resistor of 2.2M or higher so you still have some hysteresis, but less.

Another thing to try is to move the thermistor closer to the heater. It will cause the heater to cycle a lot more but may reduce the overall temperature variation depending on the thermal mass involved.

Keith.

 

[FvM]

A two-point controller will generate an "unstable" process value by design. If it's inappropriate for your application, you should go for a linear or PWM modulated PI or PID controller. Specifying the acceptable error band would be a first step to decide about it.

In some cases, a feedforward of heater to sensor (respectively bringing the sensor closer to the heater, as suggested) can reduce the temperature variation at the place of interest. But it will also introduce a permanent error and increased dependency on enviroment temperature and supply voltage variation.

 

[dr pepper]

You will allways get some temp oscillation due to the switching points and hysteresys, the oscillation will go higher than the upper set point due to overshoot caused by thermal energy still in the heater not accounted for by the circuit.
I have seen older systems in industry where the sensor is directly heated by a heater resistor while the main heating element is switched on, this kind of compensation is effective but its real hard to set up and get right.
You can get a bery stable system, but you would need a PID controller, this with heaters would use a slow pulse width system, where the heaters are turned on and off every few seconds, very tight control is possible.
The system is much more complex of course, but if you have access to a pic programmer the physical electronics part of the system is simpler than what you allready have, do a search on google see if someone has allready done it, most likely they have.

 

[keith1200rs]

I have seen older systems in industry where the sensor is directly heated by a heater resistor while the main heating element is switched on, this kind of compensation is effective but its real hard to set up and get right.

I think that was common in bi-metallic thermostats to try to reduce the inherent hysteresis in the bi-stable elements.

Keith

 

[dr pepper]

Yes it was wasnt it.
Same applies, the idea is not to overshoot the setpoint.

Clem: have you tried moving the position of the thermistor, the temp stability will change depending where you place it (depending on the power of your heater), also the power rating of your heater will affect stability, if the heater is way too powerfull then the temp will be up and down like a yo-yo, you want just enough power to keep the incubator warm (allthough it will take longer to warm up in the fisrt place).

 

[gozitano]

Hi Keith,

Many thanks for your continued support.
In my latest effort to sort out the thermostat problem,I have changed the thermistor from 10 to 4.7. It worked a miracle in stability.Power comes on for one minute and goes off for two minutes,with hardly any change in temperature.Vr2 is now functioning allowing hysteresis adjustment.However I am getting chatter on switching. Would this damage the relay?

 

[keith1200rs]

You will reduce the relay lifetime. A couple of things you could try - add extra decoupling to the opamp. Add a capacitor across the thermistor - it may make it worse but it is worth trying.

Keith.

 

[dr pepper]

Try a 47 or 100uF electrolytic cap from the base of the transistor to ground, + side to the base.
That will hold the tranny on for a little while and reduce or stop chatter.

 

[gozitano]

Hi dr.pepper,
I tried a 100uF from base of transistor to ground and the chatter became continuous.

 

[FvM]

I tried a 100uF from base of transistor to ground and the chatter became continuous.

Sounds plausible. Apparently the "chatter" is coming from a insuficiently regulated power supply. The capacitor should be better placed between base and emitter of the transistor. The fact, that you get chatter although a hysteresis is present in the circuit suggests either AC voltages interfering with the circuit, e.g. the sensor cables, or a very bad supply regulation.