I am planning on making a propagator. Wanted a decent 1x0.4 metre area to place seedling trays.
I am going to install red/blue LED strips which I am more than happy to do...but I have been trying to find "12v heating cable" and it seems to be non-existant or hard to get for a reasonable price.
Can I just use say "resistance wire" of a length that will give me the resistance required (R=V/I = 12/0.5 = 25ohms)?
If I have 5m of this wire, so there is a nice even coverage...that leaves me with 5ohms per m?
Am I doing this right...or have I lost the plot?
Would basically placing 5m of this across a 12-14V battery give me the 25W of heat I want? 28swg res wire?
Thanks for any input...I don't want to go buying extra stuff to place in the garage!
PS. I will find 28swg wire on ebay of course...not from Maplin...the massively overpriced stock and less-than-knowledgeable staff.
Constantan is fine for low temperature - nichrome is needed at high temperatures (red hot) since
it doesn't oxidize. You need to keep it isolated from the soil, note, or it will corrode electrolytically
(whatever metal is it made from) - so you ideally want high-temperature insulation on the wire.
Your maths is awful: 25/12 != 0.5A, try 2.1A instead.
R = V^2/P = 144/25 = 5.8 ohms, or 1.4 metres of the wire you link to.
If you fine a longer length or lower resisitivity wire you may be able to use silicone insulated wire,
I suspect 1.4 metres at 25W will be too hot for anything but ceramic insulation (good luck finding that!)
This calculator will help you determine the power requirements. Either NiCr60 or NiCr80 will work at low temperatures. AWG26 is common. Select the item to be calculated and set the others to what you want.
If the voltage is low then use a switching power supply as your source and a PWM DC motor controller for temperature adjust. If the voltage required is high enough, more likely, then use 120VAC and an SCR type of speed controller with zero cross.
MarkT:
Constantan is fine for low temperature - nichrome is needed at high temperatures (red hot) since
it doesn't oxidize. You need to keep it isolated from the soil, note, or it will corrode electrolytically
(whatever metal is it made from) - so you ideally want high-temperature insulation on the wire.
Your maths is awful: 25/12 != 0.5A, try 2.1A instead.
R = V^2/P = 144/25 = 5.8 ohms, or 1.4 metres of the wire you link to.
If you fine a longer length or lower resisitivity wire you may be able to use silicone insulated wire,
I suspect 1.4 metres at 25W will be too hot for anything but ceramic insulation (good luck finding that!)
Woops! I did V/P in my head while I wrote P/V.
Anyway, looked at these calculators, and I only assume I want 40W in 4 rows of 1m wire = 10W per m.
It is 4 m of wire...so I can only assume it will not melt? As far as design, placing old tiles under it and above in a wooden box bottom...like a wire+ceramic sandwich, then with some thick plastic sheeting between the soil and the top tile layer?...bad idea?
That's the beauty of the calculator. You can alter the various parameters to get what you want.
If you want adjustability, there are many choices on eBay for switching supplies and speed controllers, and the wire. If not, you can use a 12VAC transformer or a surplus wall wart of sufficient amperage.
Resistors are dirt cheap. You can always make a string of
paralleled resistors to get that amount of heat.
100 1/4 watt resistors of about 1500 ohms will give you
9.6 watts.
Just an alternative.
How many LED's are you using? You might use
the LED dropping resistors to supplement the power
and be even more efficient. How much power
do you expect to use there?
Dwight
dwightthinker:
Resistors are dirt cheap. You can always make a string of
paralleled resistors to get that amount of heat.
100 1/4 watt resistors of about 1500 ohms will give you
9.6 watts.
Just an alternative.
How many LED's are you using? You might use
the LED dropping resistors to supplement the power
and be even more efficient. How much power
do you expect to use there?
Dwight
Pretty sure the £1.50 for the 10m of 28AWG NiCr 80:20 is actually cheaper. Both financially, physically and most importantly...mentally.
It was just to show an alternative. Not many places sell short pieces
of Nichrome wire.
I buy resistors at 5 for 1 cent in hundreds from a surplus shop.
The cost of solder would most likely exceed the price you quote,
though. The time spent soldering them all would also be a pain.
Still, you might look at how you are limiting the current to the LEDs.
there may be significant power wasted there.
Dwight