Results of a new build based on the schematics
Dew Controller Feature Rich
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brownrb
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Hi,
I just got a minimal dew controller setup, got the sensors, lcd and transistors working and supplying 2 small dew strips (less than 1A together, its just for a 80mm refractor and 50mm guidescope). Some of the experiences below
1) Connecting the 12V+ to the VIN of the nano controller was not a good idea in my environment. It worked at first, but when i switched to a 12v battery that was just disconnected from a charger (and probably was well above 12V) the nano power regulator chip got fried. Although the genuine nano states that it should be able to take up to 20V, the chip present in my dccduino nano (AMS1117) specs say that the absolute max voltage is 15v.
Following that mishap i replaced the nano with another one (it was still working but i could not connect to it by serial anymore), and disconnected the 12V+ from the VIN pin (left the GND connected to the 12V- though), now everything seems to work fine fed by USB power
2) At 1A current, split between the two transitors, they are not even hot to touch when tested at room temperature, i am quite sure that outside at much lower temps i will not need that fan.
3) While the input voltage from the 12V under load looks to be around 12V, the 10A10 diode drops about 1V, and the transistors seem to drop some more, so the strips get 10.37V. It looks like the circuitry "wastes" around 15% of the voltage, does it mean that 15% of the battery get wasted as well ? Would it make sense to remove the 10A10 if i am extra sure that i am not going to get a reverse polarity connection or that diode has another function (i have not officially studied electronics, and i could not find a clear explanation for this part's task on google)
4) What is the functionality of the 1Mohm resistor between the base and emitter of the transitors ? don't seem to understand that part of the scematics
Overall, great project. Thank you for the scematics, and the great documentation. This is my first arduino project and i am super excited that one can get to such great results with just a list of parts, a few hours, and some effort
Hi Alex
Thanks for posting your feedback.
1. I had the same happen to me re 12V and VIN recently. Using a Zener diode across VIN and GND would compensate for this. The purpose was to to be able to run the controller just off 12V in the field without a computer. One could also use a 7812 voltage regulator instead of a zener diode circuit.
2. Yes the fan is very much overkill and unnecessary.
3. The 10A10 is for protection in case one connects the 12V around the wrong way. and other designs had none. Yes there is a fair bit of drop but it is a safety feature (in the dark one could easily connect the clips on a car battery the wrong way round). The 10A10 is overkill for the design. As the current draw is not large, and the circuit uses PWM, there is there is not much wasted battery power.
4. The purpose of the 1Mohm is to ensure the TP120 switches fully on and off with pwm, without it there is a tendency for the TIP120 not to fully turn on or off with short pulses, thus more voltage is dropped across the TIP120 and it runs hotter.
If you have time, please post a picture here, would like to see your final design.
Cheers
Robert
I am not happy with the use of the VR's. I am going to explore an alternative. In the meantime, I will revert back to previous versions of the schematics and layouts.
The issue is around an external supply being too high a voltage supplying VIN on the Nano and thus damaging the Nano. A Zener diode can achieve this also. For now, I would recommend ensuring the external DC input is not connected to VIN. If your controller is already working then you probably dont have an issue.
Apologies for the rapid reversal
I will be sure to post a picture when i am done with the project (right now its on a breadboard + some standalone wires for the higher current stuff), i am still waiting for the project box to arrive in the mail (should be done in a little more than a week).
Since i am not connecting directly to the battery terminals but using a cigarette lighter plug to connect to the battery unit, i have no concern of reverse polarity connection, i will just skip the 10A10 for now. And thank you for following through on the VIN voltage issue
The recent changes are of course the Zener diode/resistor feeding VIN.
Yes the 10A10 can be skipped. Once one has all the cables made up that cannot be accidently crossed over its pretty much redundant.
You will find I have added Schottky diodes across the dew controller outputs - if one is using very long dew strap leads this would be a good idea, I think mine are about 1.5m long and the last unit built is still ok without the schottky diodes so if you want an extra level of protection include them.
Cheers
Robert