This is a DIY Article on how to build a ‘cheap as possible’ propagation unit for raising seedlings, cuttings and young plants.
The unit is ‘self regulated’ i.e. it does not use a thermostat as these devices are hard to get secondhand and usually come attached to a big things like fridges or ovens. The most important thing with getting stuff done on the cheap is patience. Eventually, everything you need will turn up at the dump or the op-shop.
The important components:
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Heating source:
The most critical (and hardest to get) thing you need to keep an eye out for is the heating element. What you need to look for is a heating device that outputs low amounts of heat over a large area. These are commonly found in electric blankets (I found the one I used in my propagator in a “dinner plate warmer”). This heating wire is usually insulated which is better for obvious reasons. You can also buy new uninsulated nichrome wire from Dick Smith or Jaycar (only 4m lengths) or any size/type from E-bay or a wire merchant (just search on ‘Nichrome heating wire’). Its actually pretty cheap from a supplier, which is why I cringe when I see how much they sell “heating units” for in the hydroponic shops.
- Light source:
The second critical thing is the lighting ballast. These can be bought for about $5-10 from scrap yards or the dump always has a ready supply. They just need to be wired up to a switch/plug.
The fluoros you cant really skimp on, but day-light (more blue) or cool-white (more even blue/red mix) work great for seedling/cutting/germination work. ‘Plant lights’ are specially doped and give you more bang for your energy consumption but they also cost about three times as much. Flouros dim eventually and need to be replaced. Swap them for fluoros in less light intensity critical areas like the shed.
- Power Source:
The third critical element is the power supply, if you choose to operate at mains power you don’t need an additional power supply. However….Should you choose to build this unit using mains power then it has the potential to kill you. Ensure that all conductive (i.e. metal) components are fully grounded and protective fuses are in place to ensure continued safety. Additionally, ensure a qualified electrician checks the safety of the device prior to operation. Having said this, the cheapest way to run the unit is at mains power as any transformer will loose you energy when dropping the voltage.
You can more safely build the unit using a safer voltage (say 12 or 24V).
- Building the heating element:
- Watt density calculations:
The watt density is what I used as the basis for calculating the wattage of the heating element I needed to build.
Calc 1: I want to heat an area equivalent to about 6 seedling flats (70*95cm) or 6860 sq.cm. So if the watt density needs to be 0.01 W/sq.cm I need an element that generates 6860 sq.cm * 0.01 W/sq.cm = 68.6 watts. Lets say 70 W to round it up.
Calc 2 (calculating the watts an element will generate, this calc is not really needed but it gives you an idea of the resistance you need when buying nichrome or maybe you’ve got an electric blanket in the house and might mention to the wife that its looking a bit shabby!): If I required an element of 70 Watts and ran this from a power supply of 240V the resistance of the element would need to be (240V*240V)/70W= 822 ohms.
If you have the wire already you can measure the resistance/meter (ohm/m) and determine how long the wire needs to be. As a rule of thumb the shorter the element wire the hotter it has to get to produce the same amount of heat (watts). This must be a compromise. You need a wire long enough to evenly distribute the heat over the area but not so long it is unmanageable or so short it melts anything. Length is not really a limiting factor as if the element is too long you just have to wind the wire more around your element frame, this gives more even heat distribution and reduces thermal strain on the wire as it operates at a lower overall temperature.
A note of caution: Nichrome does not take solder, it needs to be crimped or clamped for a secure electrical contact, so get good joiners from Dick Smith or a suitable nut and bolt would suffice.
To build my element I used ‘Corflute’. This material is a 5mm corrugated PVC plastic used universally in house ‘For Sale’ signs. It also retails for ~$20 a sheet (1.8*2.4m). If you found an electric blanket suited to your needs then you could just use that no modifications necessary.
All you need to then do is thread the wire up and down the corflute to form the element. You can seal/waterproof the element with silicon, but as this part should never be wet you can leave it be.
- Finding the right power supply:
If you choose to run at 240V you don’t need an additional power supply. But this is potentially dangerous and needs a qualified electrician to certify the job. Not building the unit out of anything conductive and ensuring your wiring is enclosed within the system means your pretty safe if anything goes wrong.
- Sources of cheap power supplies: (PS)
My standard source for a switch mode PS is old computers, but wall warts, battery chargers and light dimmers can also be used. The list is endless and you only need to check the unit is capable of pumping the Amps required (see below). You then can build the rest of the unit around the Volts the supply runs at (standards are 5, 12 or 24V, the higher the volts the better). Any old electrical equipment generally has good transformers, but these obviously need more work to make them work.
- Ensuring the power supply will do the job:
A note of caution about solid state power supplies, if your running a small heating unit it may not draw enough power to operate the power supply properly. Checking the minimum amperage these devices need is not usually possible as they don’t write this info on the units. This is especially pertinent with light dimmers and computer power supplies but you can get around this problem by adding additional loading to circuit (say a small light bulb to let you know the powers on).
To check that your power supply will handle the Amps drawn by the element, you need to factor in a safety margin of 2. This is an arbitrary safety measure for determining power supply suitability.
Calc 2: I calculated above (calc 1) that the element generates 70 watts of heat; therefore I need a power supply that is rated to at least 140 Watts. However, more commonly, power supplies are rated in Amps, so a conversion is needed.
I have calculated I need a 70 Watt element (see calc 1), and I have a power supply equivalent to mains voltage (240V rated at 10A). Is my power supply big enough? Well a 70Watt element will draw 70W/240V = 0.29Amps. As my power supply is rated at 10 Amps this gives me a safety factor of 10A/0.29A= 34. So yes the power supply will not overheat when running the element.
As a rule of thumb the smaller the voltage used the higher the Amps needed to produce the same amount of power from an element. The power supply you find will be the primary determinate on how big a unit you can build. If you start getting low on the safety margin or element wire length play around with the element wattage and reduce the overall area of the propagator. It’s a matter of crunching the numbers to see what fits best. The best way to do this is to make a spreadsheet.
- Wiring the light:
This is all standard wiring to a three pin plug.
- Putting it all together:
I built the element first (cut to fit ~six seed flats) and then built a tray from framing pine and masonite (any scrap will do). I first lined the tray with polystyrene cut from broccoli boxes and then with some old foam yoga mats (you want the heat to go up, not down). To waterproof the element I used pond liner plastic as a solid barrier between the pot area and the tray/element components. This was followed by a 20kg bag of washed sand. The sand helps the liner plastic keep close contact with the element and builds up the thermal mass of the unit.
The whole thing is on an old desk and the light is suspended from the roof with a couple of pulleys and a rope.
- Operation of the unit:
As mentioned earlier the more insulation you have over the heating pad the hotter it will make the soil due to reduced heat loss to the environment. The converse happens also, so it is important if you want to get the heat built up to use wet sand in the unit to increase the heat retention. If you don’t use sand an alternative could be to use foam covers over the sections you are not using to trap the generated heat.
Equations for converting stuff
V=Voltage A=Amps R=Resistance (ohms) W=Watts
W= (V*V)/R
A= W/V
V=IR
R=(V*V)/W
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