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Arduino dew heater control

Gina

By Gina
in DIY Astronomer

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philr

philr

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What have you got left to do Jason?

Looks like the next add on to mine will be variables control via Android mobile phone. More on this in due course.

Shame about the cancellation of the Beckington Astro meeting. Will I see you in February?

Phil

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JAS

JAS

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I am all done wth the heaters Phil, I posted the finished sketch here; http://stargazerslou...ed-dew-control/

I did concider external control but I am not sure if I would use it in my setup, so I didnt take it any further. I woud be interested to see how you get on tho :smiley:

I made a rain sensor before Christmas but never did anything with it, so I am playing around with that at the moment. I have also just got a temp sensor to make up a cloud sensor.

I have had a problem with my existing auto focus board reseting the Arduino, so I am changing over to stepper motors that I salvaged from a printer, this will be controlled by a second Arduino. I am following the SGL automation groups design.

I havnt joined the Beckington group yet but I am planing to go along on the 15th to see what it is all about.

Jason.

post-11618-0-11843800-1359059961_thumb.j

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Auntystatic

Auntystatic

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One thing I have noticed from all your diagrams is that your using the hot side of the circuit for the heaters, what I mean here is that you have the 12v power running through the heater lines at all times awaiting switching, so that the cable is always live, if a fault occurs you will have a problem as the 12v grounds out and you let the smoke out of the wiring. :police:

If you put the heaters between the emitter and ground, you will have voltage on the line only when the transistor switches, making it electrically a safer system.

Comments please :grin:

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Gina

Gina

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It's a good point and not one I'd missed but it does add to the circuitry. You would need an extra transistor and two resistors in each line to convert the 5v Arduino signal to 12v (or a driver chip). I'll look into it and come up with something - I too don't like the idea of a "hot" system.

I've started mine in a very small box and it's too cramped really so I'm thinking of changing things anyway and may add the extra circuitry to have a common ground rather than the "hot" 12v.

Thank you for the suggestion :)

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Gina

Gina

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Been looking into this and there's a problem. The gate on the power MOSFET has to go at least 3v above the source to switch on, so in the ON state the source is at +12v supplying the heater. ie. the gate would be above 15v - typically 12 + 5 = 17v. So both 12v and 17v supplies would be needed. So if we want to avoid a "hot" circuit a complete rethink is required. To use N channel power MOSFETs an extra supply is required. If we don't use N channel MOSFETs we'll need something else. With a current of an amp or less we dont really need power MOSFETs with their 50 or 100A capacities.

Gotta go now - duty calls - I'll look into this later...

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Gina

Gina

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We can get over the extra supply line by using P-channel power MOSFETs but now we need to supply a switching signal between +12v (OFF) and 12-5=7v (ON) though between +12v (OFF) and 0v (ON) is fine so we still need to change logic signal levels. I'll look at that next...

P-channel power MOSFETs :- http://www.ebay.co.uk/itm/281054015528

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Gina

Gina

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Here's a circuit diagram of one section of the new heater power control. The resistor is not critical - something like 10K would be suitable. MOSFETS are 2N7000N and IRF5305 other components as previous version.

post-13131-0-40645700-1359906560_thumb.p

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Gina

Gina

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I have designed a circuit layout for the modification. This is using the prototyping board I mentioned earlier. These boards seem ideal for use with the Arduino Nano with a few external components. I am having my Arduino plugged into a 40 pin DIL socket though it only uses 30 pins (a 30 pin 0.6" DIL socket would be better).

post-13131-0-72155800-1360096809_thumb.p

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philr

philr

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If anyone is looking for a suitable 12 Volt high wattage power supply, then you need to consider what I have just done. I found that my nominally 5 Amp PSU was dipping below 11.2 V when all heaters were on. I had a look around for 10 A PSU's. but they can be quite expensive. The solution was an ATX desk top computer PSU. These have multiple outputs. The one I have just bought is this:

http://www.ebay.co.uk/itm/140903967308?ssPageName=STRK:MEWNX:IT&_trksid=p3984.m1439.l2649

Bags of power and has two 12V output levels: 12.1V and 12.2V. I simply cut off the standard PC connectors and soldered on my chosen type of connectors, I have configured it for 3 outputs, to serve other functions too.

Pin 16 (green) on the 24 way ATX connector is taken to ground, or common (black) to enable the PSU. This emulates a PC power switch.

This PSU having 18 Amp outputs does not struggle at all with all heaters running - job done.

Phil

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  • 3 weeks later...
philr

philr

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Trev, yes of course. It has 12V outputs for SATA drives and non SATA drives. The SATA connectors are the black ones moulded onto the wires. The non Sata connectors are chunky white plastic with crimp sockets. Both types have black wires for common or ground, and yellow wires for plus (+) 12 Volts. I have taken two black and two yellow wires from each, then wired each pair of wires in parallel to form two separate 12 Volt outputs. Both capable of up to 18 Amps.

I am using 4mm banana type connectors used for connecting model aircraft LiPo batteries. These are reliable, robust and don't creep apart as cigar lighter connectors can, and do.

Phil

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t50ufo

t50ufo

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Thanks Phil,

That is great - I was just about to order a Bench Power Supply rated at 7 amps at about £50 delivered - this thing is a real steal at the price!

I have been following this thread as I was interested in having a go at a Arduino based dew heater - just trying to get my head round what I need and how to do it!

Thanks for heads up on the PSU meantime and yes agree Banana plugs/sockets are nice and chunky and easy to work with. You mentioned a green wire to ground - am I right in saying that still needs doing - has it something to do with delivering or switching on the 12v correctly?

Trev

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Gina

Gina

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The green wire is the PSU ON/OFF control. It is normally connected to the ON/OFF switch on the front of a PC. Connect it to any Ground wire - black - to enable power output.

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Auntystatic

Auntystatic

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Instead of complicating the cct by adding extra mosfets etc why not go for a complementary darlington pair, the tip120 has a continuous collector current of 5 Amp and the tip132 has a continuous collector current of 8Amp which really is overkill the tip120 would do, then all you would have to do was to move the heater from the collector to the emitter side. There is tons of stuff out there about using the tip120 as a motor controller etc :grin:

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