Resistance is Fertile - Page 1
Forum section: electronics / general

author: staxquad
author_rank: Frequent Contributor
timestamp: April 08, 2014, 09:52:39 pm
content: Resistance is FertilePlanting season for some of us in the Northern HemisphereWanted a head start with germination, otherwise would have to wait for the heat of the sun outside (like last year).Stared at some plates of glass from disassembled sliding windows all winter that's been stored for a few years, have resistance wire and decided to make a seed heating bed without spending any money for the project. (Been adding insulation upstairs, so have wood, plastic, screws)My plan was to put a length of resistance wire in between two plates of glass to get the soil temperature between 20* and 25*C.Cupron (copper nickel) resistance wire, 3.3 ohms/ft. It's varnished and has a cotton wrap. Made by Wilbur and Driver Co.First go at it, 75 ohms of wire positioned with duct tape (Red Green approved) , to figure out the right resistance for the right amount of heating to raise ambient temperature about 13*C+. Need more resistance, it's too hot.adjusting voltage to get the right wattage, temp, to figure resistance requiredtest with soil, direct plug into outlet, resistance/wattage setinstalled in the unused room upstairs (it was the second day of spring, can't you tell?)the heating bed of two glass plates sandwiching resistance wire was sealed with red Tuck Tape (Canadian Technical Tape Ltee, used to seal window flanges, insulation sheets, joints in plastic vapor barriers etc.)Germinated after just 4 daysRequired a timer to compensate for daytime and nighttime house ambient temperature variations, but still needed a temp controller ($4.50, my only additional expense so far for the project, will need 2 more temp controllers for the other two beds, already had the $1.70 digital temperature probe displays) for when the sun pops out and radiates additional heat through the window, I want the heat turned off. Has a 2*C spread from when it turns off to when it turns back on, reference temp adjusted on the pot, I set it to around 24.5*C.In the first bed 30"x20" upstairs in the unused room, heats at 46W (313 Ohms of resistance wire), planted 120 tomato seeds (they don't all germinate, need about 85 plants in 13 varieties, all Heirloom) and 50 Alpine Strawberry seeds (in the middle), after 4 days heating, tomatoes were sprouting already. After only a week the plants touched the tops of the plastic containers, so had to construct a mini greenhouse.replaced timer with temp control and a 12v power supplywhen upper temp is reached, relay activated, green led on and circuit opened, heating stopped (the power supply and temp control use up maybe a few watts only;when lower temp reached, relay deactivated, green led off, circuit closed, heating in progressThis is the ambient temperature during the day in that room (it's a few degrees colder at night)Started a second bed and box in the downstairs bathroom, heats at 67W (213 Ohms of resistance wire), for the Bloody Butchers and Black Krims that didn't completely sprout in the first bed, some peppers and 20 more Alpine Strawberries and some Oregano.This bed puts out 67 Watts, so the timer is set to be one hour on,one hour off and it holds temp within pretty well (except for when the sun shines, so will get a temp controller too)Will construct a third bed 24"x17"and box for the zucchini, cucumber, beans and peas etc., seeding due soon.Ordered this temp controller to control one of the beds, then I'll transfer it to the fan in the hothouse I will construct, this temp controller can allow you to input the high and low temp for any spread required, not just 2*C.Tuck Tape: Applications include sealing of joints and seams of housewrap, insulation materials and foam underlayments for laminate flooring.temp control, $4.50http://www.ebay.com/itm/261255575739?ssPageName=STRK:MEWNX:IT&_trksid=p3984.m1439.l2649Digital LCD Thermometer, $1.66http://www.ebay.com/itm/Digital-LCD-Thermometer-for-Refrigerator-Fridge-Freezer-Temperature-50-70-C-R1-/370981621292?pt=LH_DefaultDomain_0&hash=item56603a4e2c

author: rexxar
author_rank: Frequent Contributor
timestamp: April 08, 2014, 10:46:55 pm
content: Interesting, I was just looking for a way to do this. The only thing I could think of was to bolt some power transistors to a metal plate (I only needed 2 sq ft), but this should work much better.

author: staxquad
author_rank: Frequent Contributor
timestamp: April 09, 2014, 06:00:43 pm
content: Quote from: rexxar on April 08, 2014, 10:46:55 pmthis wire would probably do the trick, it's insulated and a good ohmic value for enough length for even coverage:http://www.ebay.com/itm/Isotan-Konstantan-Cotton-Insulated-Resistance-Heating-Wire-0-20-mm-AWG-32-/190960459160?pt=UK_BOI_Electrical_Components_Supplies_ET&var=&hash=item2c76215d98from Norway, but cheap shippingIsotan Konstantan Cotton Insulated Resistance Heating Wire 0.20 mm, AWG 3220 meters = 65 feetKonstantan = Cupron65ft x 4.76 ohms/ft = 312 ohms which is exactly the same total ohms and same alloy I use in the 30"x20" (though shorter length and smaller gauge but higher ohms/ft), and if you need a little more heat, cut the length as in my second bed at 213 ohms.